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[mascara-docs.git] / hw / intel 387 programmers reference manual.txt

INTEL 80387 PROGRAMMER'S REFERENCE MANUAL 1987

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May 26, 1987

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Preface

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

This manual describes the 80387 Numeric Processor Extension (NPX) for the
80386 microprocessor. Understanding the 80387 requires an understanding of
the 80386; therefore, a brief overview of 80386 concepts is presented first.
A detailed discussion of the 80386 microprocessor can be found in the 80386
Programmer's Reference Manual.

The 80386 Microsystem

The 80386 is the basis of a new VLSI microprocessor system with exceptional
capabilities for supporting large-system applications. This powerful
microsystem is designed to support multiuser reprogrammable and real-time
multitasking applications. Its dedicated system support circuits simplify
system hardware; sophisticated hardware and software tools reduce both the
time and the cost of product development. The 80386 microsystem offers a
total-solution approach, enabling you to develop high-speed, interactive,
multiuser, multitasking‘‘even multiprocessor‘‘systems more rapidly and at
higher performance than ever before.

  Ž  Reliability and system up-time are becoming increasingly important in
     all applications. Information must be protected from misuse or
     accidental loss. The 80386 includes a sophisticated and flexible
     four-level protection mechanism that can isolate layers of operating
     system programs from application programs to maintain a high degree of
     system integrity.

  Ž  The 80386 addresses up to 4 gigabytes of physical memory to support
     today's application requirements. This large physical memory enables
     the 80386 to keep many large programs and data structures
     simultaneously in memory for high-speed access.

  Ž  For applications with dynamically changing memory requirements, such
     as multiuser business systems, the 80386 CPU provides on-chip memory
     management and virtual memory support. On an 80386-based system, each
     user can have up to 64 terabytes of virtual-address space. This large
     address space virtually eliminates restrictions on the size of programs
     that may be part of the system. The memory management features are
     subject to control of systems software; therefore, systems software
     designers can choose among a variety of memory-organization models.
     Systems designers can choose to view memory in terms of fixed-length
     pages, in terms of variable length segments, or as a combination of
     pages and segments. The sizes of segments can range from one byte to 4
     gigabytes. Virtual memory can be implemented either at the level of
     segments or at the level of pages.

  Ž  Large multiuser or real-time multitasking systems are easily supported
     by the 80386. High-performance features, such as a very high-speed task
     switch, fast interrupt-response time, intertask protection,
     page-oriented virtual memory, and a quick and direct operating system
     interface, make the 80386 highly suited to multiuser/multitasking 
     applications.

  Ž  The 80386 has two primary operating modes: real-address mode and
     protected mode. In real-address mode, the 80386/80387 is fully upward
     compatible from the 8086, 8088, 80186, and 80188 microprocessors and
     from the 80286 real-address mode; all of the extensive libraries of
     8086 and 8088 software execute 15 to 20 times faster on the 80386,
     without any modification.

  Ž  In protected-address mode, the advanced memory management
     and protection features of the 80386 become available, without any
     reduction in performance. Upgrading 8086 and 8088 application
     programs to use these new memory management and protection features
     usually requires only reassembly or recompilation (some programs may
     require minor modification). Entire 80286 protected-mode applications
     can run in this mode without modification.

  Ž  The virtual-8086 mode of the 80386 is available when the primary mode
     is protected mode. Virtual-8086 mode enables direct execution of
     multiple 8086/8088 programs within a protected-mode environment. Most
     8086 and 8088 application programs can be executed in this environment
     without alteration (refer to the 80386 Programmer's Reference Manual
     for differences from 8086). This high degree of compatibility between
     80386 and earlier members of the 8086 processor family reduces both
     the time and the cost of software development.

The Organization of This Manual

This manual describes the 80387 Numeric Processor Extension (NPX) for the
80386 microprocessor. The material in this manual is presented from the
perspective of software designers, both at an applications and at a systems
software level.

  Ž  Chapter 1, "Introduction to the 80387 Numerics Processor Extension,"
     gives an overview of the 80387 NPX and reviews the concepts of numeric
     computation using the 80387.

  Ž  Chapter 2, "80387 Numerics Processor Architecture," presents the
     registers and data types of the 80387 to both applications and systems
     programmers.

  Ž  Chapter 3, "Special Computational Situations," discusses the special
     values that can be represented in the 80387's real formats‘‘denormal
     numbers, zeros, infinities, NaNs (not a number)‘‘as well as numerics
     exceptions. This chapter should be read thoroughly by systems
     programmers, but may be skimmed by applications programmers. Many of
     these special values and exceptions may never occur in applications
     programs.

  Ž  Chapter 4, "80387 Instruction Set," provides functional information
     for software designers generating applications for systems containing
     an 80386 CPU with an 80387 NPX. The 80386/80387 instruction set
     mnemonics are explained in detail.

  Ž  Chapter 5, "Programming Numeric Applications," provides a description
     of programming facilities for 80386/80387 systems. A comparative 80387
     programming example is given.

  Ž  Chapter 6, "System-Level Numeric Programming," provides information of
     interest to systems software writers, including details of the 80387
     architecture and operational characteristics.

  Ž  Chapter 7, "Numeric Programming Examples," provides several detailed
     programming examples for the 80387, including conditional branching,
     the conversion betweenfloating-point values and their ASCII
     representations, and the use of trigonometric functions. These examples
     illustrate assembly-language programming on the 80387 NPX.

  Ž  Appendix A, "Machine Instruction Encoding and Decoding," gives
     reference information on the encoding of NPX instructions. This
     information is useful to writers of debuggers, exception handlers, and
     compilers.

  Ž  Appendix B, "Exception Summary," provides a list of the exceptions 
     that each instruction can cause. This list is valuable to both 
     applications and systems programmers.

  Ž  Appendix C, "Compatability between the 80387 and the 80287/8087,"
     describes the differences from the 80387 that are common to the 80287
     and the 8087.

  Ž  Appendix D, "Compatability between the 80387 and the 8087," describes
     the additional differences between the 80387 and the 8087 that are of
     concern when porting 8086/8087 programs directly to the 80386/80387.

  Ž  Appendix E
Please consult the most recent 80387 data sheet for these specifications, "80387 80-Bit CHMOS III Numeric Processor Extension,"
     reproduces a data sheet of 80387 specifications that is separately
     available. The table of instruction timings in this appendix will be of
     interest to many readers of this manual. (The AC specifications have
     been deliberately left out.) The specifications in data sheets are
     subject to change; consult the most recent data sheet for design-in
     information.

  Ž  Appendix F, "PC/AT-Compatible 80387 Connection," documents a
     nonstandard method of connecting an 80387 to an 80386 to achieve
     compatibility with the IBM PC/AT.

  Ž  The Glossary defines 80387 and floating-point terminology. Refer to it
     as needed.

Related Publications

To best use the material in this manual, readers should be familiar with
the operation and architecture of 80386 systems. The following manuals
contain information related to the content of this manual and of interest to
programmers of 80387 systems:

  Ž  Introduction to the 80386, order number 231252
  Ž  80386 Data Sheet, order number 231630
  Ž  80386 Hardware Reference Manual, order number 231732
  Ž  80386 Programmer's Reference Manual, order number 230985
  Ž  80387 Data Sheet, order number 231920


Notational Conventions

This manual uses special notation to represent sub and superscript
characters. Subscript characters are surrounded by {curly brackets}, for
example 10{2} = 10 base 2. Superscript characters are preceeded by a caret
and enclosed within (parentheses), for example 10^(3) = 10 to the third
power.


Table of Contents

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

Chapter 1  Introduction to the 80387 Numerics Processor Extension

1.1  History
1.2  Performance 
1.3  Ease of Use 
1.4  Applications
1.5  Upgradability
1.6  Programming Interface

Chapter 2  80387 Numerics Processor Architecture

2.1  80387 Registers 
      2.1.1  The NPX Register Stack
      2.1.2  The NPX Status Word
      2.1.3  Control Word 
      2.1.4  The NPX Tag Word 
      2.1.5  The NPX Instruction and Data Pointers 

2.2  Computation Fundamentals 
      2.2.1  Number System 
      2.2.2  Data Types and Formats 
              2.2.2.1  Binary Integers 
              2.2.2.2  Decimal Integers 
              2.2.2.3  Real Numbers

      2.2.3  Rounding Control 
      2.2.4  Precision Control

Chapter 3  Special Computational Situations

3.1  Special Numeric Values
      3.1.1  Denormal Real Numbers
              3.1.1.1  Denormals and Gradual Underflow

      3.1.2  Zeros
      3.1.3  Infinity
      3.1.4  NaN (Not-a-Number)
              3.1.4.1  Signaling NaNs
              3.1.4.2  Quiet NaNs

      3.1.5  Indefinite
      3.1.6  Encoding of Data Types
      3.1.7  Unsupported Formats

3.2  Numeric Exceptions
      3.2.1  Handling Numeric Exceptions
              3.2.1.1  Automatic Exception Handling
              3.2.1.2  Software Exception Handling

      3.2.2  Invalid Operation
              3.2.2.1  Stack Exception
              3.2.2.2  Invalid Arithmetic Operation

      3.2.3  Division by Zero
      3.2.4  Denormal Operand
      3.2.5  Numeric Overflow and Underflow
              3.2.5.1  Overflow
              3.2.5.2  Underflow

      3.2.6  Inexact (Precision)
      3.2.7  Exception Priority
      3.2.8  Standard Underflow/Overflow Exception Handler

Chapter 4  The 80387 Instruction Set

4.1  Compatibility with the 80287 and 8087
4.2  Numeric Operands
4.3  Data Transfer Instructions
      4.3.1  FLD source
      4.3.2  FST destination
      4.3.3  FSTP destination
      4.3.4  FXCH//destination
      4.3.5  FILD source
      4.3.6  FIST destination
      4.3.7  FISTP destination
      4.3.8  FBLD source
      4.3.9  FBSTP destination

4.4  Nontranscendental Instructions
      4.4.1  Addition
      4.4.2  Normal Subtraction
      4.4.3  Reversed Subtraction
      4.4.4  Multiplication
      4.4.5  Normal Division
      4.4.6  Reversed Division
      4.4.7  FSQRT
      4.4.8  FSCALE
      4.4.9  FPREM---Partial Remainder (80287/8087-Compatible)
      4.4.10 FPREM1---Partial Remainder (IEEE Std. 754-Compatible)
      4.4.11 FRNDINT
      4.4.12 FXTRACT
      4.4.13 FABS
      4.4.14 FCHS

4.5  Comparison Instructions
      4.5.1  FCOM//source
      4.5.2  FCOMP//source
      4.5.3  FCOMPP
      4.5.4  FICOM source
      4.5.5  FICOMP source
      4.5.6  FTST
      4.5.7  FUCOM//source
      4.5.8  FUCOMP//source
      4.5.9  FUCOMPP
      4.5.10 FXAM

4.6  Transcendental Instructions
      4.6.1  FCOS
      4.6.2  FSIN
      4.6.3  FSINCOS
      4.6.4  FPTAN
      4.6.5  FPATAN
      4.6.6  F2XM1
      4.6.7  FYL2X
      4.6.8  FYL2XP1

4.7  Constant Instructions
      4.7.1  FLDZ
      4.7.2  FLD1
      4.7.3  FLDPI
      4.7.4  FLDL2T
      4.7.5  FLDL2E
      4.7.6  FLDLG2
      4.7.7  FLDLN2

4.8  Processor Control Instructions
      4.8.1  FINIT/FNINIT
      4.8.2  FLDCW source
      4.8.3  FSTCW/FNSTCW destination
      4.8.4  FSTSW/FNSTSW destination
      4.8.5  FSTSW AX/FNSTSW AX
      4.8.6  FCLEX/FNCLEX
      4.8.7  FSAVE/FNSAVE destination
      4.8.8  FRSTOR source
      4.8.9  FSTENV/FNSTENV destination
      4.8.10 FLDENV source
      4.8.11 FINCSTP
      4.8.12 FDECSTP
      4.8.13 FFREE destination
      4.8.14 FNOP
      4.8.15 FWAIT (CPU Instruction)

Chapter 5  Programming Numeric Applications

5.1  Programming Facilities
      5.1.1  High-Level Languages
      5.1.2  C Programs
      5.1.3  PL/M-386
      5.1.4  ASM386
              5.1.4.1  Defining Data
              5.1.4.2  Records and Structures
              5.1.4.3  Addressing Methods

      5.1.5  Comparative Programming Example
      5.1.6  80387 Emulation

5.2  Concurrent Processing with the 80387
      5.2.1  Managing Concurrency
              5.2.1.1  Incorrect Exception Synchronization
              5.2.1.2  Proper Exception Synchronization

Chapter 6  System-Level Numeric Programming

6.1  80386/80387 Architecture
      6.1.1  Instruction and Operand Transfer
      6.1.2  Independent of CPU Addressing Modes
      6.1.3  Dedicated I/O Locations

6.2  Processor Initialization and Control
      6.2.1  System Initialization
      6.2.2  Hardware Recognition of the NPX
      6.2.3  Software Recognition of the NPX
      6.2.4  Configuring the Numerics Environment
      6.2.5  Initializing the 80387
      6.2.6  80387 Emulation
      6.2.7  Handling Numerics Exceptions
      6.2.8  Simultaneous Exception Response
      6.2.9  Exception Recovery Examples

Chapter 7  Numeric Programming Examples

7.1  Conditional Branching Example
7.2  Exception Handling Examples
7.3  Floating-Point to ASCII Conversion Examples
      7.3.1  Function Partitioning
      7.3.2  Exception Considerations 
      7.3.3  Special Instructions
      7.3.4  Description of Operation
      7.3.5  Scaling the Value
              7.3.5.1  Inaccuracy in Scaling
              7.3.5.2  Avoiding Underflow and Overflow
              7.3.5.3  Final Adjustments

      7.3.6  Output Format

7.4  Trigonometric Calculation Examples (Not Tested)

Appendix A  Machine Instruction Encoding and Decoding

Appendix B  Exception Summary

Appendix C  Compatibility Between the 80387 and the 80287/8087

Appendix D  Compatibility Between the 80387 and the 8087

Appendix E  80387 80-Bit CHMOS III Numeric Processor Extension

Appendix F  PC/AT-Compatible 80387 Connection

Glossary of 80387 and Floating-Point Terminology


Figures

1-1     Evolution and Performance of Numeric Processors

2-1     80387 Register Set
2-2     80387 Status Word
2-3     80387 Control Word Format
2-4     80387 Tag Word Format
2-5     Protected Mode 80387 Instruction and Data Pointer Image in Memory,
            32-Bit Format
2-6     Real Mode 80387 Instruction and Data Pointer Image in Memory,
            32-Bit Format
2-7     Protected Mode 80387 Instruction and Data Pointer Image in Memory,
            16-Bit Format
2-8     Real Mode 80387 Instruction and Data Pointer Image in Memory,
            16-Bit Format
2-9     80387 Double-Precision Number System
2-10    80387 Data Formats

3-1     Floating-Point System with Denormals
3-2     Floating-Point System without Denormals
3-3     Arithmetic Example Using Infinity

4-1     FSAVE/FRSTOR Memory Layout (32-Bit)
4-2     FSAVE/FRSTOR Memory Layout (16-Bit)
4-3     Protected Mode 80387 Environment, 32-Bit Format
4-4     Real Mode 80387 Environment, 32-Bit Format
4-5     Protected Mode 80387 Environment, 16-Bit Format
4-6     Real Mode 80387 Environment, 16-Bit Format

5-1     Sample C-386 Program
5-2     Sample 80387 Constants
5-3     Status Word Record Definition
5-4     Structure Definition
5-5     Sample PL/M-386 Program
5-6     Sample ASM386 Program
5-7     Instructions and Register Stack
5-8     Exception Synchronization Examples

6-1     Software Routine to Recognize the 80287

7-1     Conditional Branching for Compares
7-2     Conditional Branching for FXAM
7-3     Full-State Exception Handler
7-4     Reduced-Latency Exception Handler
7-5     Reentrant Exception Handler
7-6     Floating-Point to ASCII Conversion Routine
7-7
See page 7-22 in the printed version of this manual     Relationships between Adjacent Joints
7-8     Robot Arm Kinematics Example


Tables

1-1     Numeric Processing Speed Comparisons
1-2     Numeric Data Types
1-3     Principal NPX Instructions

2-1     Condition Code Interpretation
2-2     Correspondence between 80387 and 80386 Flag Bits
2-3     Summary of Format Parameters
2-4     Real Number Notation
2-5     Rounding Modes

3-1     Arithmetic and Nonarithmetic Instructions
3-2     Denormalization Process
3-3     Zero Operands and Results
3-4     Infinity Operands and Results
3-5     Rules for Generating QNaNs
3-6     Binary Integer Encodings
3-7     Packed Decimal Encodings
3-8     Single and Double Real Encodings
3-9     Extended Real Encodings
3-10    Masked Responses to Invalid Operations
3-11    Masked Overflow Results

4-1     Data Transfer Instructions
4-2     Nontranscendental Instructions
4-3     Basic Nontranscendental Instructions and Operands
4-4     Condition Code Interpretation after FPREM and FPREM
            Instructions
4-5     Comparison Instructions
4-6     Condition Code Resulting from Comparisons
4-7     Condition Code Resulting from FTST
4-8     Condition Code Defining Operand Class
4-9     Transcendental Instructions
4-10    Results of FPATAN
4-11    Constant Instructions
4-12    Processor Control Instructions

5-1     PL/M-386 Built-In Procedures
5-2     ASM386 Storage Allocation Directives
5-3     Addressing Method Examples

6-1     NPX Processor State Following Initialization


Chapter 1  Introduction to the 80387 Numerics Processor Extension

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

The 80387 NPX is a high-performance numerics processing element that
extends the 80386 architecture by adding significant numeric capabilities
and direct support for floating-point, extended-integer, and BCD data types.
The 80386 CPU with 80387 NPX easily supports powerful and accurate numeric
applications through its implementation of the IEEE Standard 754 for Binary
Floating-Point Arithmetic. The 80387 provides floating-point performance
comparable to that of large minicomputers while offering compatibility with
object code for 8087 and 80287.


1.1  History

The 80387 Numeric Processor Extension (NPX) is compatible with its
predecessors, the earlier Intel 8087 NPX and 80287 NPX. As the 80386 runs
8086 programs, so programs designed to use the 8087 and 80287 should run
unchanged on the 80387.

The 8087 NPX was designed for use in 8086-family systems. The 8086 was the
first microprocessor family to partition the processing unit to permit
high-performance numeric capabilities. The 8087 NPX for this processor
family implemented a complete numeric processing environment in compliance
with an early proposal for the IEEE 754 Floating-Point Standard.

With the 80287 Numeric Processor Extension, high-speed numeric computations
were extended to 80286 high-performance multitasking and multiuser systems.
Multiple tasks using the numeric processor extension were afforded the full
protection of the 80286 memory management and protection features.

The 80387 Numeric Processor Extension is Intel's third generation numerics
processor. The 80387 implements the final IEEE standard, adds new
trigonometric instructions, and uses a new design and CHMOS-III process to
allow higher clock rates and require fewer clocks per instruction. Together,
the 80387 with additional instructions and the improved standard bring even
more convenience and reliability to numerics programming and make this
convenience and reliability available to applications that need the
high-speed and large memory capacity of the 32-bit environment of the 80386
CPU.

Figure 1-1 illustrates the relative performance of 5-MHz 8086/8087,
8-MHz 80286/80287, and 20-MHz 80386/80387 systems in executing
numerics-oriented applications.


Figure 1-1.  Evolution and Performance of Numeric Processors

                  16�                       80386/80387 (20 MHz)
                  15�
                  14�
                  13�
                  12�
                  11�
     RELATIVE     10�
     PERFORMANCE   9�
                   8�
                   7�
                   6�
                   5�
                   4�
                   3�     80286/80287 (8 MHz)
                   2�
                   1�  8086/8087 (5 MHz)
                    ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                         1980        1983        1987

                                 YEAR INTRODUCED


1.2  Performance

Table 1-1 compares the execution times of several 80387 instructions with
the equivalent operations executed on an 8-MHz 80287. As indicated in the
table, the 16-MHz 80387 NPX provides about 5 to 6 times the performance of
an 8-MHz 80287 NPX. A 16-MHz 80387 multiplies 32-bit and 64-bit
floating-point numbers in about 1.9 and 2.8 microseconds, respectively. Of
course, the actual performance of the NPX in a given system depends on the
characteristics of the individual application.

Although the performance figures shown in Table 1-1 refer to operations on
real (floating-point) numbers, the 80387 also manipulates fixed-point
binary and decimal integers of up to 64 bits or 18 digits, respectively. The
80387 can improve the speed of multiple-precision software algorithms for
integer operations by 10 to 100 times.

Because the 80387 NPX is an extension of the 80386 CPU, no software
overhead is incurred in setting up the NPX for computation. The 80387 and
80386 processors coordinate their activities in a manner transparent to
software. Moreover, built-in coordination facilities allow the 80386 CPU to
proceed with other instructions while the 80387 NPX is simultaneously
executing numeric instructions. Programs can exploit this concurrency of
execution to further increase system performance and throughput.


Table 1-1.  Numeric Processing Speed Comparisons

                                           Approximate Performance Ratios:
       Floating-Point Instruction                16 MHz 80386/80387 ÷
 ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“         8 MHz 80286/80287

FADD      ST, ST(i)                Addition             6.2
FDIV      dword_var                Division             4.7
FYL2X     stack (0), (1) assumed   Logarithm            6.0
FPATAX    stack (0) assumed        Arctangent           2.6
The ratio is higher if the operand is not in range of the 80287
instruction.
F2XM1     stack (0) assumed        Exponentiation       2.7
The ratio is higher if the operand is not in range of the 80287
instruction.


1.3  East of Use

The 80387 NPX offers more than raw execution speed for
computation-intensive tasks. The 80387 brings the functionality and power of
accurate numeric computation into the hands of the general user. These
features are available in most high-level languages available for the 80386.

Like the 8087 and 80287 that preceded it, the 80387 is explicitly designed
to deliver stable, accurate results when programmed using straightforward
"pencil and paper" algorithms. The IEEE standard 754 specifically addresses
this issue, recognizing the fundamental importance of making numeric
computations both easy and safe to use.

For example, most computers can overflow when two single-precision
floating-point numbers are multiplied together and then divided by a third,
even if the final result is a perfectly valid 32-bit number. The 80387
delivers the correctly rounded result. Other typical examples of undesirable
machine behavior in straightforward calculations occur when computing
financial rate of return, which involves the expression (1 + i)^(n) or when
solving for roots of a quadratic equation:

       -b ± ¹(b² - 4ac)
       ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
              2a

If a does not equal 0, the formula is numerically unstable when the roots
are nearly coincident or when their magnitudes are wildly different. The
formula is also vulnerable to spurious over/underflows when the coefficients
a, b, and c are all very big or all very tiny. When single-precision
(4-byte) floating-point coefficients are given as data and the formula is
evaluated in the 80387's normal way, keeping all intermediate results in
its stack, the 80387 produces impeccable single-precision roots. This
happens because, by default and with no effort on the programmer's part, the
80387 evaluates all those subexpressions with so much extra precision and
range as to overwhelm any threat to numerical integrity.

If double-precision data and results were at issue, a better formula would
have to be used, and once again the 80387's default evaluation of that
formula would provide substantially enhanced numerical integrity over mere
double-precision evaluation.

On most machines, straightforward algorithms will not deliver consistently
correct results (and will not indicate when they are incorrect). To obtain
correct results on traditional machines under all conditions usually
requires sophisticated numerical techniques that are foreign to most
programmers. General application programmers using straightforward
algorithms will produce much more reliable programs using the 80387. This
simple fact greatly reduces the software investment required to develop
safe, accurate computation-based products.

Beyond traditional numerics support for scientific applications, the 80387
has built-in facilities for commercial computing. It can process decimal
numbers of up to 18 digits without round-off errors, performing exact
arithmetic on integers as large as 2^(64) or 10^(18). Exact arithmetic is
vital in accounting applications where rounding errors may introduce
monetary losses that cannot be reconciled.

The NPX contains a number of optional facilities that can be invoked by
sophisticated users. These advanced features include directed rounding,
gradual underflow, and programmed exception-handling facilities.

These automatic exception-handling facilities permit a high degree of
flexibility in numeric processing software, without burdening the
programmer. While performing numeric calculations, the NPX automatically
detects exception conditions that can potentially damage a calculation (for
example, X ÷ 0 or ¹X when X < 0). By default, on-chip exception logic
handles these exceptions so that a reasonable result is produced and
execution may proceed without program interruption. Alternatively, the NPX
can signal the CPU, invoking a software exception handler to provide special
results whenever various types of exceptions are detected.


1.4  Applications

The 80386's versatility and performance make it appropriate to a broad
array of numeric applications. In general, applications that exhibit any of
the following characteristics can benefit by implementing numeric processing
on the 80387:

  Ž  Numeric data vary over a wide range of values, or include nonintegral
     values.

  Ž  Algorithms produce very large or very small intermediate results.

  Ž  Computations must be very precise; i.e., a large number of significant
     digits must be maintained.

  Ž  Performance requirements exceed the capacity of traditional
     microprocessors.

  Ž  Consistently safe, reliable results must be delivered using a
     programming staff that is not expert in numerical techniques.

Note also that the 80387 can reduce software development costs and improve
the performance of systems that use not only real numbers, but operate on
multiprecision binary or decimal integer values as well.

A few examples, which show how the 80387 might be used in specific numerics
applications, are described below. In many cases, these types of systems
have been implemented in the past with minicomputers or small mainframe
computers. The advent of the 80387 brings the size and cost savings of
microprocessor technology to these applications for the first time.

  Ž  Business data processing‘‘The NPX's ability to accept decimal operands
     and produce exact decimal results of up to 18 digits greatly simplifies
     accounting programming. Financial calculations that use power functions
     can take advantage of the 80387's exponentiation and logarithmic
     instructions. Many business software packages can benefit from the
     speed and accuracy of the 80387; for example, Lotus* 1-2-3*,
     Multiplan*, SuperCalc*, and Framework*.

  Ž  Simulation‘‘The large (32-bit) memory space of the 80386 coupled with
     the raw speed of the 80386 and 80387 processors make 80386/80387
     microsystems suitable for attacking large simulation problems, which
     heretofore could only be executed on expensive mini and mainframe
     computers. For example, complex electronic circuit simulations using
     SPICE can now be performed on a microcomputer, the 80386/80387.
     Simulation of mechanical systems using finite element analysis can
     employ more elements, resulting in more detailed analysis or simulation
     of larger systems.

  Ž  Graphics transformations‘‘The 80387 can be used in graphics terminals
     to locally perform many functions that normally demand the attention of
     a main computer; these include rotation, scaling, and interpolation. By
     also using an 82786 Graphics Display Controller to perform high-speed
     drawing and window management, very powerful and highly self-sufficient
     terminals can be built from a relatively small number of 80386 family
     parts.

  Ž  Process control‘‘The 80387 solves dynamic range problems
     automatically, and its extended precision allows control functions to
     be fine-tuned for more accurate and efficient performance. Control
     algorithms implemented with the NPX also contribute to improved
     reliability and safety, while the 80387's speed can be exploited in
     real-time operations.

  Ž  Computer numerical control (CNC)‘‘The 80387 can move and position
     machine tool heads with accuracy in real-time. Axis positioning also
     benefits from the hardware trigonometric support provided by the 80387.

  Ž  Robotics‘‘Coupling small size and modest power requirements with
     powerful computational abilities, the 80387 is ideal for on-board
     six-axis positioning.

  Ž  Navigation‘‘Very small, lightweight, and accurate inertial guidance
     systems can be implemented with the 80387. Its built-in trigonometric
     functions can speed and simplify the calculation of position from
     bearing data.

  Ž  Data acquisition‘‘The 80387 can be used to scan, scale, and reduce
     large quantities of data as it is collected, thereby lowering storage
     requirements and time required to process the data for analysis.

The preceding examples are oriented toward traditional numerics
applications. There are, in addition, many other types of systems that do
not appear to the end user as computational, but can employ the 80387 to
advantage. Indeed, the 80387 presents the imaginative system designer with
an opportunity similar to that created by the introduction of the
microprocessor itself. Many applications can be viewed as numerically-based
if sufficient computational power is available to support this view (e.g.,
character generation for a laser printer). This is analogous to the
thousands of successful products that have been built around "buried"
microprocessors, even though the products themselves bear little
resemblance to computers.


1.5  Upgradability

The architecture of the 80386 CPU is specifically adapted to allow easy
upgradability to use an 80387, simply by plugging in the 80387 NPX. For this
reason, designers of 80386 systems may wish to incorporate the 80387 NPX
into their designs in order to offer two levels of price and performance at
little additional cost.

Two features of the 80386 CPU make the design and support of upgradable
80386 systems particularly simple:

  Ž  The 80386 can be programmed to recognize the presence of an 80387 NPX;
     that is, software can recognize whether it is running on an 80386 with
     or without an 80387 NPX.

  Ž  After determining whether the 80387 NPX is available, the 80386 CPU
     can be instructed to let the NPX execute all numeric instructions. If
     an 80387 NPX is not available, the 80386 CPU can emulate all 80387
     numeric instructions in software. This emulation is completely
     transparent to the application software‘‘the same object code may be
     used by 80386 systems both with and without an 80387 NPX. No relinking
     or recompiling of application software is necessary; the same code will
     simply execute faster with the 80387 NPX than without.

To facilitate this design of upgradable 80386 systems, Intel provides a
software emulator for the 80387 that provides the functional equivalent of
the 80387 hardware, implemented in software on the 80386. Except for timing,
the operation of this 80387 emulator (EMUL387) is the same as for the 80387
NPX hardware. When the emulator is combined as part of the systems software,
the 80386 system with 80387 emulation and the 80386 with 80387 hardware are
virtually indistinguishable to an application program. This capability
makes it easy for software developers to maintain a single set of programs
for both systems. System manufacturers can offer the NPX as a simple plug-in
performance option without necessitating any changes in the user's software.


1.6  Programming Interface

The 80386/80387 pair is programmed as a single processor; all of the 80387
registers appear to a programmer as extensions of the basic 80386 register
set. The 80386 has a class of instructions known as ESCAPE instructions, all
having a common format. These ESC instructions are numeric instructions for
the 80387 NPX. These numeric instructions for the 80387 are simply encoded
into the instruction stream along with 80386 instructions.

All of the CPU memory-addressing modes may be used in programming the NPX,
allowing convenient access to record structures, numeric arrays, and other
memory-based data structures. All of the memory management and protection
features of the CPU (both paging and segmentation) are extended to the NPX
as well.

Numeric processing in the 80387 centers around the NPX register stack.
Programmers can treat these eight 80-bit registers either as a fixed
register set, with instructions operating on explicitly-designated
registers, or as a classical stack, with instructions operating on the top
one or two stack elements.

Internally, the 80387 holds all numbers in a uniform 80-bit extended
format. Operands that may be represented in memory as 16-, 32-, or 64-bit
integers, 32-, 64-, or 80-bit floating-point numbers, or 18-digit packed BCD
numbers, are automatically converted into extended format as they are loaded
into the NPX registers. Computation results are subsequently converted back
into one of these destination data formats when they are stored into memory
from the NPX registers.

Table 1-2 lists each of the seven data types supported by the 80387,
showing the data format for each type. All operands are stored in memory
with the least significant digits starting at the initial (lowest) memory
address. Numeric instructions access and store memory operands using only
this initial address. For maximum system performance, all operands should
start at memory addresses divisible by four.

Table 1-3 lists the 80387 instructions by class. No special programming
tools are necessary to use the 80387, because all of the NPX instructions
and data types are directly supported by the ASM386 Assembler, by high-level
languages from Intel, and by assemblers and compilers produced by many
independent software vendors. Software routines for the 80387 may be written
in ASM386 Assembler or any of the following higher-level languages from
Intel:

      PL/M-386
      C-386

In addition, all of the development tools supporting the 8086/8087 and
80286/80287 can also be used to develop software for the 80386/80387.

All of these high-level languages provide programmers with access to the
computational power and speed of the 80387 without requiring an
understanding of the architecture of the 80386 and 80387 chips. Such
architectural considerations as concurrency and synchronization are handled
automatically by these high-level languages. For the ASM386 programmer,
specific rules for handling these issues are discussed in a later section
of this manual.

The following operating systems are known or expected to support the
80387: RMX-286/386, MS-DOS, Xenix-286/386, and Unix-286/386. Advanced
in-circuit debugging support is provided by ICE-386.


Table 1-2.  Numeric Data Types

Data Type       Bits  Significant  Approximate Range (Decimal)
                      Digits
                      (Decimal)

Word integer    16    4            -32,768 ¾ X ¾ +32,767
Short integer   32    9            -2*10^(9) ¾ X ¾ +2*10^(9)
Long integer    64    18           -9*10^(18) ¾ X ¾ +9*10^(18)
Packed decimal  80    18           -99...99 ¾ X ¾ +99...99 (18 digits)
Single real     32    6-7          1.18*10^(-38) ¾ �X� ¾ 3.40*10^(38)
Double real     64    15-16        2.23*10^(-308) ¾ �X� ¾ 1.80*10^(308)
Extended real
Equivalent to double extended format of IEEE Std 754  80    19           3.30*10^(-4932) ¾ �X� ¾ 1.21*10^(4932)


Table 1-3.  Principal NPX Instructions

Class                Instruction Types

Data Transfer        Load (all data types), Store (all data types), Exchange

Arithmetic           Add, Subtract, Multiply, Divide, Subtract Reversed,
                     Divide Reversed, Square Root, Scale, Remainder, Integer
                     Part, Change Sign, Absolute Value, Extract

Comparison           Compare, Examine, Test

Transcendental       Tangent, Arctangent, Sine, Cosine, Sine and Cosine,
                     2^(x) - 1, Y * Log{2}(X), Y * Log{2}(X+1)

Constants            0, 1, Ò, Log{10}2, Log{e}2, Log{2}10, Log{2}e

Processor Control    Load Control Word, Store Control Word, Store Status
                     Word, Load Environment, Store Environment, Save,
                     Restore, Clear Exceptions, Initialize



Class                Instruction Types

Data Transfer        Load (all data types), Store (all data types), Exchange

Arithmetic           Add, Subtract, Multiply, Divide, Subtract Reversed,
                     Divide Reversed, Square Root, Scale, Remainder, Integer
                     Part, Change Sign, Absolute Value, Extract

Comparison           Compare, Examine, Test

Transcendental       Tangent, Arctangent, Sine, Cosine, Sine and Cosine,
                     2^(x) - 1, Y * Log{2}(X), Y * Log{2}(X+1)

Constants            0, 1, Ò, Log{10}2, Log{e}2, Log{2}10, Log{2}e

Processor Control    Load Control Word, Store Control Word, Store Status
                     Word, Load Environment, Store Environment, Save,
                     Restore, Clear Exceptions, Initialize


Chapter 2  80387 Numerics Processor Architecture

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

To the programmer, the 80387 NPX appears as a set of additional registers,
data types, and instructions‘‘all of which complement those of the 80386.
Refer to Chapter 4 for detailed explanations of the 80387 instruction set.
This chapter explains the new registers and data types that the 80387 brings
to the architecture of the 80386.


2.1  80387 Registers

The additional registers consist of

  Ž  Eight individually-addressable 80-bit numeric registers, organized as
     a register stack

  Ž  Three sixteen-bit registers containing:

     the NPX status word
     the NPX control word
     the tag word

  Ž  Two 48-bit registers containing pointers to the current instruction
     and operand (these registers are actually located in the 80386)

All of the NPX numeric instructions focus on the contents of these NPX
registers.


2.1.1  The NPX Register Stack

The 80387 register stack is shown in Figure 2-1. Each of the eight numeric
registers in the 80387's register stack is 80 bits wide and is divided into
fields corresponding to the NPX's extended real data type.

Numeric instructions address the data registers relative to the register on
the top of the stack. At any point in time, this top-of-stack register is
indicated by the TOP (stack TOP) field in the NPX status word. Load or push
operations decrement TOP by one and load a value into the new top register.
A store-and-pop operation stores the value from the current TOP register and
then increments TOP by one. Like 80386 stacks in memory, the 80387 register
stack grows down toward lower-addressed registers.

Many numeric instructions have several addressing modes that permit the
programmer to implicitly operate on the top of the stack, or to explicitly
operate on specific registers relative to the TOP. The ASM386 Assembler
supports these register addressing modes, using the expression ST(0), or
simply ST, to represent the current Stack Top and ST(i) to specify the ith
register from TOP in the stack (0 ¾ i ¾ 7). For example, if TOP contains
011B (register 3 is the top of the stack), the following statement would add
the contents of two registers in the stack (registers 3 and 5):

FADD   ST, ST(2)

The stack organization and top-relative addressing of the numeric registers
simplify subroutine programming by allowing routines to pass parameters on
the register stack. By using the stack to pass parameters rather than using
"dedicated" registers, calling routines gain more flexibility in how they
use the stack. As long as the stack is not full, each routine simply loads
the parameters onto the stack before calling a particular subroutine to
perform a numeric calculation. The subroutine then addresses its parameters
as ST, ST(1), etc., even though TOP may, for example, refer to physical
register 3 in one invocation and physical register 5 in another.


Figure 2-1.  80387 Register Set

                            80387 DATA REGISTERS                 TAG
                                                                FIELD
             79 78    64 63                                 0    1 0
          ‚����Ð��������Ð������������������������������������ƒ  ‚���ƒ
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        R3Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  Ñ‘‘Â
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        R5Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  Ñ‘‘Â
        R6Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  Ñ‘‘Â
        R7Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  Ñ‘‘Â
          „����¤��������¤������������������������������������…  „���…

           15                0   47                                0
          ‚�������������������ƒ ‚�����������������������������������ƒ
          € CONTROL REGISTER  € €        INSTRUCTION POINTER        €
          Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
          €  STATUS REGISTER  € €            DATA POINTER           €
          Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ „�����������������������������������…
          €     TAG WORD      €
          „�������������������…


2.1.2  The NPX Status Word

The 16-bit status word shown in Figure 2-2 reflects the overall state of
the 80387. This status word may be stored into memory using the
FSTSW/FNSTSW, FSTENV/FNSTENV, and FSAVE/FNSAVE instructions, and can be
transferred into the 80386 AX register with the FSTSW AX/FNSTSW AX
instructions, allowing the NPX status to be inspected by the CPU.

The B-bit (bit 15) is included for 8087 compatibility only. It reflects the
contents of the ES bit (bit 7 of the status word), not the status of the
BUSY# output of the 80387.

The four NPX condition code bits (C{3}-C{0}) are similar to the flags in a
CPU: the 80387 updates these bits to reflect the outcome of arithmetic
operations. The effect of these instructions on the condition code bits is
summarized in Table 2-1. These condition code bits are used principally for
conditional branching. The FSTSW AX instruction stores the NPX status word
directly into the CPU AX register, allowing these condition codes to be
inspected efficiently by 80386 code. The 80386 SAHF instruction can copy
C{3}-C{0} directly to 80386 flag bits to simplify conditional branching.
Table 2-2 shows the mapping of these bits to the 80386 flag bits.

Bits 12-14 of the status word point to the 80387 register that is the
current Top of Stack (TOP). The significance of the stack top has been
described in the prior section on the register stack.

Figure 2-2 shows the six exception flags in bits 0-5 of the status word.
Bit 7 is the exception summary status (ES) bit. ES is set if any unmasked
exception bits are set, and is cleared otherwise. If this bit is set, the
ERROR# signal is asserted. Bits 0-5 indicate whether the NPX has detected
one of six possible exception conditions since these status bits were last
cleared or reset. They are "sticky" bits, and can only be cleared by the
instructions FINIT, FCLEX, FLDENV, FSAVE, and FRSTOR.

Bit 6 is the stack fault (SF) bit. This bit distinguishes invalid
operations due to stack overflow or underflow from other kinds of invalid
operations. When SF is set, bit 9 (C{1}) distinguishes between stack
overflow (C{1} = 1) and underflow (C{1} = 0).


Figure 2-2.  80387 Status Word

         ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ 80387 BUSY
         �       ’‘‘‘˜‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ TOP OF STACK POINTER
         �   ’‘‘‘�‘‘‘�‘‘‘�‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ CONDITION CODE
         \x1f   \x1f   \x1f   \x1f   \x1f   \x1f   \x1f   \x1f
        15                               7                            0
       ‚���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���ƒ
       € B � C �    TOP    � C � C � C � E � S � P � U � O � Z � D � I €
       €   � 3 �   �   �   � 2 � 1 � 0 � S � F � E � E � E � E � E � E €
       „���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���…
                                         \x1e   \x1e   \x1e   \x1e   \x1e   \x1e   \x1e   \x1e
       ERROR SUMMARY STATUS ‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �   �   �   �   �
       STACK FAULT ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �   �   �   �
       EXCEPTION FLAGS                           �   �   �   �   �   �
         PRECISION ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �   �   �
         UNDERFLOW ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �   �
         OVERFLOW ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �
         ZERO DIVIDE ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �
         DENORMALIZED OPERAND ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �
         INVALID OPERATION ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTE:
       ES IS SET IF ANY UNMASKED EXCEPTION BIT IS SET; CLEARED OTHERWISE.
       SEE TABLE 2-1 FOR INTERPRETATION OF CONDITION CODE.
       TOP VALUES:
           000 = REGISTER 0 IS TOP OF STACK
           001 = REGISTER 1 IS TOP OF STACK
                          .
                          .
                          .
           111 = REGISTER 7 IS TOP OF STACK
       FOR DEFINITIONS OF EXCEPTIONS, REFER TO CHAPTER 3.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


Table 2-1.  Condition Code Interpretation


Instruction         C0 (S)     C3 (Z)      C1 (A)      C2 (C)

FPREM, FPREM1       Three least significant bits       Reduction
                            of quotient

                    Q2         Q0          Q1          0=complete
                                           or O/U#     1=incomplete

FCOM, FCOMP,
FCOMPP, FTST,       Result of comparison   Zero        Operand is not
FUCOM, FUCOMP,                             or O/U#     comparable
FUCOMPP, FICOM,
FICOMP

FXAM                Operand class          Sign        Operand class
                                           or O/U#

FCHS, FABS,
FXCH, FINCTOP,
FDECTOP, Constant   UNDEFINED              Zero        UNDEFINED
loads, FXTRACT,                            or O/U#
FLD, FILD, FBLD,
FSTP (ext real)

FIST, FBSTP,
FRNDINT, FST,
FSTP, FADD, FMUL,
FDIV, FDIVR, FSUB,  UNDEFINED              Roundup     UNDEFINED
FSUBR, FSCALE,                             or O/U#
FSQRT, FPATAN,
F2XM1, FYL2X,
FYL2XP1

FPTAN, FSIN,        UNDEFINED              Roundup     Reduction
FCOS, FSINCOS                              or O/U#     0=complete
                                           undefined   1=incomplete
                                           if C2=1

FLDENV, FRSTOR      Each bit loaded
                    from memory


FLDCW, FSTENV,
FSTCW, FSTSW,       UNDEFINED
FCLEX, FINIT,
FSAVE


‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTES
  O/U#        When both IE and SF bits of status word are set,
              indicating a stack exception, this bit distinguishes
              between stack overflow (C1=1) and underflow (C1=0).

  Reduction   If FPREM and FPREM1 produces a remainder that is less
              than the modulus, reduction is complete.  When reduction
              is incomplete the value at the top of the stack is a
              partial remainder, which can be used as input to further
              reduction. For FPTAN, FSIN, FCOS, and FSINCOS, the
              reduction bit is set if the operand at the top of the
              stack is too large. In this case the original operand
              remains at the top of the stack.

  Roundup     When the PE bit of the status word is set, this bit
              indicates whether the last rounding in the instruction
              was upward.

  UNDEFINED   Do not rely on finding any specific value in these bits.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


Table 2-2.  Correspondence between 80387 and 80386 Flag Bits

80387 Flag                    80386 Flag

C{0}                          CF
C{1}                          (none)
C{2}                          PF
C{3}                          ZF


2.1.3  Control Word

The NPX provides the programmer with several processing options, which are
selected by loading a word from memory into the control word. Figure 2-3
shows the format and encoding of the fields in the control word.

The low-order byte of this control word configures the 80387 exception
masking. Bits 0-5 of the control word contain individual masks for each of
the six exception conditions recognized by the 80387. The high-order byte of
the control word configures the 80387 processing options, including

  Ž  Precision control
  Ž  Rounding control

The precision-control bits (bits 8-9) can be used to set the 80387 internal
operating precision at less than the default precision (64-bit significand).
These control bits can be used to provide compatibility with the
earlier-generation arithmetic processors having less precision than the
80387. The precision-control bits affect the results of only the following
five arithmetic instructions: ADD, SUB(R), MUL, DIV(R), and SQRT. No other
operations are affected by PC.

The rounding-control bits (bits 10-11) provide for the common
round-to-nearest mode, as well as directed rounding and true chop. Rounding
control affects only the arithmetic instructions (refer to Chapter 3 for
lists of arithmetic and nonarithmetic instructions).


Figure 2-3.  80387 Control Word Format

         ’‘‘‘˜‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘RESERVED
         �   �   �   ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ (INFINITY CONTROL)
This "infinity control" bit is not meaningful to the 80387. To maintain
compatibility with the 80287, this bit can be programmed; however,
regardless of its value, the 80387 treats infinity in the affine sense
(-ý < +ý).
         �   �   �   �   ’‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ ROUNDING CONTROL
         �   �   �   �   �   �   ’‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ PRECISION CONTROL
         \x1f   \x1f   \x1f   \x1f   \x1f   \x1f   \x1f   \x1f
        15                               7                            0
       ‚���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���Ð���ƒ
       € X   X   X � X �  RC   �  PC   � X   X � P � U � O � Z � D � I €
       €   �   �   �   �   �   �   �   �   �   � M � M � M � M � M � M €
       „���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���¤���…
                                         \x1e   \x1e   \x1e   \x1e   \x1e   \x1e   \x1e   \x1e
       RESERVED ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘•   �   �   �   �   �   �
       EXECEPTION MASKS                          �   �   �   �   �   �
         PRECISION ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �   �   �
         UNDERFLOW ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �   �
         OVERFLOW ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �   �
         ZERO DIVIDE ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �   �
         DENORMALIZED OPERAND ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•   �
         INVALID OPERATION ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTE:
     PRECISION CONTROL                   ROUNDING CONTROL
       00--24 BITS (SINGLE PRECISION)      00--ROUND TO NEAREST OR EVEN
       01--(RESERVED)                      01--ROUND DOWN (TOWARD -ý)
       10--53 BITS (DOUBLE PRECISION)      10--ROUND UP (TOWARD +ý)
       11--64 BITS (EXTENDED PRECISION)    11--CHOP (TRUNCATE TOWARDS ZERO)
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


2.1.4  The NPX Tag Word

The tag word indicates the contents of each register in the register stack,
as shown in Figure 2-4. The tag word is used by the NPX itself to
distinguish between empty and nonempty register locations. Programmers of
exception handlers may use this tag information to check the contents of a
numeric register without performing complex decoding of the actual data in
the register. The tag values from the tag word correspond to physical
registers 0-7. Programmers must use the current top-of-stack (TOP) pointer
stored in the NPX status word to associate these tag values with the
relative stack registers ST(0) through ST(7).

The exact values of the tags are generated during execution of the FSTENV
and FSAVE instructions according to the actual contents of the nonempty
stack locations. During execution of other instructions, the 80387 updates
the TW only to indicate whether a stack location is empty or nonempty.


Figure 2-4.  80387 Tag Word Format

    15                                                                   0
  ‚����Ð���Ð����Ð���Ð����Ð���Ð����Ð���Ð����Ð���Ð����Ð���Ð����Ð���Ð����Ð���ƒ
  € TAG (7)� TAG (6)� TAG (5)� TAG (4)� TAG (3)� TAG (2)� TAG (1)� TAG (0)€
  „����¤���¤����¤���¤����¤���¤����¤���¤����¤���¤����¤���¤����¤���¤����¤���…
                              TAG VALUES:
                              00 = VALID
                              01 = ZERO
                              10 = INVALID OR INFINITY
                              11 = EMPTY


2.1.5  The NPX Instruction and Data Pointers

The instruction and data pointers provide support for programmed
exception-handlers. These registers are actually located in the 80386, but
appear to be located in the 80387 because they are accessed by the ESC
instructions FLDENV, FSTENV, FSAVE, and FRSTOR. Whenever the 80386 decodes
an ESC instruction, it saves the instruction address, the operand address
(if present), and the instruction opcode.

When stored in memory, the instruction and data pointers appear in one of
four formats, depending on the operating mode of the 80386 (protected mode
or real-address mode) and depending on the operand-size attribute in effect
(32-bit operand or 16-bit operand). When the 80386 is in virtual-8086 mode,
the real-address mode formats are used.

Figures 2-5 through 2-8 show these pointers as they are stored following an
FSTENV instruction.

The FSTENV and FSAVE instructions store this data into memory, allowing
exception handlers to determine the precise nature of any numeric exceptions
that may be encountered.

The instruction address saved in the 80386 (as in the 80287) points to any
prefixes that preceded the instruction. This is different from the 8087, for
which the instruction address points only to the ESC instruction opcode.

Note that the processor control instructions FINIT, FLDCW, FSTCW, FSTSW,
FCLEX, FSTENV, FLDENV, FSAVE, FRSTOR, and FWAIT do not affect the data
pointer. Note also that, except for the instructions just mentioned, the
value of the data pointer is undefined if the prior ESC instruction did not
have a memory operand.


Figure 2-5.  Protected Mode 80387 Instruction and Data Pointer Image in
             Memory, 32-Bit Format

                        32-BIT PROTECTED MODE FORMAT

 31                23                15                7               0
‚�����������������Ï�����������������Ï�����������������Ï�����������������ƒ
€             RESERVED              �            CONTROL WORD           €0H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �            STATUS WORD            €4H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �              TAG WORD             €8H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€                               IP OFFSET                               €CH
Ñ‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€ 0 0 0 0 0 �     OPCODE 10..0      �            CS SELECTOR            €10H
Ñ‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€                          DATA OPERAND OFFSET                          €14H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �         OPERAND SELECTOR          €18H
„�����������������Ï�����������������Ï�����������������Ï�����������������…


Figure 2-6.  Real Mode 80387 Instruction and Data Pointer Image in 
             Memory, 32-Bit Format

                      32-BIT REAL ADDRESS MODE FORMAT

 31                23                15                7               0
‚�����������������Ï�����������������Ï�����������������Ï�����������������ƒ
€             RESERVED              �            CONTROL WORD           €0H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �            STATUS WORD            €4H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �              TAG WORD             €8H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �     INSTRUCTION POINTER 15..0     €CH
Ñ‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘˜‘˜‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€ 0 0 0 0 �      INSTRUCTION POINTER 31..16     �0�     OPCODE 10..0    €10H
Ñ‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘™‘™‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �         OPERAND POINTER           €14H
Ñ‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€ 0 0 0 0 �        OPERAND POINTER 31..16       �0 0 0 0 0 0 0 0 0 0 0 0€18H
„���������¤�������Ï�����������������Ï�����������¤�����Ï�����������������…


Figure 2-7.  Protected Mode 80387 Instruction and Data Pointer Image in
             Memory, 16-Bit Format

                        16-BIT PROTECTED MODE FORMAT

                     15                7              0
                    ‚�����������������Ï����������������ƒ
                    €           CONTROL WORD           € 0H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €            STATUS WORD           € 2H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €             TAG WORD             € 4H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €             IP OFFSET            € 6H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €            CB SELECTOR           € 8H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €          OPERAND OFFSET          € AH
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €         OPERAND SELECTOR         € CH
                    „�����������������Ï����������������…


Figure 2-8.  Real Mode 80387 Instruction and Data Pointer Image in
             Memory, 16-Bit Format

                          16-BIT REAL-ADDRESS MODE
                        AND VIRTUAL-8086 MODE FORMAT

                     15                7              0
                    ‚����������������Ï����������������ƒ
                    €          CONTROL WORD           € 0H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €           STATUS WORD           € 2H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €            TAG WORD             € 4H
                    Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €    INSTRUCTION POINTER 15..0    € 6H
                    Ñ‘‘‘‘‘‘‘‘˜‘˜‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €IP 19..16�0�      OPCODE 10..0   € 8H
                    Ñ‘‘‘‘‘‘‘‘™‘™‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €      OPERAND POINTER 15..0      € AH
                    Ñ‘‘‘‘‘‘‘‘˜‘˜‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                    €OP 19..16�0�0 0 0 0 0 0 0 0 0 0 0€ CH
                    „���������¤�¤����Ï����������������…


2.2  Computation Fundamentals

This section covers 80387 programming concepts that are common to all
applications. It describes the 80387's internal number system and the
various types of numbers that can be employed in NPX programs. The most
commonly used options for rounding and precision (selected by fields in the
control word) are described, with exhaustive coverage of less frequently
used facilities deferred to later sections. Exception conditions that may
arise during execution of NPX instructions are also described along with the
options that are available for responding to these exceptions.


2.2.1  Number System

The system of real numbers that people use for pencil and paper
calculations is conceptually infinite and continuous. There is no upper or
lower limit to the magnitude of the numbers one can employ in a calculation,
or to the precision (number of significant digits) that the numbers can
represent. When considering any real number, there are always arbitrarily
many numbers both larger and smaller. There are also arbitrarily many
numbers between (i.e., with more significant digits than) any two real
numbers. For example, between 2.5 and 2.6 are 2.51, 2.5897, 2.500001, etc.

While ideally it would be desirable for a computer to be able to operate on
the entire real number system, in practice this is not possible. Computers,
no matter how large, ultimately have fixed-size registers and memories that
limit the system of numbers that can be accommodated. These limitations
determine both the range and the precision of numbers. The result is a set
of numbers that is finite and discrete, rather than infinite and
continuous. This sequence is a subset of the real numbers that is designed
to form a useful approximation of the real number system.

Figure 2-9 superimposes the basic 80387 real number system on a real number
line (decimal numbers are shown for clarity, although the 80387 actually
represents numbers in binary). The dots indicate the subset of real numbers
the 80387 can represent as data and final results of calculations. The
80387's range of double-precision, normalized numbers is approximately
±2.23 * 10^(-308) to ±1.80 * 10^(308). Applications that are required to
deal with data and final results outside this range are rare. For reference,
the range of the IBM System 370* is about ±0.54 * 10^(-78) to
±0.72 * 10^(76).

The finite spacing in Figure 2-9 illustrates that the NPX can represent a
great many, but not all, of the real numbers in its range. There is always a
gap between two adjacent 80387 numbers, and it is possible for the result of
a calculation to fall in this space. When this occurs, the NPX rounds the
true result to a number that it can represent. Thus, a real number that
requires more digits than the 80387 can accommodate (e.g., a 20-digit
number) is represented with some loss of accuracy. Notice also that the
80387's representable numbers are not distributed evenly along the real
number line. In fact, an equal number of representable numbers exists
between successive powers of 2 (i.e., as many representable numbers exist
between 2 and 4 as between 65,536 and 131,072). Therefore, the gaps between
representable numbers are larger as the numbers increase in magnitude. All
integers in the range ±2^(64) (approximately ±10^(18)), however, are exactly
representable.

In its internal operations, the 80387 actually employs a number system that
is a substantial superset of that shown in Figure 2-9. The internal format
(called extended real) extends the 80387's range to about ±3.30 * 10^(-4932)
to ±1.21 * 10^(4932), and its precision to about 19 (equivalent decimal)
digits. This format is designed to provide extra range and precision for
constants and intermediate results, and is not normally intended for data
or final results.

From a practical standpoint, the 80387's set of real numbers is
sufficiently large and dense so as not to limit the vast majority of
microprocessor applications. Compared to most computers, including
mainframes, the NPX provides a very good approximation of the real number
system. It is important to remember, however, that it is not an exact
representation, and that arithmetic on real numbers is inherently
approximate.

Conversely, and equally important, the 80387 does perform exact arithmetic
on integer operands. That is, if an operation on two integers is valid and
produces a result that is in range, the result is exact. For example, 4 ÷ 2
yields an exact integer, 1 ÷ 3 does not, and 2^(40) * 2^(30) + 1 does not,
because the result requires greater than 64 bits of precision.


Figure 2-9.  80387 Double-Precision Number System

 |\x11‘‘‘ NEGATIVE RANGE (NORMALIZED) ‘‘\x10|
 |                                    |
 |               -5  -4  -3  -2  -1   |
 ’‘‘‘˜‘‘‘˜‘‘˜“’‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘“
 �   �   �  ���›››�›››�œœœ�œœœ���������
 ”‘‘‘™‘‘‘™‘‘™•”‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘•
 \x1e                                    \x1e
 �                   -2.23 X 10^(-308)•
 ” -1.80 X 10^(308)
                                         ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
                                         �   ‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘            �
                                         �   ��������œœœœœœœœœ            �
|\x11‘‘ POSITIVE RANGE (NORMALIZED) ‘‘‘\x10|   �   ��������œœœœœœœœœ            �
|                                    |   �   ‘¨‘‘‘‘‘¨‘‘‘‘‘¨‘‘‘            �
|   1   2   3   4   5                |   �    �\x11‘˜‘\x10�                     �
’‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘“’˜‘‘˜‘‘‘˜‘‘‘“   �    �  �  ”2.00000000000000000  �
���������œœœ�œœœ�›››�›››���  �   �   �   �    �  ” (NOT REPRESENTABLE)    �
”‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘•”™‘‘™‘‘‘™‘‘‘•   �    ”‘‘‘‘‘‘1.99999999999999999  �
\x1e     ”‘‘‘—                          \x1e   �  PRECISION�\x11‘  18 DIGITS  ‘\x10�  �
�         ”‘‘‘‘‘‘‘‘“  1.80 X 10^(308)•   �                                �
” 2.23 X 10^(-308) ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•


2.2.2  Data Types and Formats

The 80387 recognizes seven numeric data types for memory-based values,
divided into three classes: binary integers, packed decimal integers, and
binary reals. A later section describes how these formats are stored in
memory (the sign is always located in the highest-addressed byte).

Figure 2-10 summarizes the format of each data type. In the figure, the
most significant digits of all numbers (and fields within numbers) are the
leftmost digits.


Figure 2-10.  80387 Data Formats

 ’‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
 �         �         �         �MOST                  HIGHEST ADDRESSED    �
 �DATA     � RANGE   �PRECISION�SIGNIFICANT BYTE                   BYTE    �
 �FORMATS  �         �         –‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘“   �
 �         �         �         �7 0�7 0�7 0�7 0�7 0�7 0�7 0�7 0�7 0�7 0�   �
 –‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘—
 �WORD     �         �         –‘‘‘‘‘‘“(TWO'S                              �
 �INTEGER  � 10^(4)  � 16 BITS –‘‘‘‘‘‘•COMPLEMENT)                         �
 �         �         �         �15   0                                     �
 –‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
 �SHORT    �         �         –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“(TWO'S                     �
 �INTEGER  � 10^(2)  � 32 BITS –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•COMPLEMENT)                �
 �         �         �         �31            0                            �
 –‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
 �LONG     �         �         –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“(TWO'S     �
 �INTEGER  � 10^(19) � 64 BITS –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•COMPLEMENT)�
 �         �         �         �6                             0            �
 –‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
 �         �         �         �                MAGNITUDE                  �
 �PACKED   �         �         –‘˜‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘¨¨¨‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“ �
 �BCD      � 10^(18) �18 DIGITS�S� X �d{17} d{16}        d{2}  d{1}  d{0}� �
 �         �         �         –‘™‘‘‘™‘‘‘‘‘™‘‘‘‘‘™‘¨¨¨‘™‘‘‘‘‘™‘‘‘‘‘™‘‘‘‘‘• �
 �         �         �         �    72                                  0  �
 –‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
 �         �         �         –‘˜‘‘‘‘‘˜‘‘‘‘‘‘‘“                           �
 �SINGLE   � 10^(±38)� 24 BITS �S� BE  � SIGN. �                           �
 �PRECISION�         �         –‘™‘‘‘‘‘™‘‘‘‘‘‘‘•                           �
 �         �         �         �31     23     0                            �
 –‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
 �         �         �         –‘˜‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“           �
 �DOUBLE   �10^(±308)� 53 BITS �S� BE     �     SIGNIFICAND    �           �
 �PRECISION�         �         –‘™‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•           �
 �         �         �         �63       52                   0            �
 –‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
 �         �         �         –‘˜‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“�
 �EXTENDED �10^(4932)� 64 BITS �S� BE         –‘“        SIGNIFICAND      ��
 �PRECISION�         �         –‘™‘‘‘‘‘‘‘‘‘‘‘‘™I™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•�
 �         �         �         �79          64 63\x1e                       0 �
 ”‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTE:
   (1) BE = BIASED EXPONENT
   (2) S = SIGN BIT (0 = positive, 1 = negative)
   (3) d{n} = DECIMAL DIGIT (TWO PER TYPE)
   (4) X = BITS HAVE NO SIGNIFICANCE; 80387 IGNORES WHEN LOADING,
           ZEROS IN WHEN STORING
   (5) \x1e = POSITION OF IMPLICIT BINARY POINT
   (6) I = INTEGER BIT OF SIGNIFICAND; STORED IN TEMPORARY REAL,
       IMPLICIT IN SINGLE AND DOUBLE PRECISION
   (7) EXPONENT BIAS (NORMALIZED VALUES):
       SINGLE: 127 (7FH)
       DOUBLE: 1023 (3FFH)
       EXTENDED REAL: 16383 (3FFFH)
   (8) PACKED BCD: (-1)^(S) (D{17}...D{0})
   (9) REAL: (-1)^(S) (2^(E-BIAS)) (F{0}F{1}...)
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


2.2.2.1  Binary Integers

The three binary integer formats are identical except for length, which
governs the range that can be accommodated in each format. The leftmost bit
is interpreted as the number's sign: 0 = positive and 1 = negative. Negative
numbers are represented in standard two's complement notation (the binary
integers are the only 80387 format to use two's complement). The quantity
zero is represented with a positive sign (all bits are 0). The 80387 word
integer format is identical to the 16-bit signed integer data type of the
80386; the 80387 short integer format is identical to the 32-bit signed
integer data type of the 80386.

The binary integer formats exist in memory only. When used by the 80387,
they are automatically converted to the 80-bit extended real format. All
binary integers are exactly representable in the extended real format.


2.2.2.2  Decimal Integers

Decimal integers are stored in packed decimal notation, with two decimal
digits "packed" into each byte, except the leftmost byte, which carries the
sign bit (0 = positive, 1 = negative). Negative numbers are not stored in
two's complement form and are distinguished from positive numbers only by
the sign bit. The most significant digit of the number is the leftmost
digit. All digits must be in the range 0-9.

The decimal integer format exists in memory only. When used by the 80387,
it is automatically converted to the 80-bit extended real format. All
decimal integers are exactly representable in the extended real format.


2.2.2.3  Real Numbers

The 80387 represents real numbers of the form:

  (-1)^(s)2^(E)(b{0\x1e}b{1}b{2}b{3}..b{p-1})

...where...

  s = 0 or 1
  E = any integer between Emin and Emax, inclusive
  b{i} = 0 or 1
  p = number of bits of precision

Table 2-3 summarizes the parameters for each of the three real-number
formats.

The 80387 stores real numbers in a three-field binary format that resembles
scientific, or exponential, notation. The format consists of the following
fields:

  Ž  The number's significant digits are held in the significand field,
     b{0\x1e} b{1} b{2} b{3}..b{p-1}. (The term "significand" is analogous
     to the term "mantissa" used to describe floating point numbers on some
     computers.)

  Ž  The exponent field, e = E+bias, locates the binary point within the
     significant digits (and therefore determines the number's magnitude).
     (The term "exponent" is analogous to the term "characteristic" used to
     describe floating point numbers on somecomputers.)

  Ž  The 1-bit sign field indicates whether the number is positive or
     negative. Negative numbers differ from positive numbers only in the
     sign bits of their significands.

Table 2-4 shows how the real number 178.125 (decimal) is stored in the
80387 single real format. The table lists a progression of equivalent
notations that express the same value to show how a number can be converted
from one form to another. (The ASM386 and PL/M-386 language translators
perform a similar process when they encounter programmer-defined real number
constants.) Note that not every decimal fraction has an exact binary
equivalent. The decimal number 1/10, for example, cannot be expressed
exactly in binary (just as the number 1/3 cannot be expressed exactly in
decimal). When a translator encounters such a value, it produces a rounded
binary approximation of the decimal value.

The NPX usually carries the digits of the significand in normalized form.
This means that, except for the value zero, the significand contains an
integer bit and fraction bits as follows:

1{\x1e}fff...ff

where {\x1e} indicates an assumed binary point. The number of fraction bits
varies according to the real format: 23 for single, 52 for double, and 63
for extended real. By normalizing real numbers so that their integer bit is
always a 1, the 80387 eliminates leading zeros in small values (�X� < 1).
This technique maximizes the number of significant digits that can be
accommodated in a significand of a given width. Note that, in the single
and double formats, the integer bit is implicit and is not actually stored;
the integer bit is physically present in the extended format only.

If one were to examine only the significand with its assumed binary point,
all normalized real numbers would have values greater than or equal to 1 and
less than 2. The exponent field locates the actual binary point in the
significant digits. Just as in decimal scientific notation, a positive
exponent has the effect of moving the binary point to the right, and a
negative exponent effectively moves the binary point to the left, inserting
leading zeros as necessary. An unbiased exponent of zero indicates that the
position of the assumed binary point is also the position of the actual
binary point. The exponent field, then, determines a real number's
magnitude.

In order to simplify comparing real numbers (e.g., for sorting), the 80387
stores exponents in a biased form. This means that a constant is added to
the true exponent described above. As Table 2-3 shows, the value of this
bias is different for each real format. It has been chosen so as to
force the biased exponent to be a positive value. This allows two real
numbers (of the same format and sign) to be compared as if they are unsigned
binary integers. That is, when comparing them bitwise from left to right
(beginning with the leftmost exponent bit), the first bit position that
differs orders the numbers; there is no need to proceed further with the
comparison. A number's true exponent can be determined simply by
subtracting the bias value of its format.

The single and double real formats exist in memory only. If a number in one
of these formats is loaded into an 80387 register, it is automatically
converted to extended format, the format used for all internal operations.
Likewise, data in registers can be converted to single or double real for
storage in memory. The extended real format may be used in memory also,
typically to store intermediate results that cannot be held in registers.

Most applications should use the double format to store real-number data
and results; it provides sufficient range and precision to return correct
results with a minimum of programmer attention. The single real format is
appropriate for applications that are constrained by memory, but it should
be recognized that this format provides a smaller margin of safety. It is
also useful for the debugging of algorithms, because roundoff problems will
manifest themselves more quickly in this format. The extended real format
should normally be reserved for holding intermediate results, loop
accumulations, and constants. Its extra length is designed to shield final
results from the effects of rounding and overflow/underflow in intermediate
calculations. However, the range and precision of the double format are
adequate for most microcomputer applications.


Table 2-3.  Summary of Format Parameters

Parameter                 ’‘‘‘‘‘‘‘‘ Format ‘‘‘‘‘‘‘‘“
                          Single   Double   Extended

Format width in bits          32       64         80
p (bits of precision)         24       53         64
Exponent width in bits         8       11         15
Emax                        +127    +1023     +16383
Emin                        -126    -1022     -16382
Exponent bias               +127    +1023     +16383


Table 2-4.  Real Number Notation

Notation                Value

Ordinary Decimal        178.125
Scientific Decimal      1{\x1e}78125E2
Scientific Binary       1{\x1e}0110010001E111
Scientific Binary       1{\x1e}0110010001E10000110
(Biased Exponent)
80387 Single Format     Sign    Biased Exponent     Significand
(Normalized)            0       10000110            01100100010000000000000
                                                    1{\x1e}(implicit)


2.2.3  Rounding Control

Internally, the 80387 employs three extra bits (guard, round, and sticky
bits) that enable it to round numbers in accord with the infinitely precise
true result of a computation; these bits are not accessible to programmers.
Whenever the destination can represent the infinitely precise true result,
the 80387 delivers it. Rounding occurs in arithmetic and store operations
when the format of the destination cannot exactly represent the infinitely
precise true result. For example, a real number may be rounded if it is
stored in a shorter real format, or in an integer format. Or, the infinitely
precise true result may be rounded when it is returned to a register.

The NPX has four rounding modes, selectable by the RC field in the control
word (see Figure 2-3). Given a true result b that cannot be represented by
the target data type, the 80387 determines the two representable numbers a
and c that most closely bracket b in value (a < b < c). The processor then
rounds (changes) b to a or to c according to the mode selected by the RC
field as shown in Table 2-5. Rounding introduces an error in a result that
is less than one unit in the last place to which the result is rounded.

  Ž  "Round to nearest" is the default mode and is suitable for most
     applications; it provides the most accurate and statistically unbiased
     estimate of the true result.

  Ž  The "chop" or "round toward zero" mode is provided for integer
     arithmeticapplications.

  Ž  "Round up" and "round down" are termed directed rounding and can be
     used to implement interval arithmetic. Interval arithmetic generates a
     certifiable result independent of the occurrence of rounding and other
     errors. The upper and lower bounds of an interval may be computed by
     executing an algorithm twice, rounding up in one pass and down in the
     other.

Rounding control affects only the arithmetic instructions (refer to Chapter
3 for lists of arithmetic and nonarithmetic instructions).


2.2.4  Precision Control

The 80387 allows results to be calculated with either 64, 53, or 24 bits of
precision in the significand as selected by the precision control (PC) field
of the control word. The default setting, and the one that is best suited
for most applications, is the full 64 bits of significance provided by the
extended real format. The other settings are required by the IEEE standard
and are provided to obtain compatibility with the specifications of certain
existing programming languages. Specifying less precision nullifies the
advantages of the extended format's extended fraction length. When reduced
precision is specified, the rounding of the fractional value clears the
unused bits on the right to zeros.


Table 2-5.  Rounding Modes

RC Field    Rounding Mode            Rounding Action

00          Round to nearest         Closer to b of a or c; if equally
                                     close, select even number (the one
                                     whose least significant bit is zero).
01          Round down (toward -ý)   a
10          Round up (toward +ý)     c
11          Chop (toward 0)          Smaller in magnitude of a or c.

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTE
  a < b < c; a and c are successive representable numbers; b is not
  representable.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


Chapter 3  Special Computational Situations

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

Besides being able to represent positive and negative numbers, the 80387
data formats may be used to describe other entities. These special values
provide extra flexibility, but most users will not need to understand them
in order to use the 80387 successfully. This section describes the special
values that may occur in certain cases and the significance of each. The
80387 exceptions are also described, for writers of exception handlers and
for those interested in probing the limits of computation using the 80387.

The material presented in this section is mainly of interest to programmers
concerned with writing exception handlers. Many readers will only need to
skim this section.

When discussing these special computational situations, it is useful to
distinguish between arithmetic instructions and nonarithmetic instructions.
Nonarithmetic instructions are those that have no operands or transfer their
operands without substantial change; arithmetic instructions are those that
make significant changes to their operands. Table 3-1 defines these two
classes of instructions.


Table 3-1.  Arithmetic and Nonarithmetic Instructions


Nonarithmetic Instructions             Arithmetic Instructions

FABS                                   F2XM1
FCHS                                   FADD (P)
FCLEX                                  FBLD
FDECSTP                                FBSTP
FFREE                                  FCOMP(P)(P)
FINCSTP                                FCOS
FINIT                                  FDIV(R)(P)
FLD (register-to-register)             FIADD
FLD (extended format from memory)      FICOM(P)
FLD constant                           FIDIV(R)
FLDCW                                  FILD
FLDENV                                 FIMUL
FNOP                                   FIST(P)
FRSTOR                                 FISUB(R)
FSAVE                                  FLD (conversion)
FST(P) (register-to-register)          FMUL(P)
FSTP (extended format to memory)       FPATAN
FSTCW                                  FPREM
FSTENV                                 FPREM1
FSTSW                                  FPTAN
FWAIT                                  FRNDINT
FXAM                                   FSCALE
FXCH                                   FSIN
                                       FSINCOS
                                       FSQRT
                                       FST(P) (conversion)
                                       FSUB(R)(P)
                                       FTST
                                       FUCOM(P)(P)
                                       FXTRACT
                                       FYL2X
                                       FYL2XP1



3.1  Special Numeric Values

The 80387 data formats encompass encodings for a variety of special values
in addition to the typical real or integer data values that result from
normal calculations. These special values have significance and can express
relevant information about the computations or operations that produced
them. The various types of special values are

  Ž  Denormal real numbers
  Ž  Zeros
  Ž  Positive and negative infinity
  Ž  NaN (Not-a-Number)
  Ž  Indefinite
  Ž  Unsupported formats

The following sections explain the origins and significance of each of
these special values. Tables 3-6 through 3-9 at the end of this section
show how each of these special values is encoded for each of the numeric
data types.


3.1.1  Denormal Real Numbers

The 80387 generally stores nonzero real numbers in normalized
floating-point form; that is, the integer (leading) bit of the significand
is always a one. (Refer to Chapter 2 for a review of operand formats.) This
bit is explicitly stored in the extended format, and is implicitly assumed
to be a one (1{\x1e}) in the single and double formats. Since leading zeros are
eliminated, normalized storage allows the maximum number of significant
digits to be held in a significand of a given width.

When a numeric value becomes very close to zero, normalized floating-point
storage cannot be used to express the value accurately. The term tiny is
used here to precisely define what values require special handling by the
80387. A number R is said to be tiny when -2{Emin} < R < 0 or
0 < R < +2{Emin}. (As defined in Chapter 2, Emin is -126 for single format,
-1022 for double format, and -16382 for extended format.) In other words, a
nonzero number is tiny if its exponent would be too negative to store in the
destination format.

To accommodate these instances, the 80387 can store and operate on reals
that are not normalized, i.e., whose significands contain one or more
leading zeros. Denormals typically arise when the result of a calculation
yields a value that is tiny.

Denormal values have the following properties:

  Ž  The biased floating-point exponent is stored at its smallest value
     (zero)

  Ž  The integer bit of the significand (whether explicit or implicit) is
     zero

The leading zeros of denormals permit smaller numbers to be represented, at
the possible cost of some lost precision (the number of significant bits is
reduced by the leading zeros). In typical algorithms, extremely small values
are most likely to be generated as intermediate, rather than final, results.
By using the NPX's extended real format for holding intermediate values,
quantities as small as ±3.4*10{-4932} can be represented; this makes the
occurrence of denormal numbers a rare phenomenon in 80387 applications.
Nevertheless, the NPX can load, store, and operate on denormalized real
numbers when they do occur.

Denormals receive special treatment by the 80387 in three respects:

  Ž  The 80387 avoids creating denormals whenever possible. In other words,
     it always normalizes real numbers except in the case of tiny numbers.

  Ž  The 80387 provides the unmasked underflow exception to permit
     programmers to detect cases when denormals would be created.

  Ž  The 80387 provides the denormal exception to permit programmers to
     detect cases when denormals enter into further calculations.

Denormalizing means incrementing the true result's exponent and inserting a
corresponding leading zero in the significand, shifting the rest of the
significand one place to the right. Denormal values may occur in any of the
single, double, or extended formats. Table 3-2 illustrates how a result
might be denormalized to fit a single format destination.

Denormalization produces either a denormal or a zero. Denormals are readily
identified by their exponents, which are always the minimum for their
formats; in biased form, this is always the bit string: 00..00. This same
exponent value is also assigned to the zeros, but a denormal has a nonzero
significand. A denormal in a register is tagged special. Tables 3-8 and
3-9 show how denormal values are encoded in each of the real data formats.

The denormalization process causes loss of significance if low-order
one-bits bits are shifted off the right of the significand. In a severe
case, all the significand bits of the true result are shifted out and
replaced by the leading zeros. In this case, the result of denormalization
is a true zero, and, if the value is in a register, it is tagged as a zero.

Denormals are rarely encountered in most applications. Typical debugged
algorithms generate extremely small results during the evaluation of
intermediate subexpressions; the final result is usually of an appropriate
magnitude for its single or double format real destination. If intermediate
results are held in temporary real, as is recommended, the great range of
this format makes underflow very unlikely. Denormals are likely to arise
only when an application generates a great many intermediates, so many that
they cannot be held on the register stack or in extended format memory
variables. If storage limitations force the use of single or double format
reals for intermediates, and small values are produced, underflow may occur,
and, if masked, may generate denormals.

When a denormal number is single or double format is used as a source
operand and the denormal exception is masked, the 80387 automatically
normalizes the number when it is converted to extended format.


Table 3-2.  Denormalization Process

Operation          Sign    Exponent    Significand

True Result        0       -129        1{\x1e}01011100..00
Denormalize        0       -128        0{\x1e}101011100..00
Denormalize        0       -127        0{\x1e}0101011100..00
Denormalize        0       -126        0{\x1e}00101011100..00
Denormal Result    0       -126        0{\x1e}00101011100..00


3.1.1.1  Denormals and Gradual Underflow

Floating-point arithmetic cannot carry out all operations exactly for all
operands; approximation is unavoidable when the exact result is not
representable as a floating-point variable. To keep the approximation
mathematically tractable, the hardware is made to conform to accuracy
standards that can be modeled by certain inequalities instead of equations.
Let the assignment

  X \e Y @ Z    (where @ is some operation)

represent a typical operation. In the default rounding mode (round to
nearest), each operation is carried out with an absolute error no larger
than half the separation between the two floating-point numbers closest to
the exact results. Let x be the value stored for the variable whose name in
the program is X, and similarly y for Y, and z for Z. Normally y and z will
differ by accumulated errors from what is desired and from what would have
been obtained in the absence of error. For the calculation of x we assume
that y and z are the best approximations available, and we seek to compute x
as well as we can. If y@z is representable exactly, then we expect x = y@z,
and that is what we get for every algebraic operation on the 80387 (i.e.,
when y@z is one of y+z, y-z, y*z, y÷z, sqrt z). But if y@z must be
approximated, as is usually the case, then x must differ from y@z by no
more than half the difference between the two representable numbers that
straddle y@z. That difference depends on two factors:

  1.  The precision to which the calculation is carried out, as determined
      either by the precision control bits or by the format used in memory.
      On the 80387, the precisions are single (24 significant bits), double
      (53 significant bits), and extended (64 significant bits).

  2.  How close y@z is to zero. In this respect the presence of denormal
      numbers on the 80387 provides a distinct advantage over systems that
      do not admit denormal numbers.

In any floating-point number system, the density of representable numbers
is greater near zero than near the largest representable magnitudes.
However, machines that do not use denormal numbers suffer from an enormous
gap between zero and its closest neighbors. Figures 3-1 and 3-2 show what
happens near zero in two kinds of floating-point number systems.

Figure 3-1 shows a floating-point number system that (like the 80387)
admits denormal numbers. For simplicity, only the non-negative numbers
appear and the figure illustrates a number system that carries just four
significant bits instead of the 24, 53, or 64 significant bits that the
80387 offers.

Each vertical mark stands for a number representable in four significant
bits, and the bolder marks stand for the normal powers of 2. The denormal
numbers lie between 0 and the nearest normal power of 2. They are no less
dense than the remaining normal nonzero numbers.

Figure 3-2 shows a floating-point number system that (unlike the 80387)
does not admit denormal numbers. There are two yawning gaps, one on the
positive side of zero (as illustrated) and one on the negative side of zero
(not illustrated). The gap between zero and the nearest neighbor of zero
differs from the gap between that neighbor and the next bigger number by a
factor of about 8.4 * 10^(6) for single, 4.5 * 10^(15) for double, and
9.2*10^(18) for extended format. Those gaps would horribly complicate error
analysis.

The advantage of denormal numbers is apparent when one considers what
happens in either case when the underflow exception is masked and y@z falls
into the space between zero and the smallest normal magnitude. The 80387
returns the nearest denormal number. This action might be called "gradual
underflow." The effect is no different than the rounding that can occur when
y@z falls in the normal range.

On the other hand, the system that does not have denormal numbers returns
zero as the result, an action that can be much more inaccurate than
rounding. This action could be called "abrupt underflow."


Figure 3-1.  Floating-Point System with Denormals

 0+++++++�+++++++�-+-+-+-+-+-+-+-�---+---+---+---+---+---+---+---�------+...

  ”‘‘˜‘‘• - - - - - - - - Normal Numbers - - - - - -\x10
  Denormals


Figure 3-2.  Floating-Point System without Denormals

 0    �+++++++�-+-+-+-+-+-+-+-�---+---+---+---+---+---+---+---�------+---...

       - - - - - - - - Normal Numbers - - - - - -\x10


3.1.2  Zeros

The value zero in the real and decimal integer formats may be signed either
positive or negative, although the sign of a binary integer zero is always
positive. For computational purposes, the value of zero always behaves
identically, regardless of sign, and typically the fact that a zero may be
signed is transparent to the programmer. If necessary, the FXAM instruction
may be used to determine a zero's sign.

If a zero is loaded or generated in a register, the register is tagged
zero. Table 3-3 lists the results of instructions executed with zero
operands and also shows how a zero may be created from nonzero operands.


Table 3-3.  Zero Operands and Results


Key to symbols used in this table
X and Y denote nonzero operand.
*  Sign of original zero operand.
#  Sign of original X operand.
-# Compliment of sign of original X operand.
Þ  Exclusive OR of the signs of the operands.


Operation          Operands                    Result

FLD,FBLD           +0                          +0
                   -0                          -0
FILD               +0                          +0
FST,FSTP           +0                          +0
                   -0                          -0
                   +X                          +0
When extreme underflow denormalizes the result to zero.


                   -X                          -0
When extreme underflow denormalizes the result to zero.


FBSTP              +0                          +0
                   -0                          -0
FIST,FISTP         +0                          +0
                   -0                          -0
                   +X                          +0
When 0 < X < 1 and rounding mode is not up.


                   -X                          -0
When 0 < X < 1 and rounding mode is not up.


Addition           +0 plus +0                  +0
                   -0 plus -0                  -0
                   +0 plus -0, -0 plus +0      ±0
Sign determined by rounding mode: + for nearest, up, or chop, - for down.


                   -X plus +X, +X plus -X      ±0
Sign determined by rounding mode: + for nearest, up, or chop, - for down.


                   ±0 plus ±X, ±X plus ±0      #X
Subtraction        +0 minus                    -0+0
                   -0 minus +0                 -0
                   +0 minus +0, -0 minus -0    ±0
Sign determined by rounding mode: + for nearest, up, or chop, - for down.


                   +X minus +X, -X minus -X    ±0
Sign determined by rounding mode: + for nearest, up, or chop, - for down.


                   ±0 minus ±X                 -#X
                   ±X minus ±0                 #X
Multiplication     +0 * +0, -0 * -0            +0
                   +0 * -0, -0 * +0            -0
                   +0 * +X, +X * +0            +0
                   +0 * -X, -X * +0            -0
                   -0 * +X, -X * +0            -0
Multiplication     -0 * -X, -X * -0            +0
                   +X * +Y, -X * -Y            +0
When extreme underflow denormalizes the result to zero.


                   +X * -Y, -X * +Y            -0
When extreme underflow denormalizes the result to zero.


Division           ±0 ÷ ±0                     Invalid Operation
                   ±X ÷ ±0                     Þý (Zero Divide)
                   +0 ÷ +X, -0 ÷ -X            +0
                   +0 ÷ -X, -0 ÷ +X            -0
                   -X ÷ -Y, +X ÷ +Y            +0
When extreme underflow denormalizes the result to zero.


                   -X ÷ +Y, +X ÷ -Y            -0
When extreme underflow denormalizes the result to zero.


FPREM, FPREM1      ±0 rem ±0                   Invalid Operation
                   ±X rem ±0                   Invalid Operation
                   +0 rem ±X                   +0
                   -0 rem ±X                   -0
FPREM              +X rem ±Y                   +0 Y exactly divides X
                   -X rem ±Y                   -0 Y exactly divides X
FPREM1             +X rem ±Y                   +0 Y exactly divides X
                   -X rem ±Y                   -0 Y exactly divides X
FSQRT              +0                          +0
                   -0                          -0
Compare            ±0 : +X                     ±0 < +X
                   ±0 : ±0                     ±0 = ±0
                   ±0 : -X                     ±0 > -X
FTST               ±0                          ±0 = 0
                   +0                          C{3}=1; C{2}=C{1}=C{0}=0
                   -0                          C{3}=C{1}=1; C{2}=C{0}=0
FCHS               +0                          -0
                   -0                          +0
FABS               ±0                          +0
F2XM1              +0                          +0
                   -0                          -0
FRNDINT            +0                          +0
                   -0                          -0
FSCALE             ±0 scaled by -ý             *0
                   ±0 scaled by +ý             Invalid Operation
                   ±0 scaled by X              *0
FXTRACT            +0                          ST=+0,ST(1)=-ý, Zero divide
                   -0                          ST=-0,ST(1)=-ý, Zero divide
FPTAN±0            *0
FSIN (or           ±0                          *0
SIN result of
FSINCOS)
FCOS (or           ±0                          +1
COS result of
FSINCOS)
FPATAN             ±0 ÷ +X                     *0
                   ±0 ÷ -X                     *Ò
                   ±X ÷ ±0                     #Ò/2
                   ±0 ÷ +0                     *0
                   ±0 ÷ -0                     *Ò
                   +ý ÷ ±0                     +Ò/2
                   -ý ÷ ±0                     -Ò/2
                   ±0 ÷ +ý                     *0
                   ±0 ÷ -ý                     *Ò
FYL2X              ±Y * log(±0)                Zero Divide
                   ±0 * log(±0)                Invalid Operation
FYL2XP1            +Y * log(±0+1)              *0
                   -Y * log(±0+1)              -0


3.1.3  Infinity

The real formats support signed representations of infinities. These values
are encoded with a biased exponent of all ones and a significand of
1{\x1e}00..00; if the infinity is in a register, it is tagged special.

A programmer may code an infinity, or it may be created by the NPX as its
masked response to an overflow or a zero divide exception. Note that
depending on rounding mode, the masked response may create the largest valid
value representable in the destination rather than infinity.

The signs of the infinities are observed, and comparisons are possible.
Infinities are always interpreted in the affine sense; that is, -ý < (any
finite number) < +ý. Arithmetic on infinities is always exact and,
therefore, signals no exceptions, except for the invalid operations
specified in Table 3-4.


Table 3-4.  Infinity Operands and Results


Key to symbols used in this table
X  Zero or nonzero positive oprand.
Y  Nonzero positive operand.
*  Sign of original infinity operand.
-* Compliment of sign of original infinity operand.
$  Sign of original operand.
#  Sign of the original Y operand.
Þ  Exclusive OR of signs of operands.


Operation           Operands            Result

Addition            +ý plus +ý          +ý
                    -ý plus -ý          -ý
                    +ý plus -ý          Invalid Operation
                    -ý plus +ý          Invalid Operation
                    ±ý plus ±X          *ý
                    ±X plus ±ý          *ý
Subtraction         +ý minus -ý         +ý
                    -ý minus +ý         -ý
                    +ý minus +ý         Invalid Operation
                    -ý minus -ý         Invalid Operation
                    ±ý minus ±X         *ý
                    ±X minus ±ý         -*ý
Multiplication      ±ý * ±ý             Þý
                    ±ý * ±Y, ±Y * ±ý    Þý
                    ±0 * ±ý, ±ý * ±0    Invalid Operation
Division            ±ý ÷ ±ý             Invalid Operation
                    ±ý ÷ ±X             Þý
                    ±X ÷ ±ý             Þ0
                    ±ý ÷ ±0             Þý
FSQRT               -ý                  Invalid Operation
                    +ý                  +ý
FPREM, FPREM1       ±ý rem ±ý           Invalid Operation
                    ±ý rem ±X           Invalid Operation
                    ±X rem ±ý           $X, Q = 0
FRNDINT             ±ý                  *ý
FSCALE              ±ý scaled by --ý    Invalid Operation
                    ±ý scaled by +ý     *ý
                    ±ý scaled by ±X     *ý
                    ±0 scaled by -ý     ±0
Sign of original zero operand.


                    ±0 scaled by ýI     Invalid Operation
                    ±Y scaled by +ý     #ý
                    ±Y scaled by -ý     #0
FXTRACT             ±ý                  ST = *ý, ST(1) = +ý
Compare             +ý : +ý             +ý = +ý
                    -ý : -ý             -ý = -ý
                    +ý : -ý             +ý > -ý
                    -ý : +ý             -ý < +ý
                    +ý : ±X             +ý > X
                    -ý : ±X             -ý < X
                    ±X : +ý             X < +ý
                    ±X : -ý             X > +ý
FTST                +ý                  +ý > 0
                    -ý                  -ý < 0
FPATAN              ±ý ÷ ±X             *Ò/2
                    ±Y ÷ +ý             #0
                    ±Y ÷ -ý             #Ò
                    ±ý ÷ +ý             *Ò/4
                    ±ý ÷ -ý             *3Ò/4
                    ±ý ÷ ±0             *Ò/2
                    +0 ÷ +ý             +0
                    +0 ÷ -ý             +Ò
                    -0 ÷ +ý             -0
                    -0 ÷ -ý             -Ò
F2XM1               +ý                  +ý
                    -ý                  -1
FYL2X, FYL2XP1      ±ý * log(1)         Invalid Operation
                    ±ý * log(Y>1)       *ý
                    ±ý * log(0<Y<1)     -*ý
                    ±Y * log(+ý)        #ý
                    ±0 * log(+ý)        Invalid Operation
                    ±Y * log(-ý)        Invalid Operation


3.1.4  NaN (Not-a-Number)

A NaN (Not a Number) is a member of a class of special values that exists
in the real formats only. A NaN has an exponent of 11..11B, may have either
sign, and may have any significand except 1{\x1e}00..00B, which is assigned to
the infinities. A NaN in a register is tagged special.

There are two classes of NaNs: signaling (SNaN) and quiet (QNaN). Among the
QNaNs, the value real indefinite is of special interest.


3.1.4.1  Signaling NaNs

A signaling NaN is a NaN that has a zero as the most significant bit of its
significand. The rest of the significand may be set to any value. The 80387
never generates a signaling NaN as a result; however, it recognizes
signaling NaNs when they appear as operands. Arithmetic operations (as
defined at the beginning of this chapter) on a signaling NaN cause an
invalid-operation exception (except for load operations, FXCH, FCHS, and
FABS).

By unmasking the invalid operation exception, the programmer can use
signaling NaNs to trap to the exception handler. The generality of this
approach and the large number of NaN values that are available provide the
sophisticated programmer with a tool that can be applied to a variety of
special situations.

For example, a compiler could use signaling NaNs as references to
uninitialized (real) array elements. The compiler could preinitialize each
array element with a signaling NaN whose significand contained the index
(relative position) of the element. If an application program attempted to
access an element that it had not initialized, it would use the NaN placed
there by the compiler. If the invalid operation exception were unmasked, an
interrupt would occur, and the exception handler would be invoked. The
exception handler could determine which element had been accessed, since the
operand address field of the exception pointers would point to the NaN, and
the NaN would contain the index number of the array element.


3.1.4.2  Quiet NaNs

A quiet NaN is a NaN that has a one as the most significant bit of its
significand. The 80387 creates the quiet NaN real indefinite (defined below)
as its default response to certain exceptional conditions. The 80387 may
derive other QNaNs by converting an SNaN. The 80387 converts a SNaN by
setting the most significant bit of its significand to one, thereby
generating an QNaN. The remaining bits of the significand are not changed;
therefore, diagnostic information that may be stored in these bits of the
SNaN is propagated into the QNaN.

The 80387 will generate the special QNaN, real indefinite, as its masked
response to an invalid operation exception. This NaN is signed negative; its
significand is encoded 1{\x1e}100..00. All other NaNs represent values created
by programmers or derived from values created by programmers.

Both quiet and signaling NaNs are supported in all operations. A QNaN is
generated as the masked response for invalid-operation exceptions and as the
result of an operation in which at least one of the operands is a QNaN. The
80387 applies the rules shown in Table 3-5 when generating a QNaN:

Note that handling of a QNaN operand has greater priority than all
exceptions except certain invalid-operation exceptions (refer to the section
"Exception Priority" in this chapter).

Quiet NaNs could be used, for example, to speed up debugging. In its early
testing phase, a program often contains multiple errors. An exception
handler could be written to save diagnostic information in memory whenever
it was invoked. After storing the diagnostic data, it could supply a quiet
NaN as the result of the erroneous instruction, and that NaN could point to
its associated diagnostic area in memory. The program would then continue,
creating a different NaN for each error. When the program ended, the NaN
results could be used to access the diagnostic data saved at the time the
errors occurred. Many errors could thus be diagnosed and corrected in one
test run.


Table 3-5.  Rules for Generating QNaNs

Operation                          Action

Real operation on an SNaN and      Deliver the QNaN operand.
a QNaN

Real operation on two SNaNs        Deliver the QNaN that results from
                                   converting the SNaN that has the larger
                                   significand.

Real operation on two QNaNs        Deliver the QNaN that has the larger
                                   significand.

Real operation on an SNaN and      Deliver the QNaN that results from
another number                     converting the SNaN.

Real operation on a QNaN and       Deliver the QNaN.
another number

Invalid operation that does not    Deliver the default QNaN real indefinite.
involve NaNs


3.1.5  Indefinite

For every 80387 numeric data type, one unique encoding is reserved for
representing the special value indefinite. The 80387 produces this encoding
as its response to a masked invalid-operation exception.

In the case of reals, the indefinite value is a QNaN as discussed in the
prior section.

Packed decimal indefinite may be stored by the NPX in a FBSTP instruction;
attempting to use this encoding in a FBLD instruction, however, will have an
undefined result; thus indefinite cannot be loaded from a packed decimal
integer.

In the binary integers, the same encoding may represent either indefinite
or the largest negative number supported by the format (-2^(15), -2^(31), or
-2^(63)). The 80387 will store this encoding as its masked response to
an invalid operation, or when the value in a source register represents or
rounds to the largest negative integer representable by the destination. In
situations where its origin may be ambiguous, the invalid-operation
exception flag can be examined to see if the value was produced by an
exception response. When this encoding is loaded or used by an integer
arithmetic or compare operation, it is always interpreted as a negative
number; thus indefinite cannot be loaded from a binary integer.


3.1.6  Encoding of Data Types

Tables 3-6 through 3-9 show how each of the special values just
described is encoded for each of the numeric data types. In these tables,
the least-significant bits are shown to the right and are stored in the
lowest memory addresses. The sign bit is always the left-most bit of the
highest-addressed byte.


3.1.7  Unsupported Formats

The extended format permits many bit patterns that do not fall into any of
the previously mentioned categories. Some of these encodings were supported
by the 80287 NPX; however, most of them are not supported by the 80387 NPX.
These changes are required due to changes made in the final version of the
IEEE 754 standard that eliminated these data types.

The categories of encodings formerly known as pseudozeros, pseudo-NaNs,
pseudoinfinities, and unnormal numbers are not supported by the 80387. The
80387 raises the invalid-operation exception when they are encountered as
operands.

The encodings formerly known as pseudodenormal numbers are not generated by
the 80387; however, they are correctly utilized when encountered in operands
to 80387 instructions. The exponent is treated as if it were 00..01 and the
mantissa is unchanged. The denormal exception is raised.


Table 3-6. Binary Integer Encodings

              Class                   Sign         Magnitude
    ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �         (Largest)                0            11...11
    �                                  ¨               ¨
Positives                              ¨               ¨
    �                                  ¨               ¨
    �         (Smallest)               0            00...01
    ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
               Zero                    0            00...00
    ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �         (Smallest)               1            11...11
    �                                  ¨               ¨
Negatives                              ¨               ¨
    �                                  ¨               ¨
    �         (Largest/Indefinite
If this encoding is used as a source operand (as in an integer load or
integer arithmetic instruction), the 80387 interprets it as the largest
negative number representable in the format: -2^(15), -2^(31), or -2^(63).
The 80387 will deliver this encoding to an integer destination in two
cases:
    1.  If the result is the largest negative number
    2.  As the response to a masked invalid operation exception, in which
        case it represents the special value integer indefinite.)    1            00...00
    ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                    Word:        ‘‘‘15 bits‘‘‘
                                    Short:       ‘‘‘31 bits‘‘‘
                                    Long:        ‘‘‘63 bits‘‘‘


Table 3-7. Packed Decimal Encodings



                                           ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Magnitude ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
            Class          Sign             digit      digit      digit      digit    . . . digit
    ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �       (Largest)      0       0000000  1 0 0 1    1 0 0 1    1 0 0 1    1 0 0 1  . . . 1 0 0 1
    �                      ¨          ¨                              ¨
    �                      ¨          ¨                              ¨
Positives                  ¨          ¨                              ¨
    �       (Smallest)     0       0000000  0 0 0 0    0 0 0 0    0 0 0 0    0 0 0 0  . . . 0 0 0 1
    �
    �       Zero           0       0000000  0 0 0 0    0 0 0 0    0 0 0 0    0 0 0 0  . . . 0 0 0 0
    ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �       Zero           1       0000000  0 0 0 0    0 0 0 0    0 0 0 0    0 0 0 0  . . . 0 0 0 0
    � 
    �       (Smallest)     1       0000000  0 0 0 0    0 0 0 0    0 0 0 0    0 0 0 0  . . . 0 0 0 1
Negatives                  ¨          ¨                              ¨
    �                      ¨          ¨                              ¨
    �                      ¨          ¨                              ¨
    �       (Largest)      1       0000000  1 0 0 1    1 0 0 1    1 0 0 1    1 0 0 1  . . . 1 0 0 1
    ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
       Indefinite
The packed decimal indefinite encoding is stored by FBSTP in response to a
masked invalid operation exception. Attempting to load this value via FBLD
produces an undefined result.         1       1111111  1 1 1 1    1 1 1 1    U U U U
UUUU means bit values are undefined and may contain any value   U U U U  . . . U U U U
                           ‘‘‘‘ 1 byte ‘‘‘  ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ 9 bytes ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


Table 3-8. Single and Double Real Encodings


                                          Biased      Significand
            Class                Sign     Exponent    ff--ff
Integer bit is implied and not stored.




   ’‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Quiet       0       11...11     11...11
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                       0       11...11     10...00
   �    NaNs        ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Signaling   0       11...11     01...11
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                       0       11...11     00...01
   �      ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �                  ý           0       11...11     00...00
   �      ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Normals     0       11...10     11...11
   �      �                                  ¨           ¨
Positives �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                       0       00...01     00...00
   �      �         ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �    Reals         Denormals   0       00...00     11...11
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                       0       00...00     00...01
   �      �         ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Zero        0       00...00     00...00
   ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Zero        1       00...00     00...00
   �      �         ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Denormals   1       00...00     00...01
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �    Reals                                ¨           ¨
   �      �                       1       00...00     11...11
   �      �         ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Normals     1       00...01     00...00
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                                  ¨           ¨
   �      �                       1       11...10     11...11
Negatives ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �                  ý           1       11...11     00...00
   �      ’‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �         �             1       11...11     00...01
   �      �         �                        ¨           ¨
   �      �    Signaling                     ¨           ¨
   �      �         �                        ¨           ¨
   �      �         �             1       11...11     01...11
   �    NaNs        –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �         � Indefinite  1       11...11     10...00
   �      �         �    ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �         �                        ¨           ¨
   �      �      Quiet                       ¨           ¨
   �      �         �                        ¨           ¨
   �      �         �             1       11...11     11...11
   ”‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                             Double:  � ‘‘‘8 bits‘‘ � ‘‘23 bits‘‘ �
                             Single:  � ‘‘11 bits‘‘ � ‘‘52 bits‘‘ �


Table 3-9. Extended Real Encodings


                                        Biased     Significand
            Class              Sign     Exponent   1.ff--ff
    ’‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    0       11...11    1 11..11
    �      �     Quiet          ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    0       11...11    1 10..01
    �    NaNs   ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    0       11...11    1 01..11
    �      �     Signaling      ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    0       11...11    1 00..01
    �      ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �         ý                 0       11...11    1 00..00
    �      ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    0       11...10    1 11..11
    �      �     Normals        ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    0       00...01    1 00..00
    �      �    ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Positives  �                    0       11...10    0 11..11
    �    Reals   Unsupported    ¨          ¨          ¨
    �      �     8087 Unnormals ¨          ¨          ¨
    �      �                    0       00...01    0 00..00
    �      �    ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    0       00...00    1 11..11
    �      �     Pseudo-        ¨          ¨          ¨
    �      �       normals      ¨          ¨          ¨
    �      �                    0       00...00    1 00..00
    �      �    ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    0       00...00    0 11..11
    �      �     Denormals      ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    0       00...00    0 00..01
    �      –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �     Zero           0       00...00    000...00
    ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �     Zero           1       00...00    000...00
    �      –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    1       00...00    0 00..01
    �      �     Denormals      ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    1       00...00    0 11..11
    �      �    ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    1       00...00    1 00..00s
    �    Reals   Pseudo-        ¨          ¨          ¨
    �      �       normals      ¨          ¨          ¨
    �      �                    1       00...00    1 11..11
    �      �    ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    1       00...00    0 00..00
Negatives  �     Unsupported    ¨          ¨          ¨
    �      �     8087 Unnormals ¨          ¨          ¨
    �      �                    1       11...10    0 11..11
    �      �    ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    1       00...01    1 00..00
    �      �     Normals        ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    1       11...10    1 11..11
    �      ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �         ý                 1       11...11    1 00..00
    �      ’‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �   �                1       11...11    1 00..01
    �      �  Signaling         ¨          ¨          ¨
    �      �   �                ¨          ¨          ¨
    �      �   �                1       11...11    1 01..11
    �      �   –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �     NaNs �    Indefinite  1       11...11    110...00
    �      �   �     ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �   �                1       11...11    1 10..00
    �      �  Quiet             ¨          ¨          ¨
    �      �   �                ¨          ¨          ¨
    �      �   �                1       11...11    1 11..11
    ”‘‘‘‘‘‘™‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                     �‘‘15 bits‘‘�‘‘64 bits‘‘�


3.2  Numeric Exceptions

The 80387 can recognize six classes of numeric exception conditions while
executing numeric instructions:

  1.  I‘‘ Invalid operation
          Ž  Stack fault
          Ž  IEEE standard invalid operation
  2.  Z‘‘ Divide-by-zero
  3.  D‘‘ Denormalized operand
  4.  O‘‘ Numeric overflow
  5.  U‘‘ Numeric underflow
  6.  P‘‘ Inexact result (precision)


3.2.1  Handling Numeric Exceptions

When numeric exceptions occur, the NPX takes one of two possible courses of
action:

  Ž  The NPX can itself handle the exception, producing the most reasonable
     result and allowing numeric program execution to continue undisturbed.

  Ž  A software exception handler can be invoked by the CPU to handle the
     exception.

Each of the six exception conditions described above has a corresponding
flag bit in the 80387 status word and a mask bit in the 80387 control word.
If an exception is masked (the corresponding mask bit in the control
word = 1), the 80387 takes an appropriate default action and continues with
the computation. If the exception is unmasked (mask = 0), the 80387 asserts
the ERROR# output to the 80386 to signal the exception and invoke a software
exception handler.

Note that when exceptions are masked, the NPX may detect multiple
exceptions in a single instruction, because it continues executing the
instruction after performing its masked response. For example, the 80387
could detect a denormalized operand, perform its masked response to this
exception, and then detect an underflow.


3.2.1.1  Automatic Exception Handling

The 80387 NPX has a default fix-up activity for every possible exception
condition it may encounter. These masked-exception responses are designed to
be safe and are generally acceptable for most numeric applications.

As an example of how even severe exceptions can be handled safely and
automatically using the NPX's default exception responses, consider a
calculation of the parallel resistance of several values using only the
standard formula (Figure 3-3). If R{1} becomes zero, the circuit resistance
becomes zero. With the divide-by-zero and precision exceptions masked, the
80387 NPX will produce the correct result.

By masking or unmasking specific numeric exceptions in the NPX control
word, NPX programmers can delegate responsibility for most exceptions to the
NPX, reserving the most severe exceptions for programmed exception handlers.
Exception-handling software is often difficult to write, and the NPX's
masked responses have been tailored to deliver the most reasonable result
for each condition. For the majority of applications, masking all
exceptions other than invalid-operation yields satisfactory results with the
least programming effort. An invalid-operation exception normally indicates
a program error that must be corrected; this exception should not normally
be masked.

The exception flags in the NPX status word provide a cumulative record of
exceptions that have occurred since these flags were last cleared. Once set,
these flags can be cleared only by executing the FCLEX (clear exceptions)
instruction, by reinitializing the NPX, or by overwriting the flags with an
FRSTOR or FLDENV instruction. This allows a programmer to mask all
exceptions (except invalid operation), run a calculation, and then inspect
the status word to see if any exceptions were detected at any point in the
calculation.


Figure 3-3.  Arithmetic Example Using Infinity

                           ‘‘‘˜‘‘‘‘‘‘˜‘‘‘‘‘‘“
                              �      �      �
                              �      �      �
                              �      �      �
                             R{1}   R{2}   R{3}
                              �      �      �
                              �      �      �
                              �      �      �
                           ‘‘‘™‘‘‘‘‘‘™‘‘‘‘‘‘•

                                                1
          EQUIVALENT RESISTANCE = ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                   1/R{1}  +  1/R{2}  +  1/R{3}


3.2.1.2  Software Exception Handling

If the NPX encounters an unmasked exception condition, it signals the
exception to the 80386 CPU using the ERROR# status line between the two
processors.

The next time the 80386 CPU encounters a WAIT or ESC instruction in its
instruction stream, the 80386 will detect the active condition of the ERROR#
status line and automatically trap to an exception response routine using
interrupt #16, the "processor extension error" exception.

This exception response routine is normally a part of the systems software.
Typical exception responses may include:

  Ž  Incrementing an exception counter for later display or printing

  Ž  Printing or displaying diagnostic information (e.g., the 80387
     environment andregisters)

  Ž  Aborting further execution

  Ž  Using the exception pointers to build an instruction that will run
     without exception and executing it

For 80386 systems having systems software support for the 80387 NPX,
applications programmers should consult the operating system's reference
manuals for the appropriate system response to NPX exceptions. For systems
programmers, specific details on writing software exception handlers are
included in Chapter 6.


3.2.2  Invalid Operation

This exception may occur in response to two general classes of operations:

  1.  Stack operations
  2.  Arithmetic operations

The stack flag (SF) of the status word indicates which class of operation
caused the exception. When SF is 1 a stack operation has resulted in stack
overflow or underflow; when SF is 0, an arithmetic instruction has
encountered an invalid operand.


3.2.2.1  Stack Exception

When SF is 1, indicating a stack operation, the O/U# bit of the condition
code (bit C{1}) distinguishes between stack overflow and underflow as
follows:

O/U# = 1   Stack overflow‘‘ an instruction attempted to push down a
           nonempty stack location.

O/U# = 0   Stack underflow‘‘ an instruction attempted to read an
           operand from an empty stack location.

When the invalid-operation exception is masked, the 80387 returns the QNaN
indefinite. This value overwrites the destination register, destroying
its original contents.

When the invalid-operation exception is not masked, the 80386 exception
"processor extension error" is triggered. TOP is not changed, and the source
operands remain unaffected.


3.2.2.2  Invalid Arithmetic Operation

This class includes the invalid operations defined in IEEE Std 754. The
80387 reports an invalid operation in any of the cases shown in Table 3-10.
Also shown in this table are the 80387's responses when the invalid
exception is masked. When unmasked, the 80386 exception "processor extension
error" is triggered, and the operands remain unaltered. An invalid operation
generally indicates a program error.


Table 3-10.  Masked Responses to Invalid Operations


Condition                           Masked Response

Any arithmetic operation            Return the QNaN indefinite.
on an unsupported format.

Any arithmetic operation            Return a QNaN (refer to the section
on a signaling NaN.                 "Rules for Generating QNaNs").

Compare and test operations:        Set condition codes "not comparable."
one or both operands is a NaN.

Addition of opposite-signed         Return the QNaN indefinite.
infinities or subtraction of
like-signed infinities.

Multiplication: ý * 0; or 0 * ý.    Return the QNaN indefinite.

Division: ý ÷ ý; or 0 ÷ 0.          Return the QNaN indefinite.

Remainder instructions FPREM,       Return the QNaN indefinite; set C{2}.
FPREM1 when modulus (divisor)
is zero or dividend is ý.

Trigonometric instructions FCOS,    Return the QNaN indefinite; set  C{2}.
FPTAN, FSIN, FSINCOS when
argument is ý.

FSQRT of negative operand (except   Return the QNaN indefinite.
FSQRT (-0) = -0), FYL2X of
negative operand (except FYL2X
(-0) = -ý), FYL2XP1 of operand
more negative than -1.

FIST(P) instructions when source    Store integer indefinite.
register is empty, a NaN, ý,
or exceeds representable range
of destination.

FBSTP instruction when source       Store packed decimal indefinite.
register is empty, a NaN, ý, or
exceeds 18 decimal digits.

FXCH instruction when one or        Change empty registers to the QNaN
both registers are tagged empty.    indefinite and then perform exchange.


3.2.3  Division by Zero

If an instruction attempts to divide a finite nonzero operand by zero, the
80387 will report a zero-divide exception. This is possible for
F(I)DIV(R)(P) as well as the other instructions that perform division
internally: FYL2X and FXTRACT. The masked response for FDIV and FYL2X is to
return an infinity signed with the exclusive OR of the signs of the
operands. For FXTRACT, ST(1) is set to -ý; ST is set to zero with the same
sign as the original operand. If the divide-by-zero exception is unmasked,
the 80386 exception "processor extension error" is triggered; the operands
remain unaltered.


3.2.4  Denormal Operand

If an arithmetic instruction attempts to operate on a denormal operand, the
NPX reports the denormal-operand exception. Denormal operands may have
reduced significance due to lost low-order bits, therefore it may be
advisable in certain applications to preclude operations on these operands.
This can be accomplished by an exception handler that responds to unmasked
denormal exceptions. Most users will mask this exception so that
computation may proceed; any loss of accuracy will be analyzed by the user
when the final result is delivered.

When this exception is masked, the 80387 sets the D-bit in the status word,
then proceeds with the instruction. Gradual underflow and denormal numbers
as handled on the 80387 will produce results at least as good as, and often
better than what could be obtained from a machine that flushes underflows to
zero. In fact, a denormal operand in single- or double-precision format will
be normalized to the extended-real format when loaded into the 80387.
Subsequent operations will benefit from the additional precision of the
extended-real format used internally.

When this exception is not masked, the D-bit is set and the exception
handler is invoked. The operands are not changed by the instruction and are
available for inspection by the exception handler.

If an 8087/80287 program uses the denormal exception to automatically
normalize denormal operands, then that program can run on an 80387 by
masking the denormal exception. The 8087/80287 denormal exception handler
would not be used by the 80387 in this case. A numerics program runs faster
when the 80387 performs normalization of denormal operands. A program can
detect at run-time whether it is running on an 80387 or 8087/80287 and
disable the denormal exception when an 80387 is used. The following code
sequence is recommended to distinguish between an 80387 and an 8087/80287.

      FINIT              ; Use default infinity mode:
                         ;  projective for 8087/80287,
                         ;  affine for 80387
      FLD1               ; Generate infinty
      FLDZ
      FDIV
      FLD    ST   
                         ; Form negative infinity
      FCHS
      FCOMPP             ; Compare +infinity with -infinity
      FSTSW  temp        ; 8087/80287 will say they are equal
      MOV    AX, temp
      SAHF
      JNZ    Using_80387

The denormal-operand exception of the 80387 permits emulation of arithmetic
on unnormal operands as provided by the 8087/80287. The standard does not
require the denormal exception nor does it recognize the unnormal data type.


3.2.5  Numeric Overflow and Underflow

If the exponent of a numeric result is too large for the destination real
format, the 80387 signals a numeric overflow. Conversely, if the exponent of
a result is too small to be represented in the destination format, a numeric
underflow is signaled. If either of these exceptions occur, the result of
the operation is outside the range of the destination real format.

Typical algorithms are most likely to produce extremely large and small
numbers in the calculation of intermediate, rather than final, results.
Because of the great range of the extended-precision format (recommended as
the destination format for intermediates), overflow and underflow are
relatively rare events in most 80387 applications.


3.2.5.1  Overflow

The overflow exception can occur whenever the rounded true result would
exceed in magnitude the largest finite number in the destination format. The
exception can occur in the execution of most of the arithmetic instructions
and in some of the conversion instructions; namely, FST(P), F(I)ADD(P),
F(I)SUB(R)(P), F(I)MUL(P), FDIV(R)(P), FSCALE, FYL2X, and FYL2XP1.

The response to an overflow condition depends on whether the overflow
exception is masked:

  Ž  Overflow exception masked. The value returned depends on the rounding
     mode as Table 3-11 illustrates.

  Ž  Overflow exception not masked. The unmasked response depends on
     whether the instruction is supposed to store the result on the stack
     or in memory:

     ‘‘ Destination is the stack. The true result is divided by 2^(24,576)
        and rounded. (The bias 24,576 is equal to 3 * 2^(13).) The
        significand is rounded to the appropriate precision (according to
        the precision control (PC) bit of the control word, for those
        instructions controlled by PC, otherwise to extended precision).
        The roundup bit (C{1}) of the status word is set if the
        significand was rounded upward.

        The biasing of the exponent by 24,576 normally translates the
        number as nearly as possible to the middle of the exponent range
        so that, if desired, it can be used in subsequent scaled
        operations with less risk of causing further exceptions. With the
        instruction FSCALE, however, it can happen that the result is too
        large and overflows even after biasing. In this case, the unmasked
        response is exactly the same as the masked round-to-nearest
        response, namely ± infinity. The intention of this feature is to
        ensure the trap handler will discover that a translation of the
        exponent by -24574 would not work correctly without obliging the
        programmer of Decimal-to-Binary or Exponential functions to
        determine which trap handler, if any, should be invoked.

     ‘‘ Destination is memory (this can occur only with the store
        instructions). No result is stored in memory. Instead, the operand
        is left intact in the stack. Because the data in the stack is in
        extended-precision format, the exception handler has the option
        either of reexecuting the store instruction after proper
        adjustment of the operand or of rounding the significand on the
        stack to the destination's precision as the standard requires. The
        exception handler should ultimately store a value into the
        destination location in memory if the program is to continue.


Table 3-11.  Masked Overflow Results

Rounding              Sign of
Mode                True Result         Result

To nearest               +               +ý
                         -               -ý
Toward -ý                +               Largest finite positive number
                         -               -ý
Toward +ý                +               +ý
                         -               Largest finite negative number
Toward zero              +               Largest finite positive number
                         -               Largest finite negative number


3.2.5.2  Underflow

Underflow can occur in the execution of the instructions FST(P), FADD(P),
FSUB(RP), FMUL(P), F(I)DIV(RP), FSCALE, FPREM(1), FPTAN, FSIN, FCOS,
FSINCOS, FPATAN, F2XM1, FYL2X, and FYL2XP1.

Two related events contribute to underflow:

  1.  Creation of a tiny result which, because it is so small, may cause
      some other exception later (such as overflow upon division).

  2.  Creation of an inexact result; i.e. the delivered result differs from
      what would have been computed were both the exponent range and
      precision unbounded.

Which of these events triggers the underflow exception depends on whether
the underflow exception is masked:

  1.  Underflow exception masked. The underflow exception is signaled when
      the result is both tiny and inexact.

  2.  Underflow exception not masked. The underflow exception is signaled
      when the result is tiny, regardless of inexactness.

The response to an underflow exception also depends on whether the
exception is masked:

  1.  Masked response. The result is denormal or zero. The precision
      exception is also triggered.

  2.  Unmasked response. The unmasked response depends on whether the
      instruction is supposed to store the result on the stack or in memory:

      Ž  Destination is the stack. The true result is multiplied by
         2^(24,576) and rounded. (The bias 24,576 is equal to 3 * 2^(13).)
         The significand is rounded to the appropriate precision (according
         to the precision control (PC) bit of the control word, for those
         instructions controlled by PC, otherwise to extended precision).
         The roundup bit (C{1}) of the status word is set if the significand
         was rounded upward.

         The biasing of the exponent by 24,576 normally translates the
         number as nearly as possible to the middle of the exponent range so
         that, if desired, it can be used in subsequent scaled operations
         with less risk of causing further exceptions. With the instruction
         FSCALE, however, it can happen that the result is too tiny and
         underflows even after biasing. In this case, the unmasked response
         is exactly the same as the masked round-to-nearest response, namely
         ±0. The intention of this feature is to ensure the trap handler
         will discover that a translation by +24576 would not work correctly
         without obliging the programmer of Decimal-to-Binary or Exponential
         functions to determine which trap handler, if any, should be
         invoked.

      Ž  Destination is memory (this can occur only with the store
         instructions). No result is stored in memory. Instead, the operand
         is left intact in the stack. Because the data in the stack is in
         extended-precision format, the exception handler has the option
         either of reexecuting the store instruction after proper adjustment
         of the operand or of rounding the significand on the stack to the
         destination's precision as the standard requires. The exception
         handler should ultimately store a value into the destination
         location in memory if the program is to continue.


3.2.6  Inexact (Precision)

This exception condition occurs if the result of an operation is not
exactly representable in the destination format. For example, the fraction
1/3 cannot be precisely represented in binary form. This exception occurs
frequently and indicates that some (generally acceptable) accuracy has been
lost.

All the transcendental instructions are inexact by definition; they always
cause the inexact exception.

The C{1} (roundup) bit of the status word indicates whether the inexact
result was rounded up (C{1} = 1) or chopped (C{1} = 0).

The inexact exception accompanies the underflow exception when there is
also a loss of accuracy. When underflow is masked, the underflow exception
is signaled only when there is a loss of accuracy; therefore the precision
flag is always set as well. When underflow is unmasked, there may or may not
have been a loss of accuracy; the precision bit indicates which is the case.

This exception is provided for applications that need to perform exact
arithmetic only. Most applications will mask this exception. The 80387
delivers the rounded or over/underflowed result to the destination,
regardless of whether a trap occurs.


3.2.7  Exception Priority

The 80387 deals with exceptions according to a predetermined precedence.
Precedence in exception handling means that higher-priority exceptions are
flagged and results are delivered according to the requirements of that
exception. Lower-priority exceptions may not be flagged even if they occur.
For example, dividing an SNaN by zero causes an invalid-operand exception
(due to the SNaN) and not a zero-divide exception; the masked result is the
QNaN real indefinite, not ý. A denormal or inexact (precision) exception,
however, can accompany a numeric underflow or overflow exception.

The exception precedence is as follows:

  1.  Invalid operation exception, subdivided as follows:

      a. Stack underflow.
      b. Stack overflow.
      c. Operand of unsupported format.
      d. SNaN operand.

  2.  QNaN operand. Though this is not an exception, if one operand is a
      QNaN, dealing with it has precedence over lower-priority exceptions.
      For example, a QNaN divided by zero results in a QNaN, not a
      zero-divide exception.

  3.  Any other invalid-operation exception not mentioned above or zero
      divide.

  4.  Denormal operand. If masked, then instruction execution continues,
      and a lower-priority exception can occur as well.

  5.  Numeric overflow and underflow. Inexact result (precision) can be
      flagged as well.

  6.  Inexact result (precision).


3.2.8  Standard Underflow/Overflow Exception Handler

As long as the underflow and overflow exceptions are masked, no additional
software is required to cause the output of the 80387 to conform to the
requirements of IEEE Std 754. When unmasked, these exceptions give the
exception handler an additional option in the case of store instructions. No
result is stored in memory; instead, the operand is left intact on the
stack. The handler may round the significand of the operand on the stack to
the destination's precision as the standard requires, or it may adjust the
operand and reexecute the faulting instruction.



Chapter 4  The 80387 Instruction Set

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

This chapter describes the operation of all 80387 instructions. Within this
section, the instructions are divided into six functional classes:

  Ž  Data Transfer instructions
  Ž  Nontranscendental instructions
  Ž  Comparison instructions
  Ž  Transcendental instructions
  Ž  Constant instructions
  Ž  Processor Control instructions

Throughout this chapter, the instruction set is described as it appears to
the ASM386 programmer who is coding a program. Not included in this chapter
are details of instruction format, encoding, and execution times. This
detailed information may be found in Appendix A. Refer also to Appendix B
for a summary of the exceptions caused by each instruction.


4.1  Compatibility With the 80287 and 8087

The instruction set for the 80387 NPX is largely the same as that for the
80287 NPX (used with 80286 systems) and that for the 8087 NPX (used with
8086 and 8088 systems). Most object programs generated for the 80287 or 8087
will execute without change on the 80387. Several instructions are new to
the 80387, and several 80287 and 8087 instructions perform no useful
function on the 80387. Appendix C and Appendix D give details of these
instruction set differences.


4.2  Numeric Operands

The typical NPX instruction accepts one or two operands as inputs, operates
on these, and produces a result as an output. An operand is most often the
contents of a register or of a memory location. The operands of some
instructions are predefined; for example, FSQRT always takes the square root
of the number in the top NPX stack element. Others allow, or require, the
programmer to explicitly code the operand(s) along with the instruction
mnemonic. Still others accept one explicit operand and one implicit
operand, which is usually the top NPX stack element. All 80387 instructions
that have a data operand use ST as one operand or as the only operand.

Whether supplied by the programmer or utilized automatically, the two basic
types of operands are sources and destinations. A source operand simply
supplies one of the inputs to an instruction; it is not altered by the
instruction. Even when an instruction converts the source operand from one
format to another (e.g., real to integer), the conversion is actually
performed in an internal work area to avoid altering the source operand. A
destination operand may also provide an input to an instruction. It is
distinguished from a source operand, however, because its content may be
altered when it receives the result produced by the operation; that is, the
destination is replaced by the result.

Many instructions allow their operands to be coded in more than one way.
For example, FADD (add real) may be written without operands, with only a
source or with a destination and a source. The instruction descriptions in
this section employ the simple convention of separating alternative operand
forms with slashes; the slashes, however, are not coded. Consecutive slashes
indicate an option of no explicit operands. The operands for FADD are thus
described as

  //source/destination, source

This means that FADD may be written in any of three ways:

Written Form               Action

FADD                       Add ST to ST(1), put result in ST(1), then pop ST
FADD source                Add source to ST(0)
FADD destination, source   Add source to destination

The assembler can allow the same instruction to be specified in different
ways; for example:

FADD = FADDP ST(1), ST
FADD ST(1) = FADD ST, ST(1)

When reading this section, it is important to bear in mind that memory
operands may be coded with any of the CPU's memory addressing methods
provided by the ModR/M byte. To review these methods (BASE + (INDEX * SCALE)
+ DISPLACEMENT) refer to the 80386 Programmer's Reference Manual.
Chapter 5 also provides several addressing mode examples.


4.3  Data Transfer Instructions

These instructions (summarized in Table 4-1) move operands among elements
of the register stack, and between the stack top and memory. Any of the
seven data types can be converted to extended real and loaded (pushed) onto
the stack in a single operation; they can be stored to memory in the same
manner. The data transfer instructions automatically update the 80387 tag
word to reflect whether the register is empty or full following the
instruction.

Table 4-1.  Data Transfer Instructions

Real Transfers
FLD          Load Real
FST          Store real
FSTP         Store real and pop
FXCH         Exchange registers
Integer Transfers
FILD         Integer load
FIST         Integer store
FISTP        Integer store and pop
Packed Decimal Transfers
FBLD         Packed decimal (BCD) load
FBSTP        Packed decimal (BCD) store and pop


4.3.1  FLD source

FLD (load real) loads (pushes) the source operand onto the top of the
register stack. This is done by decrementing the stack pointer by one and
then copying the content of the source to the new stack top. ST(7) must be
empty to avoid causing an invalid-operation exception. The new stack top is
tagged nonempty. The source may be a register on the stack (ST(i)) or any of
the real data types in memory. If the source is a register, the register
number used is that before TOP is decremented by the instruction. Coding FLD
ST(0) duplicates the stack top. Single and double real source operands are
converted to extended real automatically. Loading an extended real operand
does not require conversion; therefore, the I and D exceptions do not occur
in this case.


4.3.2  FST destination

FST (store real) copies the NPX stack top to the destination, which
may be another register on the stack or a single or double (but not
extended-precision) memory operand. If the destination is single or double
real, the copy of the significand is rounded to the width of the destination
according to the RC field of the control word, and the copy of the exponent
is converted to the width and bias of the destination format. The
over/underflow condition is checked for as well.

If, however, the stack top contains zero, ±ý, or a NaN, then the stack
top's significand is not rounded but is chopped (on the right) to fit the
destination. Neither is the exponent converted, rather it also is chopped on
the right and transferred "as is". This preserves the value's identification
as ý or a NaN (exponent all ones) so that it can be properly loaded and used
later in the program if desired.

Note that the 80387 does not signal the invalid-operation exception when
the destination is a nonempty stack element.


4.3.3  FSTP destination

FSTP (store real and pop) operates identically to FST except that the NPX
stack is popped following the transfer. This is done by tagging the top
stack element empty and then incrementing TOP. FSTP also permits storing to
an extended-precision real memory variable, whereas FST does not. If the
source operand is a register, the register number used is that before TOP is
incremented by the instruction. Coding FSTP ST(0) is equivalent to popping
the stack with no data transfer.


4.3.4  FXCH //destination

FXCH (exchange registers) swaps the contents of the destination and the
stack top registers. If the destination is not coded explicitly, ST(1) is
used. Many 80387 instructions operate only on the stack top; FXCH provides a
simple means of effectively using these instructions on lower stack
elements. For example, the following sequence takes the square root of the
third register from the top (assuming that ST is nonempty):

FXCH ST(3)
FSQRT
FXCH ST(3)


4.3.5  FILD source

FILD (integer load) converts the source memory operand from its binary
integer format (word, short, or long) to extended real and pushes the result
onto the NPX stack. ST(7) must be empty to avoid causing an exception. The
(new) stack top is tagged nonempty. FILD is an exact operation; the source
is loaded with no rounding error.


4.3.6  FIST destination

FIST (integer store) stores the content of the stack top to an integer
according to the RC field (rounding control) of the control word and
transfers the result to the destination, leaving the stack top unchanged.
The destination may define a word or short integer variable. Negative zero
is stored in the same encoding as positive zero: 0000...00.


4.3.7  FISTP destination

FISTP (integer and pop) operates like FIST except that it also pops the NPX
stack following the transfer. The destination may be any of the binary
integer data types.


4.3.8  FBLD source

FBLD (packed decimal (BCD) load) converts the content of the source operand
from packed decimal to extended real and pushes the result onto the NPX
stack. ST(7) must be empty to avoid causing an exception. The sign of the
source is preserved, including the case where the value is negative zero.
FBLD is an exact operation; the source is loaded with no rounding error.

The packed decimal digits of the source are assumed to be in the range 0-9.
The instruction does not check for invalid digits (A-FH), and the result of
attempting to load an invalid encoding is undefined.


4.3.9  FBSTP destination

FBSTP (packed decimal (BCD) store and pop) converts the content of the
stack top to a packed decimal integer, stores the result at the destination
in memory, and pops the stack. FBSTP rounds a nonintegral value according to
the RC (rounding control) field of the control word.


4.4  Nontranscendental Instructions

The 80387's nontranscendental instruction set (Table 4-2) provides a wealth
of variations on the basic add, subtract, multiply, and divide operations,
and a number of other useful functions. These range from a simple absolute
value to a square root instruction that executes faster than ordinary
division; 80387 programmers no longer need to spend valuable time
eliminating square roots from algorithms because they run too slowly. Other
nontranscendental instructions perform exact modulo division, round real
numbers to integers, and scale values by powers of two.

The 80387's basic nontranscendental instructions (addition, subtraction,
multiplication, and division) are designed to encourage the development of
very efficient algorithms. In particular, they allow the programmer to
reference memory as easily as the NPX register stack.

Table 4-3 summarizes the available operation/operand forms that are
provided for basic arithmetic. In addition to the four normal operations,
two "reversed" instructions make subtraction and division "symmetrical" like
addition and multiplication. The variety of instruction and operand forms
give the programmer unusual flexibility:

  Ž  Operands may be located in registers or memory.

  Ž  Results may be deposited in a choice of registers.

  Ž  Operands may be a variety of NPX data types: extended real, double
     real, single real, short integer or word integer, with automatic
     conversion to extended real performed by the 80387.

Five basic instruction forms may be used across all six operations, as
shown in Table 4-3. The classical stack form may be used to make the 80387
operate like a classical stack machine. No operands are coded in this form,
only the instruction mnemonic. The NPX picks the source operand from the
stack top and the destination from the next stack element. It then pops the
stack, performs the operation, and returns the result to the new stack top,
effectively replacing the operands by the result.

The register form is a generalization of the classical stack form; the
programmer specifies the stack top as one operand and any register on the
stack as the other operand. Coding the stack top as the destination provides
a convenient way to access a constant, held elsewhere in the stack, from the
stack top. The destination need not always be ST, however. All two operand
instructions allow use of another register as the destination. This coding
(ST is the source operand) allows, for example, adding the stack top into a
register used as an accumulator.

Often the operand in the stack top is needed for one operation but then is
of no further use in the computation. The register pop form can be used to
pick up the stack top as the source operand, and then discard it by popping
the stack. Coding operands of ST(1), ST with a register pop mnemonic is
equivalent to a classical stack operation: the top is popped and the result
is left at the new top.

The two memory forms increase the flexibility of the 80387's
nontranscendental instructions. They permit a real number or a binary
integer in memory to be used directly as a source operand. This is useful in
situations where operands are not used frequently enough to justify holding
them in registers. Note that any memory addressing method may be used to
define these operands, so they may be elements in arrays, structures, or
other data organizations, as well as simple scalars.

The six basic operations are discussed further in the next paragraphs, and
descriptions of the remaining seven operations follow.


Table 4-2.  Nontranscendental Instructions

Addition
FADD              Add real
FADDP             Add real and pop
FIADD             Integer add

Subtraction
FSUB              Subtract real
FSUBP             Subtract real and pop
FISUB             Integer subtract
FSUBR             Subtract real reversed
FSUBRP            Subtract real reversed and pop
FISUBR            Integer subtract reversed

Multiplication
FMUL              Multiply real
FMULP             Multiply real and pop
FIMUL             Integer multiply

Division
FDIV              Divide real
FDIVP             Divide real and pop
FIDIV             Integer divide
FDIVR             Divide real reversed
FDIVRP            Divide real reversed and pop
FIDIVR            Integer divide reversed

Other Operations
FSQRT             Square root
FSCALE            Scale
FPREM             Partial remainder
FPREM1            IEEE standard partial remainder
FRNDINT           Round to integer
FXTRACT           Extract exponent and significand
FABS              Absolute value
FCHS              Change sign


Table 4-3.  Basic Nontranscendental Instructions and Operands

Instruction Form           Mnemonic  Operand Forms
                           Form      destination, source    ASM386 Example

Classical stack            Fop       [ST(1), ST]            FADD
Classical stack, extra pop FopP      [ST(1), ST]            FADDP
Register                   Fop       ST(i), ST or ST, ST(i) FSUB   ST, ST(3)
Register pop               FopP      ST(i), ST              FMULP  ST(2), ST
Real memory                Fop       [ST,] single/double    FDIV   AZIMUTH
Integer memory             FIop      [ST,] word-integer/    FIDIV  PULSES
                                           short-integer

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTES
  Brackets ([]) surround implicit operands; these are not coded, and are
  shown here for information only.

  op=              ADD    destination \e destination + source
                   SUB    destination \e destination - source
                   SUBR   destination \e source - destination
                   MUL    destination \e destination * source
                   DIV    destination \e destination ÷ source
                   DIVR   destination \e source ÷ destination
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


4.4.1  Addition

FADD     //source/destination,source
FADDP    //destination,source
FIADD    source

The addition instructions (add real, add real and pop, integer add) add the
source and destination operands and return the sum to the destination. The
operand at the stack top may be doubled by coding:

FADD ST, ST(0)

If the source operand is in memory, conversion of an integer, a single
real, or a double real operand to extended real is performed automatically.


4.4.2  Normal Subtraction

FSUB     //source/destination,source
FSUBP    //destination,source
FISUB    source

The normal subtraction instructions (subtract real, subtract real and pop,
integer subtract) subtract the source operand from the destination and
return the difference to the destination.


4.4.3  Reversed Subtraction

FSUBR    //source/destination,source
FSUBRP   //destination,source
FISUBR   source

The reversed subtraction instructions (subtract real reversed, subtract
real reversed and pop, integer subtract reversed) subtract the destination
from the source and return the difference to the destination. For example,
FSUBR ST, ST(1) means subtract ST from ST(1) and leave the result in ST.


4.4.4  Multiplication

FMUL     //source/destination,source
FMULP    //destination,source
FIMUL    source

The multiplication instructions (multiply real, multiply real and pop,
integer multiply) multiply the source and destination operands and return
the product to the destination. Coding FMUL ST, ST(0) squares the content of
the stack top.


4.4.5  Normal Division

FDIV     //source/destination,source
FDIVP    //destination,source
FIDIV    source

The normal division instructions (divide real, divide real and pop, integer
divide) divide the destination by the source and return the quotient to the
destination.


4.4.6  Reversed Division

FDIVR    //source/destination,source
FDIVRP   //destination,source
FIDIVR   source

The reversed division instructions (divide real reversed, divide real
reversed and pop, integer divide reversed) divide the source operand by the
destination and return the quotient to the destination.


4.4.7  FSQRT

FSQRT (square root) replaces the content of the top stack element with its
square root. (Note: The square root of -0 is defined to be -0.)


4.4.8  FSCALE

FSCALE (scale) interprets the value contained in ST(1) as an integer and
adds this value to the exponent of the number in ST. This is equivalent to

ST \e ST * 2^(ST(1))

Thus, FSCALE provides rapid multiplication or division by integral powers
of 2. It is particularly useful for scaling the elements of a vector.

There is no limit on the range of the scale factor in ST(1). If the value
is not integral, FSCALE uses the nearest integer smaller in magnitude; i.e.,
it chops the value toward 0. If the resulting integer is zero, the value in
ST is not changed.


4.4.9  FPREM ‘‘ Partial Remainder (80287/8087-Compatible)

FPREM computes the remainder of division of ST by ST(1) and leaves the
result in ST. FPREM finds a remainder REM and a quotient Q such that

REM = ST - ST(1)*Q

The quotient Q is chosen to be the integer obtained by chopping the exact
value of ST/ST(1) toward zero. The sign of the remainder is the same as the
sign of the original dividend from ST.

By ignoring precision control, the 80387 produces an exact result with
FPREM. The precision (inexact) exception does not occur and the rounding
control has no effect.

The FPREM instruction is not the remainder operation specified in the IEEE
standard. To get that remainder, the FPREM1 instruction should be used.

The FPREM instruction is designed to be executed iteratively in a
software-controlled loop. It operates by performing successive scaled
subtractions; therefore, obtaining the exact remainder when the operands
differ greatly in magnitude can consume large amounts of execution time.
Because the 80387 can only be preempted between instructions, the remainder
function could seriously increase interrupt latency in these cases. For
this reason, the maximum number of iterations is limited. The instruction
may terminate before it has completely terminated the calculation. The C2
bit of the status word indicates whether the calculation is complete or
whether the instruction must be executed again.

FPREM can reduce the exponent of ST by up to (but not including) 64 in one
execution. If FPREM produces a remainder that is less than the modulus
(i.e., the divisor), the function is complete and bit C2 of the status word
condition code is cleared. If the function is incomplete, C2 is set to 1;
the result in ST is then called the partial remainder. Software can inspect
C2 by storing the status word following execution of FPREM, reexecuting the
instruction (using the partial remainder in ST as the dividend) until C2 is
cleared. A higher priority interrupting routine that needs the 80387 can
force a context switch between the instructions in the remainder loop.

An important use for FPREM is to reduce arguments (operands) of
transcendental functions to the range permitted by these instructions. For
example, the FPTAN (tangent) instruction requires its argument ST to be less
than 2^(63). For Ò/4 < �ST� < 2^(63), FPTAN (as well as the other
trigonometric instructions) performs an internal reduction of ST to a value
less than Ò/4 using an internally stored Ò/4 divisor that has 67 significant
bits. Because of its greater accuracy, this method of reduction is
recommended when the argument is within the required range.

However, when �ST� � 2^(63), FPREM can be employed to reduce ST. With Ò/4 as
a modulus, FPREM can reduce an argument so that it is within range of FPTAN
and so that no further reduction is required by FPTAN.

Because FPREM produces an exact result, the argument reduction does not
introduce roundoff error into the calculation, even if several iterations
are required to bring the argument into range. However, Ò is never accurate.
The rounding of Ò, when it is used by FPREM to reduce an argument for a
periodic trigonometric function, does not create the effect of a rounded
argument, but of a rounded period.

When reduction is complete, FPREM provides the least-significant three bits
of the quotient generated by FPREM (in C{3}, C{1}, C{0}). This is also
important for transcendental argument reduction, because it locates the
original angle in the correct one of eight Ò/4 segments of the unit circle
(see Table 4-4).


Table 4-4.  Condition Code Interpretation after FPREM and FPREM1
            Instructions

’‘‘ Condition Code ‘‘“             Interpretation after
C2(PF)  C3    C1    C0             FPREM and FPREM1
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                   Incomplete Reduction:
  1     X     X     X        ‘‘‘\x10   further interation required
                                    or complete reduction
        Q1    Q0    Q2  Q MOD 8

        0     0     0     0  ‘“
        0     1     0     1   �
        1     0     0     2   �    Complete Reduction:
  0     1     1     0     3   –‘\x10   C0, C3, C1 contain three least
        0     0     1     4   �     significant bits of quotient
        0     1     1     5   �
        1     0     1     6   �
        1     1     1     7  ‘•


4.4.10  FPREM1‘‘Partial Remainder (IEEE Std. 754-Compatible)

FPREM1 computes the remainder of division of ST by ST(1) and leaves the
result in ST. FPREM1 finds a remainder REM1 and a quotient Q1 such that

REM1 = ST - ST(1)*Q1

The quotient Q1 is chosen to be the integer nearest to the exact value of
ST/ST(1). When ST/ST(1) is exactly N + 1/2 (for some integer N), there are
two integers equally close to ST/ST(1). In this case the value chosen for Q1
is the even integer.

The result produced by FPREM1 is always exact; no rounding is necessary,
and therefore the precision exception does not occur and the rounding
control has no effect.

The FPREM1 instruction is designed to be executed iteratively in a
software-controlled loop. FPREM1 operates by performing successive scaled
subtractions; therefore, obtaining the exact remainder when the operands
differ greatly in magnitude can consume large amounts of execution time.
Because the 80387 can only be preempted between instructions, the remainder
function could seriously increase interrupt latency in these cases. For
this reason, the maximum number of iterations is limited. The instruction
may terminate before it has completely terminated the calculation. The C2
bit of the status word indicates whether the calculation is complete or
whether the instruction must be executed again.

FPREM1 can reduce the exponent of ST by up to (but not including) 64 in one
execution. If FPREM1 produces a remainder that is less than the modulus
(i.e., the divisor), the function is complete and bit C2 of the status word
condition code is cleared. If the function is incomplete, C2 is set to 1;
the result in ST is then called the partial remainder. Software can inspect
C2 by storing the status word following execution of FPREM1, reexecuting
the instruction (using the partial remainder in ST as the dividend) until C2
is cleared. When C2 is cleared, FPREM1 also provides the least-significant
three bits of the quotient generated by FPREM1 (in C{3}, C{1}, C{0}).

The uses for FPREM1 are the same as those for FPREM.

FPREM1 differs from FPREM it these respects:

  Ž  FPREM and FPREM1 choose the value of the quotient differently; the
     low-order three bits of the quotient as reported in bits C3, C1, C0 of
     the status word may differ by one in some cases.

  Ž  FPREM and FPREM1 may produce different remainders. FPREM produces a
     remainder R such that 0 ¾ R < �ST(1)� or -�ST(1)� < R ¾ 0, depending
     on the sign of the dividend. FPREM1 produces a remainder R1 such that
     -�ST(1)�/2 < R1 < +�ST(1)�/2.


4.4.11  FRNDINT

FRNDINT (round to integer) rounds the top stack element to an integer
according to the RC bits of the control word. For example, assume that ST
contains the 80387 real number encoding of the decimal value 155.625.
FRNDINT will change the value to 155 if the RC field of the control word is
set to down or chop, or to 156 if it is set to up or nearest.


4.4.12  FXTRACT

FXTRACT (extract exponent and significand) performs a superset of the
IEEE-recommended logb(x) function by "decomposing" the number in the stack
top into two numbers that represent the actual value of the operand's
exponent and significand fields. The "exponent" replaces the original
operand on the stack and the "significand" is pushed onto the stack. (ST(7)
must be empty to avoid causing the invalid-operation exception.) Following
execution of FXTRACT, ST (the new stack top) contains the value of the
original significand expressed as a real number: its sign is the same as the
operand's, its exponent is 0 true (16,383 or 3FFFH biased), and its
significand is identical to the original operand's. ST(1) contains the value
of the original operand's true (unbiased) exponent expressed as a real
number.

If the original operand is zero, FXTRACT leaves -ý in ST(1) (the exponent)
while ST is assigned the value zero with a sign equal to that of the
original operand. The zero-divide exception is raised in this case, as well.

To illustrate the operation of FXTRACT, assume that ST contains a number
whose true exponent is +4 (i.e., its exponent field contains 4003H). After
executing FXTRACT, ST(1) will contain the real number +4.0; its sign will be
positive, its exponent field will contain 4001H (+2 true) and its
significand field will contain 1{\x1e}00...00B. In other words, the value in
ST(1) will be 1.0 * 2² = 4. If ST contains an operand whose true exponent
is -7 (i.e., its exponent field contains 3FF8H), then FXTRACT will return an
"exponent" of -7.0; after the instruction executes, ST(1)'s sign and
exponent fields will contain C001H (negative sign, true exponent of 2), and
its significand will be 1{\x1e}1100...00B. In other words, the value in ST(1)
will be -1.75 * 2² = -7.0. In both cases, following FXTRACT, ST's sign and
significand fields will be the same as the original operand's, and its
exponent field will contain 3FFFH (0 true).

FXTRACT is useful for power and range scaling operations. Both FXTRACT and
the base 2 exponential instruction F2XM1 are needed to perform a general
power operation. Converting numbers in 80387 extended real format to decimal
representations (e.g., for printing or displaying) requires not only FBSTP
but also FXTRACT to allow scaling that does not overflow the range of the
extended format. FXTRACT can also be useful for debugging, because it allows
the exponent and significand parts of a real number to be examined
separately.


4.4.13  FABS

FABS (absolute value) changes the top stack element to its absolute value
by making its sign positive. Note that the invalid-operation exception is
not signaled even if the operand is a signaling NaN or has a format that is
not supported.


4.4.14  FCHS

FCHS (change sign) complements (reverses) the sign of the top stack
element. Note that the invalid-operation exception is not signaled even if
the operand is a signaling NaN or has a format that is not supported.


4.5  Comparison Instructions

The instructions of this class allow comparison of numbers of all supported
real and integer data types. Each of these instructions (Table 4-5)
analyzes the top stack element, often in relationship to another operand,
and reports the result as a condition code in the status word.

The basic operations are compare, test (compare with zero), and examine
(report type, sign, and normalization). Special forms of the compare
operation are provided to optimize algorithms by allowing direct comparisons
with binary integers and real numbers in memory, as well as popping the
stack after a comparison.

The FSTSW (store status word) instruction may be used following a
comparison to transfer the condition code to memory or to the 80386 AX
register for inspection. The 80386 SAHF instruction is recommended for
copying the 80387 flags from AX to the 80386 flags for easy conditional
branching.

Note that instructions other than those in the comparison group may update
the condition code. To ensure that the status word is not altered
inadvertently, store it immediately following a comparison operation.


Table 4-5.  Comparison Instructions

FCOM           Compare real
FCOMP          Compare real and pop
FCOMPP         Compare real and pop twice
FICOM          Integer compare
FICOMP         Integer compare and pop
FTST           Test
FUCOM          Unordered compare real
FUCOMP         Unordered compare real and pop
FUCOMPP        Unordered compare real and pop twice
FXAM           Examine


4.5.1  FCOM //source

FCOM (compare real) compares the stack top to the source operand. The
source operand may be a register on the stack, or a single or double real
memory operand. If an operand is not coded, ST is compared to ST(1). The
sign of zero is ignored, so that +0 = -0. Following the instruction, the
condition codes reflect the order of the operands as shown in Table 4-6.

If either operand is a NaN (either quiet or signaling) or an undefined
format, or if a stack fault occurs, the invalid-operation exception is
raised and the condition bits are set to "unordered."


Table 4-6.  Condition Code Resulting from Comparisons

                                                80386
Order           C3 (ZF)   C2 (PF)   C0 (CF)     Conditional
                                                Branch

ST > Operand    0         0         0           JA
ST < Operand    0         0         1           JB
ST = Operand    1         0         0           JE
Unordered       1         1         1           JP


4.5.2  FCOMP //source

FCOMP (compare real and pop) operates like FCOM, and in addition pops the
stack.


4.5.3  FCOMPP

FCOMPP (compare real and pop twice) operates like FCOM and additionally
pops the stack twice, discarding both operands. FCOMPP always compares ST to
ST(1); no operands may be explicitly specified.


4.5.4  FICOM source

FICOM (integer compare) converts the source operand, which may reference a
word or short binary integer variable, to extended real and compares the
stack top to it. The condition code bits in the status word are set as for
FCOM.


4.5.5  FICOMP source

FICOMP (integer compare and pop) operates identically to FICOM and
additionally discards the value in ST by popping the NPX stack.


4.5.6  FTST

FTST (test) tests the top stack element by comparing it to zero. The result
is posted to the condition codes as shown in Table 4-7.


Table 4-7.  Condition Code Resulting from FTST

                                                83086
Order           C3 (ZF)   C2 (ZF)   C0 (ZF)     Conditional
                                                Branch

ST > 0.0        0         0         0           JA
ST < 0.0        0         0         1           JB
ST = 0.0        1         0         0           JE
Unordered       1         1         1           JP


4.5.7  FUCOM //source

FUCOM (unordered compare real) operates like FCOM, with two differences:

  1.  It does not cause an invalid-operation exception when one of the
      operands is a NaN. If either operand is a NaN, the condition bits of
      the status word are set to unordered as shown in Table 4-6.

  2.  Only operands on the NPX stack can be compared.


4.5.8  FUCOMP //source

FUCOMP (unordered compare real and pop) operates like FUCOM and in addition
pops the NPX stack.


4.5.9  FUCOMPP

FUCOMPP (unordered compare real and pop) operates like FUCOM and in
addition pops the NPX stack twice, discarding both operands. FUCOMPP always
compares ST to ST(1); no operands can be explicitly specified.


4.5.10  FXAM

FXAM (examine) reports the content of the top stack element as
positive/negative and NaN, denormal, normal, zero, infinity, unsupported, or
empty. Table 4-8 lists and interprets all the condition code values that
FXAM generates.


4.6  Transcendental Instructions

The instructions in this group (Table 4-9) perform the time-consuming core
calculations for all common trigonometric, inverse trigonometric,
hyperbolic, inverse hyperbolic, logarithmic, and exponential functions. The
transcendentals operate on the top one or two stack elements, and they
return their results to the stack. The trigonometric operations assume their
arguments are expressed in radians. The logarithmic and exponential
operations work in base 2.

The results of transcendental instructions are highly accurate. The
absolute value of the relative error of the transcendental instructions is
guaranteed to be less than 2^(-62). (Relative error is the ratio between the
absolute error and the exact value.)

The trigonometric functions accept a practically unrestricted range of
operands, whereas the other transcendental instructions require that
arguments be more restricted in range. FPREM or FPREM1 may be used to bring
the otherwise valid operand of a periodic function into range. Prologue and
epilogue software may be used to reduce arguments for other instructions to
the expected range and to adjust the result to correspond to the original
arguments if necessary. The instruction descriptions in this section
document the allowed operand range for each instruction.


Table 4-8.  Condition Code Defining Operand Class

C3  C2  C1  C0    Value at TOP

0   0   0   0    +Unsupported
0   0   0   1    +NaN
0   0   1   0    -Unsupported
0   0   1   1    -NaN
0   1   0   0    +Normal
0   1   0   1    +Infinity
0   1   1   0    -Normal
0   1   1   1    -Infinity
1   0   0   0    +0
1   0   0   1    +Empty
1   0   1   0    -0
1   0   1   1    -Empty
1   1   0   0    +Denormal
1   1   1   0    -Denormal


Table 4-9.  Transcendental Instructions

FSIN         Sine
FCOS         Cosine
FSINCOS      Sine and cosine
FPTAN        Tangent of ST
FPATAN       Arctangent of ST(1)/ST
F2XM1        2{X-1}
FYL2X        Y * log{2}X;        Y is ST(1), X is ST
FYL2XP1      Y * log{2}(X + 1);  Y is ST(1), X is ST


4.6.1  FCOS

When complete, this function replaces the contents of ST with COS(ST). ST,
expressed in radians, must lie in the range �Ú� < 2^(63) (for most practical
purposes unrestricted). If ST is in range, C2 of the status word is cleared
and the result of the operation is produced.

If the operand is outside of the range, C2 is set to one (function
incomplete) and ST remains intact (i.e., no reduction of the operand is
performed). It is the programmers responsibility to reduce the operand to an
absolute value smaller than 2^(63). The instructions FPREM1 and FPREM are
available for this purpose.


4.6.2  FSIN

When complete, this function replaces the contents of ST with SIN(ST). FSIN
is equivalent to FCOS in the way it reduces the operand. ST is expressed in
radians.


4.6.3  FSINCOS

When complete, this instruction replaces the contents of ST with SIN(ST),
then pushes COS(ST) onto the stack. (ST(7) must be empty to avoid an invalid
exception.) FSINCOS is equivalent to FCOS in the way it reduces the operand.
ST is expressed in radians.


4.6.4  FPTAN

When complete, FPTAN (partial tangent) computes the function Y = TAN (ST).
ST is expressed in radians. Y replaces ST, then the value 1 is pushed,
becoming the new stack top. (ST(7) must be empty to avoid an invalid
exception.) When the function is complete ST(1) = TAN (arg) and ST = 1.
FPTAN is equivalent to FCOS in the way it reduces the operand.

The fact that FPTAN places two results on the stack maintains compatibility
with the 8087/80287 and aids the calculation of other trigonometric
functions that can be derived from tan via standard trigonometric
identities. For example, the cot function is given by this identity:

cot x = 1/tan x.

Therefore, simply executing the reverse divide instruction FDIVR after
FPTAN yields the cot function.


4.6.5  FPATAN

FPATAN (arctangent) computes the function Ú = ARCTAN (Y/X). X is taken from
ST(0) and Y from ST(1). The instruction pops the NPX stack and returns Ú to
the (new) stack top, overwriting the Y operand. The result is expressed in
radians. The range of operands is not restricted; however, the range of the
result depends on the relationship between the operands according to Table
4-10.

The fact that the argument of FPATAN is a ratio aids calculation of other
trigonometric functions, including Arcsin and Arccos. These can be derived
from Arctan via standard trigonometric identities. For example, the Arcsin
function can be easily calculated using this identity:

Arcsin x = Arctan (x / ¹(1 - x²)).

Thus, to find Arcsin (Y), push Y onto the NPX stack, then calculate
X = ¹(1 - Y²), pushing the result X onto the stack. Executing FPATAN then
leaves Arcsin (Y) at the top of the stack.


4.6.6  F2XM1

F2XM1 (2 to the X minus 1) calculates the function Y = 2^(X) - 1. X is taken
from the stack top and must be in the range -1 ¾ X ¾ 1. The result Y
replaces the argument X at the stack top. If the argument is out of range,
the results are undefined.

This instruction is designed to produce a very accurate result even when X
is close to 0. For values of the argument very close in magnitude to 1, a
larger error will be incurred. To obtain Y = 2^(X), add 1 to the result
delivered by F2XM1.

The following formulas show how values other than 2 may be raised to a
power of X:

10^(X) = 2^(X * LOG2(10))

e^(X) = 2^(X * LOG2(e))

y^(X) = 2^(X * LOG2(Y))

As shown in the next section, the 80387 has built-in instructions for
loading the constants LOG{2}10 and LOG{2}e, and the FYL2X instruction may be
used to calculate X*LOG{2}Y.


Table 4-10.  Results of FPATAN

Sign(Y)    Sign(X)     �Y� < �X�?      Final Result

+          +           Yes                   0 < atan(Y/X) < Ò/4
+          +           No                  Ò/4 < atan(Y/X) < Ò/2
+          -           No                  Ò/2 < atan(Y/X) < 3 * Ò/4
+          -           Yes             3 * Ò/4 < atan(Y/X) < Ò
-          +           Yes                -Ò/4 < atan(Y/X) < 0
-          +           No                 -Ò/2 < atan(Y/X) < -Ò/4
-          -           No             -3 * Ò/4 < atan(Y/X) < -Ò/2
-          -           Yes                  -Ò < atan(Y/X) < -3 * Ò/4


4.6.7  FYL2X

FYL2X (Y log base 2 of X) calculates the function Z = Y * LOG{2}X. X is
taken from the stack top and Y from ST(1). The operands must be in the
following ranges:

0 ¾ X < +ý
-ý < Y < +ý

The instruction pops the NPX stack and returns Z at the (new) stack top,
replacing the Y operand. If the operand is out of range (i.e., in negative)
the invalid-operation exception occurs.

This function optimizes the calculations of log to any base other than two,
because a multiplication is always required:

LOG{N}x = (LOG{2}N){-1} * LOG{2}x


4.6.8  FYL2XP1

FYL2XP1 (Y log base 2 of (X + 1)) calculates the function Z = Y*LOG{2}
(X+1). X is taken from the stack top and must be in the range -(1-SQRT(2)/2)
< X <1-SQRT(2)/2. Y is taken from  ST(1) and is unlimited in range (-ý < Y
< +ý). FYL2XP1 pops the stack and returns Z at the (new) stack top,
replacing Y. If the argument is out of range, the results are undefined.

This instruction provides improved accuracy over FYL2X when computing the
logarithm of a number very close to 1, for example 1 + ¯ where ¯ << 1.
Providing ¯ rather than 1 + ¯ as the input to the function allows more
significant digits to be retained.


Table 4-11.  Constant Instructions

FLDZ     Load + 0.0
FLD1     Load + 1.0
FLDPI    Load Ò
FLDL2T   Load log{2}10
FLDL2E   Load log{2}e
FLDLG2   Load log{10}2
FLDLN2   Load log{e}2


4.7  Constant Instructions

Each of these instructions (Table 4-11) loads (pushes) a commonly used
constant onto the stack. (ST(7) must be empty to avoid an invalid
exception.) The values have full extended real precision (64 bits) and are
accurate to approximately 19 decimal digits. Because an external real
constant occupies 10 memory bytes, the constant instructions, which are
only two bytes long, save storage and improve execution speed, in addition
to simplifying programming.

The constants used by these instructions are stored internally in a format
more precise even than extended real. When loading the constant, the 80387
rounds the more precise internal constant according the RC (rounding
control) bit of the control word. However, in spite of this rounding, the
precision exception is not raised (to maintain compatibility). When the
rounding control is set to round to nearest on the 80387, the 80387
produces the same constant that is produced by the 80287.


4.7.1  FLDZ

FLDZ (load zero) loads (pushes) +0.0 onto the NPX stack.


4.7.2  FLD1

FLD1 (load one) loads (pushes) +1.0 onto the NPX stack.


4.7.3  FLDPI

FLDPI (load Ò) loads (pushes) Ò onto the NPX stack.


4.7.4  FLDL2T

FLDL2T (load log base 2 of 10) loads (pushes) the value LOG{2}10 onto the
NPX stack.


4.7.5  FLDL2E

FLDL2E (load log base 2 of e) loads (pushes) the value LOG{2}e onto the NPX
stack.


4.7.6  FLDLG2

FLDLG2 (load log base 10 of 2) loads (pushes) the value LOG{10}2 onto the
NPX stack.


4.7.7  FLDLN2

FLDLN2 (load log base e of 2) loads (pushes) the value LOG{e}2 onto the NPX
stack.


4.8  Processor Control Instructions

The processor control instructions are shown in Table 4-12. The instruction
FSTSW is commonly used for conditional branching. The remaining instructions
are not typically used in calculations; they provide control over the 80387
NPX for system-level activities. These activities include initialization,
exception handling, and task switching.

As shown in Table 4-12, many of the NPX processor control instructions have
two forms of assembler mnemonic:

  1.  A wait form, where the mnemonic is prefixed only with an F, such as
      FSTSW. This form checks for unmasked numeric exceptions.

  2.  A no-wait form, where the mnemonic is prefixed with an FN, such as
      FNSTSW. This form ignores unmasked numeric exceptions.

When the control instruction is coded using the no-wait form of the
mnemonic, the ASM386 assembler does not precede the ESC instruction with a
wait instruction, and the CPU does not test the ERROR# status line from the
NPX before executing the processor control instruction.

Only the processor control class of instructions have this alternate
no-wait form. All numeric instructions are automatically synchronized by the
80386; the CPU transfers all operands before initiating the next
instruction. Because of this automatic synchronization by the 80386, numeric
instructions for the 80387 need not be preceded by a CPU wait instruction
in order to execute correctly.

It should also be noted that the 8087 instructions FENI and FDISI and the
80287 instruction FSETPF perform no function in the 80387. If these opcodes
are detected in an 80386/80387 instruction stream, the 80387 performs no
specific operation and no internal states are affected. For programmers
interested in porting numeric software from 80287 or 8087 environments to
the 80386, however, it should be noted that program sections containing
these exception-handling instructions are not likely to be completely
portable to the 80387. Appendix C and Appendix D contains a more complete
description of the differences between the 80387 and the 80287/8087.


Table 4-12.  Processor Control Instructions

FINIT/FNINIT           Initialize processor
FLDCW                  Load control word
FSTCW/FNSTCW           Store control word
FSTSW/FNSTSW           Store status word
FSTSW AX/FNSTSW AX     Store status word to AX
FCLEX/FNCLEX           Clear exceptions
FSTENV/FNSTENV         Store environment
FLDENV                 Load environment
FSAVE/FNSAVE           Save state
FRSTOR                 Restore state
FINCSTP                Increment stack pointer
FDECSTP                Decrement stack pointer
FFREE                  Free register
FNOP                   No operation
FWAIT                  CPU Wait


4.8.1  FINIT/FNINIT

FINIT/FNINIT (initialize processor) sets the 80387 NPX into a known state,
unaffected by any previous activity. It sets the control word to its default
value 037FH (round to nearest, all exceptions masked, 64 bits of precision),
clears the status word, and empties all floating-point stack registers. The
no-wait form of this instruction causes the 80387 to abort any previous
numeric operations currently executing in the NEU.

This instruction performs the functional equivalent of a hardware RESET,
with one exception: RESET causes the IM bit of the control word to be reset
and the ES and IE bits of the status word to be set as a means of signaling
the presence of an 80387; FINIT puts the opposite values in these bits.

FINIT checks for unmasked numeric exceptions, FNINIT does not. Note that if
FNINIT is executed while a previous 80387 memory-referencing instruction is
running, 80387 bus cycles in progress are aborted. This instruction may be
necessary to clear the 80387 if a processor-extension segment-overrun
exception (interrupt 9) is detected by the CPU.


4.8.2  FLDCW source

FLDCW (load control word) replaces the current processor control word with
the word defined by the source operand. This instruction is typically used
to establish or change the 80387's mode of operation. Note that if an
exception bit in the status word is set, loading a new control word that
unmasks that exception will activate the ERROR# output of the 80387. When
changing modes, the recommended procedure is to first clear any exceptions
and then load the new control word.


4.8.3  FSTCW/FNSTCW destination

FSTCW/FNSTCW (store control word) writes the processor control word to the
memory location defined by the destination. FSTCW checks for unmasked
numeric exceptions; FNSTCW does not.


4.8.4  FSTSW/FNSTSW destination

FSTSW/FNSTSW (store status word) writes the current value of the 80387
status word to the destination operand in memory. The instruction is used to

  Ž  Implement conditional branching following a comparison, FPREM, or
     FPREM1 instruction (FSTSW).

  Ž  Invoke exception handlers (by polling the exception bits) in
     environments that do not use interrupts (FSTSW).

FSTSW checks for unmasked numeric exceptions, FNSTSW does not.


4.8.5  FSTSW AX/FNSTSW AX

FSTSW AX/FNSTSW AX (store status word to AX) is a special 80387 instruction
that writes the current value of the 80387 status word directly into the
80386 AX register. This instruction optimizes conditional branching in
numeric programs, where the 80386 CPU must test the condition of various NPX
status bits. The waited form FSTSW AX checks for unmasked numeric
exceptions, the non-waited form FNSTSW AX does not.

When this instruction is executed, the 80386 AX register is updated with
the NPX status word before the CPU executes any further instructions. The
status stored is that from the completion of the prior ESC instruction.


4.8.6  FCLEX/FNCLEX

FCLEX/FNCLEX (clear exceptions) clears all exception flags, the exception
status flag and the busy flag in the status word. As a consequence, the
80387's ERROR# line goes inactive. FCLEX checks for unmasked numeric
exceptions, FNCLEX does not.


4.8.7  FSAVE/FNSAVE destination

FSAVE/FNSAVE (save state) writes the full 80387 state‘‘environment plus
register stack‘‘to the memory location defined by the destination operand.
Figure 4-1 and Figure 4-2 show the layout of the save area; the size and
layout of the save the operating mode of the 80386 (real-address mode or
protected mode) and on the operand-size attribute in effect for the
instruction (32-bit operand or 16-bit operand). When the 80386 is in
virtual-8086 mode, the real-address mode formats are used. Typically the
instruction is coded to save this image on the CPU stack.

The values in the tag word in memory are determined during the execution of
FSAVE/FNSAVE. If the tag in the status register indicates that the
corresponding register is nonempty, the 80387 examines the data in the
register and stores the appropriate tag in memory. Thus the tag that is
stored always reflects the actual content of the register.

FNSAVE delays its execution until all NPX activity completes normally.
Thus, the save image reflects the state of the NPX following the completion
of any running instruction. After writing the state image to memory,
FSAVE/FNSAVE initializes the 80387 as if FINIT/FNINIT had been executed.

FSAVE/FNSAVE is useful whenever a program wants to save the current state
of the NPX and initialize it for a new routine. Three examples are

  1.  An operating system needs to perform a context switch (suspend the
      task that had been running and give control to a new task).

  2.  An exception handler needs to use the 80387.

  3.  An application task wants to pass a "clean" 80387 to a subroutine.

FSAVE checks for unmasked numeric exceptions before executing, FNSAVE does
not.


Figure 4-1.  FSAVE/FRSTOR Memory Layout (32-Bit)

               41          23          15          7         0
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              Ñ‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘ ENVIRONMENT ‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘Â+CH
              Ñ‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘       ‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘Â+10H
              Ñ‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘Â+14H
              Ñ‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘‘Â+18H
              „�����������Ï�����������Ï�����������Ï�����������…

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   ST(0)€SIGN�EXPONENT�                  SIGNIFICAND                 €+1CH
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   ST(2)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+30H
   ST(3)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+3AH
   ST(4)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+44H
   ST(5)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+4EH
   ST(6)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+58H
   ST(7)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+62H
        „����¤��������¤����������������������������������������������…
           79 78    64 63                                           0


Figure 4-2.  FSAVE/FRSTOR Memory Layout (16-Bit)

                          15         7        0
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                         Ñ‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘Â+2H
                         Ñ‘‘‘‘‘‘       ‘‘‘‘‘‘‘Â+4H
                         Ñ‘‘‘ ENVIRONMENT ‘‘‘‘Â+6H
                         Ñ‘‘‘‘‘‘       ‘‘‘‘‘‘‘Â+8H
                         Ñ‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘Â+AH
                         Ñ‘‘‘‘‘‘‘‘‘|‘‘‘‘‘‘‘‘‘‘Â+CH
                         „����������Ï����������…

        ‚����Ð��������Ð����������������������������������������������ƒ
   ST(0)€SIGN�EXPONENT�                  SIGNIFICAND                 €+EH
   ST(1)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+18H
   ST(2)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+22H
   ST(3)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+2CH
   ST(4)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+36H
   ST(5)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+40H
   ST(6)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+4AH
   ST(7)Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â+54H
        „����¤��������¤����������������������������������������������…
           79 78    64 63                                           0


4.8.8  FRSTOR source

FRSTOR (restore state) reloads the 80387 state from the memory area defined
by the source operand. This information should have been written by a
previous FSAVE/FNSAVE instruction and not altered by any other instruction.
FRSTOR automatically waits checking for interrupts until all data transfers
are completed before continuing to the next instruction.

Note that the 80387 "reacts" to its new state at the conclusion of the
FRSTOR. It generates an exception request, for example, if the exception and
mask bits in the memory image so indicate when the next WAIT or
exception-checking ESC instruction is executed.


4.8.9  FSTENV/FNSTENV destination

FSTENV/FNSTENV (store environment) writes the 80387's basic
status‘‘control, status, and tag words, and exception pointers‘‘to the
memory location defined by the destination operand. Typically, the
environment is saved on the CPU stack. FSTENV/FNSTENV is often used by
exception handlers because it provides access to the exception pointers
that identify the offending instruction and operand. After saving the
environment, FSTENV/FNSTENV sets all exception masks in the 80387 control
word (i.e., masks all exceptions). FSTENV checks for pending exceptions
before executing, FNSTENV does not.

Figures 4-3 through 4-6 show the format of the environment data in memory;
the size and layout of the save area depends on the operating mode of the
80386 (real-address mode or protected mode) and on the operand-size
attribute in effect for the instruction (32-bit operand or 16-bit operand).
When the 80386 is in virtual-8086 mode, the real-address mode formats are
used. FNSTENV does not store the environment until all NPX activity has
completed. Thus, the data saved by the instruction reflects the 80387 after
any previously decoded instruction has been executed.

The values in the tag word in memory are determined during the execution of
FNSTENV/FSTENV. If the tag in the status register indicates that the
corresponding register is nonempty, the 80387 examines the data in the
register and stores the appropriate tag in memory. Thus the tag that is
stored always reflects the actual content of the register.


Figure 4-3.  Protected Mode 80387 Environment, 32-Bit Format

                      32-BIT PROTECTED MODE FORMAT

 31                23                15                7               0
‚�����������������Ï�����������������Ð�����������������Ï�����������������ƒ
€             RESERVED              �            CONTROL WORD           €0H
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€             RESERVED              �            STATUS WORD            €4H
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€             RESERVED              �              TAG WORD             €8H
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€                               IP OFFSET                               €CH
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€ 0 0 0 0 0�      OPCODE 10..0      �            CS SELECTOR            €10H
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€                          DATA OPERAND OFFSET                          €14H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �          OPERAND SELECTOR         €18H
„�����������������Ï�����������������¤�����������������Ï�����������������…


Figure 4-4.  Real Mode 80387 Environment, 32-Bit Format

                       32-BIT PROTECTED MODE FORMAT

 31                23                15                7               0
‚�����������������Ï�����������������Ð�����������������Ï�����������������ƒ
€             RESERVED              �            CONTROL WORD           €0H
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€             RESERVED              �            STATUS WORD            €4H
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€             RESERVED              �              TAG WORD             €8H
Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€             RESERVED              �      INSTRUCTION POINTER 15..0    €CH
Ñ‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘˜‘˜‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
€ 0 0 0 0 �     INSTRUCTION POINTER 31..16      �0�    OPCODE 10..0     €10H
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€             RESERVED              �       OPERAND POINTER 15..0       €14H
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Figure 4-5.  Protected Mode 80387 Environment, 16-Bit Format

                        16-BIT PROTECTED MODE FORMAT

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Figure 4-6.  Real Mode 80387 Environment, 16-Bit Format

                          16-BIT REAL-ADDRESS MODE
                        AND VIRTUAL-8086 MODE FORMAT

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                    €            TAG WORD             € 4H
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                    €    INSTRUCTION POINTER 15..0    € 6H
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4.8.10  FLDENV source

FLDENV (load environment) reloads the environment from the memory area
defined by the source operand. This data should have been written by a
previous FSTENV/FNSTENV instruction. CPU instructions (that do not reference
the environment image) may immediately follow FLDENV. FLDENV automatically
waits for all data transfers to complete before executing the next
instruction.

Note that loading an environment image that contains an unmasked exception
causes a numeric exception when the next WAIT or exception-checking ESC
instruction is executed.


4.8.11  FINCSTP

FINCSTP (increment NPX stack pointer) adds 1 to the stack top pointer (TOP)
in the status word. It does not alter tags or register contents, nor does it
transfer data. It is not equivalent to popping the stack, because it does
not set the tag of the previous stack top to empty. Incrementing the stack
pointer when ST=7 produces ST=0.


4.8.12  FDECSTP

FDECSTP (decrement NPX stack pointer) subtracts 1 from ST, the stack top
pointer in the status word. No tags or registers are altered, nor is any
data transferred. Executing FDECSTP when ST=0 produces ST=7.


4.8.13  FFREE destination

FFREE (free register) changes the destination register's tag to empty; the
content of the register is unaffected.


4.8.14  FNOP

FNOP (no operation) effectively performs no operation.


4.8.15  FWAIT (CPU Instruction)

FWAIT is not actually an 80387 instruction, but an alternate mnemonic for
the 80386 WAIT instruction. The FWAIT or WAIT mnemonic should be coded
whenever the programmer wants to check for a pending error before modifying
a variable used in the previous floating-point instruction. Coding an FWAIT
instruction after an 80387 instruction ensures that unmasked numeric
exceptions occur and exception handlers are invoked before the next
instruction has a chance to examine the results of the 80387 instruction.

More information on when to code an FWAIT instruction is given in Chapter 5
in the section "Concurrent Processing with the 80387."



Chapter 5  Programming Numeric Applications

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

5.1  Programming Facilities

As described previously, the 80387 NPX is programmed simply as an extension
of the 80386 CPU. This section describes how programmers in ASM386 and in a
variety of higher-level languages can work with the 80387.

The level of detail in this section is intended to give programmers a basic
understanding of the software tools that can be used with the 80387, but
this information does not document the full capabilities of these
facilities. Complete documentation is available with each program
development product.


5.1.1  High-Level Languages

For programmers using high-level languages, the programming and operation
of the NPX is handled automatically by the compiler. A variety of Intel
high-level languages are available that automatically make use of the 80387
NPX when appropriate. These languages include C-386 and PL/M-386. In
addition many high-level language compilers are available from independent
software vendors.

Each of these high-level languages has special numeric libraries allowing
programs to take advantage of the capabilities of the 80387 NPX. No special
programming conventions are necessary to make use of the 80387 NPX when
programming numeric applications in any of these languages.

Programmers in PL/M-386 and ASM386 can also make use of many of these
library routines by using routines contained in the 80387 Support Library.
These libraries implement many of the functions provided by higher-level
languages, including exception handlers, ASCII-to-floating-point
conversions, and a more complete set of transcendental functions than that
provided by the 80387 instruction set.


5.1.2  C Programs

C programmers automatically cause the C compiler to generate 80387
instructions when they use the double and float data types. The float type
corresponds to the 80387's single real format; the double type corresponds
to the 80387's double real format. The statement #include <math.h> causes
mathematical functions such as sin and sqrt to return values of type
double. Figure 5-1 illustrates the ease with which C programs interface
with the 80387.


Figure 5-1.  Sample C-386 Program

XENIX286 C386 COMPILER,  V0.2 COMPILATION OF MODULE SAMPLE
OBJECT MODULE PLACED IN  sample.obj
COMPILER INVOKED BY:  c386 sample.c

stmt level

      1         /******************************************************
      2         *                                                      *
      3         *                  SAMPLE C PROGRAM                    *
      4         *                                                      *
      5         ******************************************************/
      6
      7         /** Include /usr/include/stdio.h if necessary **/
      8         /** Include math declarations for transcendenatals and others **/
      9
     10         #include </usr/include/math.h>
     36         #define PI 3.141592654
     37
     38         main()
     39         {
     40   1     double        sin_result, cos_result;
     41   1     double        angle_deg = 0.0, angle_rad;
     42   1     int           i, no_of_trial = 4;
     43
     44   1          for( i = 1; i <= no_of_trial; i++){
     45   2              angle_rad = angle_deg * PI / 180.0;
     46   2              sin_result = sin (angle_rad);
     47   2              cos_result = cos (angle_rad);
     48   2              printf("sine of %f degrees equals %f\n", angle_deg, sin_result);
     49   2              printf("cosine of %f degrees equals %f\n\n", angle_deg, cos_result);
     50   2              angle_deg = angle_deg + 30.0;
     51   2              }
     52   1     /** etc. **/
     53   1     }

C386 COMPILATION COMPLETE. 0 WARNINGS, 0 ERRORS


5.1.3  PL/M-386

Programmers in PL/M-386 can access a very useful subset of the 80387's
numeric capabilities. The PL/M-386 REAL data type corresponds to the NPX's
single real (32-bit) format. This data type provides a range of about
8.43 * 10^(-37) ¾ �X� ¾ 3.38 * 10^(38), with about seven significant decimal
digits. This representation is adequate for the data manipulated by many
microcomputer applications.

The utility of the REAL data type is extended by the PL/M-386 compiler's
practice of holding intermediate results in the 80387's extended real
format. This means that the full range and precision of the processor are
utilized for intermediate results. Underflow, overflow, and rounding
exceptions are most likely to occur during intermediate computations rather
than during calculation of an expression's final result. Holding
intermediate results in extended-precision real format greatly reduces the
likelihood of overflow and underflow and eliminates roundoff as a serious
source of error until the final assignment of the result is performed.

The compiler generates 80387 code to evaluate expressions that contain REAL
data types, whether variables or constants or both. This means that
addition, subtraction, multiplication, division, comparison, and assignment
of REALs will be performed by the NPX. INTEGER expressions, on the other
hand, are evaluated on the CPU.

Five built-in procedures (Table 5-1) give the PL/M-386 programmer access to
80387 functions manipulated by the processor control instructions. Prior to
any arithmetic operations, a typical PL/M-386 program will set up the NPX
using the INIT$REAL$MATH$UNIT procedure and then issue SET$REAL$MODE to
configure the NPX. SET$REAL$MODE loads the 80387 control word, and its
16-bit parameter has the format shown for the control word in Chapter 2.
The recommended value of this parameter is 033EH (round to nearest, 64-bit
precision, all exceptions masked except invalid operation). Other settings
may be used at the programmer's discretion.

If any exceptions are unmasked, an exception handler must be provided in
the form of an interrupt procedure that is designated to be invoked via CPU
interrupt vector number 16. The exception handler can use the GET$REAL$ERROR
procedure to obtain the low-order byte of the 80387 status word and to then
clear the exception flags. The byte returned by GET$REAL$ERROR contains the
exception flags; these can be examined to determine the source of the
exception.

The SAVE$REAL$STATUS and RESTORE$REAL$STATUS procedures are provided
for multitasking environments where a running task that uses the 80387 may
be preempted by another task that also uses the 80387. It is the
responsibility of the operating system to issue SAVE$REAL$STATUS before it
executes any statements that affect the 80387; these include the
INIT$REAL$MATH$UNIT and SET$REAL$MODE procedures as well as arithmetic
expressions. SAVE$REAL$STATUS saves the 80387 state (registers, status, and
control words, etc.) on the CPU's stack. RESTORE$REAL$STATUS reloads the
state information; the preempting task must invoke this procedure before
terminating in order to restore the 80387 to its state at the time the
running task was preempted. This enables the preempted task to resume
execution from the point of its preemption.


Table 5-1.  PL/M-386 Built-In Procedures

Procedure                80387         Description
                         Instruction

INIT$REAL$MATH$UNIT      FINIT         Initialize processor.
SET$REAL$MODE            FLDCW         Set exception masks, rounding
                                       precision, and infinity controls.
GET$REAL$ERROR           FNSTSW        Store, then clear, exception flags.
                         & FNCLEX
SAVE$REAL$STATUS         FNSAVE        Save processor state.
RESTORE$REAL$STATUS      FRSTOR        Restore processor state.


5.1.4  ASM386

The ASM386 assembly language provides programmers with complete access to
all of the facilities of the 80386 and 80387 processors.

The programmer's view of the 80386/80387 hardware is a single machine with
these resources:

  Ž  160 instructions
  Ž  12 data types
  Ž  8 general registers
  Ž  6 segment registers
  Ž  8 floating-point registers, organized as a stack


5.1.4.1  Defining Data

The ASM386 directives shown in Table 5-2 allocate storage for 80387
variables and constants. As with other storage allocation directives, the
assembler associates a type with any variable defined with these directives.
The type value is equal to the length of the storage unit in bytes (10 for
DT, 8 for DQ, etc.). The assembler checks the type of any variable coded in
an instruction to be certain that it is compatible with the instruction.
For example, the coding FIADD ALPHA will be flagged as an error if ALPHA's
type is not 2 or 4, because integer addition is only available for word and
short integer (doubleword) data types. The operand's type also tells the
assembler which machine instruction to produce; although to the programmer
there is only an FIADD instruction, a different machine instruction is
required for each operand type.

On occasion it is desirable to use an instruction with an operand that has
no declared type. For example, if register BX points to a short integer
variable, a programmer may want to code FIADD [BX]. This can be done by
informing the assembler of the operand's type in the instruction, coding
FIADD DWORD PTR [BX]. The corresponding overrides for the other storage
allocations are WORD PTR, QWORD PTR, and TBYTE PTR.

The assembler does not, however, check the types of operands used in
processor control instructions. Coding FRSTOR [BP] implies that the
programmer has set up register BP to point to the location (probably in the
stack) where the processor's 94-byte state record has been previously saved.

The initial values for 80387 constants may be coded in several different
ways. Binary integer constants may be specified as bit strings, decimal
integers, octal integers, or hexadecimal strings. Packed decimal values are
normally written as decimal integers, although the assembler will accept and
convert other representations of integers. Real values may be written as
ordinary decimal real numbers (decimal point required), as decimal numbers
in scientific notation, or as hexadecimal strings. Using hexadecimal strings
is primarily intended for defining special values such as infinities, NaNs,
and denormalized numbers. Most programmers will find that ordinary decimal
and scientific decimal provide the simplest way to initialize 80387
constants. Figure 5-2 compares several ways of setting the various 80387
data types to the same initial value.

Note that preceding 80387 variables and constants with the ASM386 EVEN
directive ensures that the operands will be word-aligned in memory. The best
performance is obtained when data transfers are double-word aligned. All
80387 data types occupy integral numbers of words so that no storage is
"wasted" if blocks of variables are defined together and preceded by a
single EVEN declarative.


Table 5-2.  ASM386 Storage Allocation Directives

Directive   Interpretation       Data Types

DW          Define Word          Word integer
DD          Define Doubleword    Short integer, short real
DQ          Dfine Quadword       Long integer, long real
DT          Define Tenbyte       Packed decimal, temporary real


Figure 5-2.  Sample 80387 Constants

; THE FOLLOWING ALL ALLOCATE THE CONSTANT: -126
; NOTE TWO'S COMPLETE STORAGE OF NEGATIVE BINARY INTEGERS.
;
; EVEN                                  ; FORCE WORD ALIGNMENT
WORD_INTEGER    DW  111111111000010B    ; BIT STRING
SHORT_INTEGER   DD  0FFFFFF82H          ; HEX STRING MUST START
                                        ; WITH DIGIT
LONG_INTEGER    DQ  -126                ; ORDINARY DECIMAL
SINGLE_REAL     DD  -126.0              ; NOTE PRESENCE OF '.'
DOUBLE_REAL     DD  -1.26E2             ; "SCIENTIFIC"
PACKED_DECIMAL  DT  -126                ; ORDINARY DECIMAL INTEGER
;
; IN THE FOLLOWING, SIGN AND EXPONENT IS 'C005'
;    SIGNIFICAND IS '7E00...00', 'R' INFORMS ASSEMBLER THAT
;    THE STRING REPRESENTS A REAL DATA TYPE.
;
EXTENDED_REAL   DT  0C0057E00000000000000R  ; HEX STRING


5.1.4.2  Records and Structures

The ASM386 RECORD and STRUC (structure) declaratives can be very useful in
NPX programming. The record facility can be used to define the bit fields of
the control, status, and tag words. Figure 5-3 shows one definition of the
status word and how it might be used in a routine that polls the 80387 until
it has completed an instruction.

Because structures allow different but related data types to be grouped
together, they often provide a natural way to represent "real world" data
organizations. The fact that the structure template may be "moved" about in
memory adds to its flexibility. Figure 5-4 shows a simple structure that
might be used to represent data consisting of a series of test score
samples. A structure could also be used to define the organization of the
information stored and loaded by the FSTENV and FLDENV instructions.


Figure 5-3.  Status Word Record Definition

; RESERVE SPACE FOR STATUS WORD
STATUS_WORD
; LAY OUT STATUS WORD FIELDS
STATUS RECORD
&   BUSY:           1,
&   COND_CODE3:     1,
&   STACK_TOP:      3,
&   COND_CODE2:     1,
&   COND_CODE1:     1,
&   COND_CODE0:     1,
&   INT_REQ:        1,
&   S_FLAG:         1,
&   P_FLAG:         1,
&   U_FLAG:         1,
&   O_FLAG:         1,
&   Z_FLAG:         1,
&   D_FLAG:         1,
&   I_FLAG:         1
; REDUCE UNTIL COMPLETE
REDUCE: FPREM1
        FNSTSW  STATUS_WORD
        TEST    STATUS_WORD, MASK_COND_CODE2
        JNZ     REDUCE


Figure 5-4.  Structure Definition

SAMPLE     STRUC
    N_OBS   DD  ?   ; SHORT INTEGER
    MEAN    DQ  ?   ; DOUBLE REAL
    MODE    DW  ?   ; WORD INTEGER
    STD_DEV DQ  ?   ; DOUBLE REAL
    ; ARRAY OF OBSERVATIONS -- WORD INTEGER
    TEST_SCORES DW 1000 DUP (?)
SAMPLE     ENDS


5.1.4.3  Addressing Methods

80387 memory data can be accessed with any of the memory addressing methods
provided by the ModR/M byte and (optionally) the SIB byte. This means that
80387 data types can be incorporated in data aggregates ranging from simple
to complex according to the needs of the application. The addressing methods
and the ASM386 notation used to specify them in instructions make the
accessing of structures, arrays, arrays of structures, and other
organizations direct and straightforward. Table 5-3 gives several examples
of 80387 instructions coded with operands that illustrate different
addressing methods.


Table 5-3.  Addressing Method Examples

Coding                    Interpretation

FIADD ALPHA               ALPHA is a simple scalar (mode is direct).

FDIVR ALPHA.BETA          BETA is a field in a structure that is
                          "overlaid" on ALPHA (mode is direct).

FMUL QWORD PTR [BX]       BX contains the address of a long real
                          variable (mode is register indirect).

FSUB ALPHA [SI]           ALPHA is an array and SI contains the
                          offset of an array element from the start of
                          the array (mode is indexed).

FILD [BP].BETA            BP contains the address of a structure on
                          the CPU stack and BETA is a field in the
                          structure (mode is based).

FBLD TBYTE PTR [BX] [DI]  BX contains the address of a packed
                          decimal array and DI contains the offset of
                          an array element (mode is based indexed).


5.1.5  Comparative Programming Example

Figures 5-5 and 5-6 show the PL/M-386 and ASM386 code for a simple 80387
program, called ARRSUM. The program references an array (X$ARRAY), which
contains 0-100 single real values; the integer variable N$OF$X indicates the
number of array elements the program is to consider. ARRSUM steps through
X$ARRAY accumulating three sums:

  Ž  SUM$X, the sum of the array values

  Ž  SUM$INDEXES, the sum of each array value times its index, where the
     index of the first element is 1, the second is 2, etc.

  Ž  SUM$SQUARES, the sum of each array element squared

(A true program, of course, would go beyond these steps to store and use
the results of these calculations.) The control word is set with the
recommended values: round to nearest, 64-bit precision, interrupts enabled,
and all exceptions masked except invalid operation. It is assumed that an
exception handler has been written to field the invalid operation if it
occurs, and that it is invoked by interrupt pointer 16. Either version of
the program will run on an actual or an emulated 80387 without altering the
code shown.

The PL/M-386 version of ARRSUM (Figure 5-5) is very straightforward and
illustrates how easily the 80387 can be used in this language. After
declaring variables, the program calls built-in procedures to initialize the
processor (or its emulator) and to load to the control word. The program
clears the sum variables and then steps through X$ARRAY with a DO-loop. The
loop control takes into account PL/M-386's practice of considering the
index of the first element of an array to be 0. In the computation of
SUM$INDEXES, the built-in procedure FLOAT converts I+1 from integer to real
because the language does not support "mixed mode" arithmetic. One of the
strengths of the NPX, of course, is that it does support arithmetic on mixed
data types (because all values are converted internally to the 80-bit
extended-precision real format).

The ASM386 version (Figure 5-6) defines the external procedure INIT387,
which makes the different initialization requirements of the processor and
its emulator transparent to the source code. After defining the data and
setting up the segment registers and stack pointer, the program calls
INIT387 and loads the control word. The computation begins with the next
three instructions, which clear three registers by loading (pushing) zeros
onto the stack. As shown in Figure 5-7, these registers remain at the
bottom of the stack throughout the computation while temporary values are
pushed on and popped off the stack above them.

The program uses the CPU LOOP instruction to control its iteration through
X_ARRAY; register ECX, which LOOP automatically decrements, is loaded with
N_OF_X, the number of array elements to be summed. Register ESI is used to
select (index) the array elements. The program steps through X_ARRAY from
back to front, so ESI is initialized to point at the element just beyond the
first element to be processed. The ASM386 TYPE operator is used to determine
the number of bytes in each array element. This permits changing X_ARRAY to
a double-precision real array by simply changing its definition (DD to DQ)
and reassembling.

Figure 5-7 shows the effect of the instructions in the program loop on the
NPX register stack. The figure assumes that the program is in its first
iteration, that N_OF_X is 20, and that X_ARRAY(19) (the 20th element)
contains the value 2.5. When the loop terminates, the three sums are left as
the top stack elements so that the program ends by simply popping them into
memory variables.


Figure 5-5.  Sample PL/M-386 Program

XENIX286 PL/M-386 DEBUG X291a COMPILATION OF MODULE ARRAYSUM
OBJECT MODULE PLACED IN arraysum.obj
COMPILER INVOKED BY:  plm386 arraysum.plm


            /***********************************************************
            *                                                          *
            *                      ARRAYSUM  MODDULE                   *
            *                                                          *
            ***********************************************************/

  1         array$sum:      do;

  2   1        declare (sum$x, sum$indexes, sum$squares) real;
  3   1        declare x$array(100) real;
  4   1        declare (n$of$x, i) integer;
  5   1        declare control$387 literally `033eh';

               /* Assume x$array and n$of$x are initialized */
  6   1        call init$real$math$unit;
  7   1        call set$real$mode(control$387);

               /* Clear sums */
  8   1        sum$x, sum$indexes, sum$squares = 0.0;

               /* Loop through array, accumulating sums */
  9   1        do i = 0 to n$of$x - 1;
 10   2             sum$x = sum$x + x$array(i);
 11   2             sum$indexes = sum$indexes + (x$array(i)*float(i+1));
 12   2             sum$squares = sum$squares + (x$array(i)*x$array(i));
 13   2        end;

               /* etc. */

 14   1     end array$sum;


 MODULE INFORMATION:

   CODE AREA SIZE      = 000000A0H       160D
   CONSTANT AREA SIZE  = 00000004H         4D
   VARIABLE AREA SIZE  = 000001A4H       420D
   MAXIMUM  STACK SIZE = 00000004H         4D
   32 LINES READ
   0 PROGRAM WARNINGS
   0 PROGRAM ERRORS

 DICTIONARY SUMMARY:

   8KB MEMORY USED
   0KB DISK SPACE USED

 END OF PL/M-386 COMPILATION


Figure 5-6.  Sample ASM386 Program

XENIX286 80386 MACRO ASSEMBLER V1.0, ASSEMBLY OF MODULE ARRAYSUM
OBJECT MODULE PLACED IN arraysum.obj
ASSEMBLER INVOKED BY: asm386 arraysum.asm

LOC       OBJ                         LINE    SOURCE

                               1    name       arraysum
                               2
                               3    ; Define initialization routine
                               4
                               5    extrn      init387:far
                               6
                               7    ; Allocate space for data
                               8
--------                       9    data       segment rw public
00000000  3E03                10    control_387        dw 033eh
00000002  ????????            11    n_of_x             dd ?
00000006  (100                12    x_array            cd 100 dup (?)
          ????????
          )
00000196  ????????            13    sum_squares        dd ?
0000019A  ????????            14    sum_indexes        dd ?
0000019E  ????????            15    sum_x              dd ?
--------                      16    data       ends
                              17
                              18    ; Allocate CPU stack space
                              19
--------                      20    stack      stackseg   400
                              21
                              22    ; Begin code
                              23
--------                      24    code       segment er public
                              25
                              26    assume  ds:data, ss:stack
                              27
00000000                      28    start:
00000000  66B8----        R   29         mov     ax, data
00000004  8ED8                30         mov     ds, ax
00000006  66B8----        R   31         mov     ax, stack
0000000A  B800000000          32         mov     eax, 0h
0000000F  8E00                33         mov     ss, ax
00000011  BC00000000      R   34         mov     esp, stackstart stack
                              35
                              36    ; Assume x_array and n_of_x have
                              37    ; been initialized
                              38
                              39    ; Prepare the 80387 or its emulator
                              40
00000016  9A00000000----  E   41         call    init387
0000001D  D92D00000000    R   42         fldcw   control_387
                              43
                              44    ; Clear three registers to hold
                              45    ; running sums
                              46
00000023  D9EE                47         fldz
00000025  D9EE                48         fldz
00000027  D9EE                49         fldz
                              50
                              51    ; Setup ECX as loop counter and ESI
                              52    ; as index into x array
                              53
00000029  8B0D02000000    R   54         mov     ecx, n of x
0000002F  F7E9                55         imul    ecx
00000031  8BF0                56         mov     esi, eax
                              57
                              58    ; ESI now contains index of last
                              59    ; element + 1
                              60    ; Loop through x_array and
                              61    ; accumulate sum
                              62
00000033                      43    sum_next:
                              64    ; backup one element and push on
                              65    ; the stack
                              66
00000033  83EE04              67         sub     esi,  type x_array
00000036  D98606000000    R   68         fld     x_array[esi]
                              69
                              70    ; add to the sum and duplicate x
                              71    ; on the stack
                              72
0000003C  DCC3                73         fadd    st(3), st
0000003E  D9C0                74         fld     st
                              75
                              76    ; square it and add into the sum of
                              77    ; (index+1) and discard
                              78
00000040  DCC8                79         fmul    st, st
00000042  DEC2                80         facdp   st(2), st
                              81
                              82    ; reduce index for next iteration
                              83
00000044  FF0D02000000    R   84         dec     n_of_x
0000004A  E2E7                85         loop    sum_next
                              86
                              87    ; Pop sums into memory
                              88
0000004C                      89    pop_results:
0000004C  D91D96010000    R   90         fstp    sum_squares
00000052  D91D9A010000    R   91         fstp    sum_indexes
00000058  D91D9E010000    R   92         fstp    sum_x
0000005E  9B                  93         fwait
                              94
                              95    ;
                              96    ; Etc.
                              97    ;
--------                      98    code       ends
                              99    end     start, ds:data, ss:stack

ASSEMBLY COMPLETE,    NO WARNINGS,    NO ERRORS.


Figure 5-7.  Instructions and Register Stack

         FLDZ, FLDZ, FLDZ                       FLD X_ARRAY[SI]
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         †��������������‡                      †��������������‡
    ST(1)€      0.0     € SUM_INDEXES     ST(1)€              € SUM_SQUARES
         †��������������‡                      †��������������‡
    ST(2)€      0.0     € SUM_X           ST(2)€      0.0     € SUM_INDEXES
         „��������������…                      †��������������‡
                                          ST(3)€      0.0     € SUM_X
                           ’ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ „��������������…
                           �
          FADD_ST(3), ST \x11‘•                        FLD_ST
         ‚��������������ƒ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘\x10‚��������������ƒ
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         †��������������‡                      †��������������‡
    ST(1)€      0.0     € SUM_SQUARES     ST(1)€      2.5     € X_ARRAY(19)
         †��������������‡                      †��������������‡
    ST(2)€      0.0     € SUM_INDEXES     ST(2)€      0.0     € SUM_SQUARES
         †��������������‡                      †��������������‡
    ST(3)€      2.5     € SUM_X           ST(3)€      0.0     € SUM_INDEXES
         „��������������…                      †��������������‡
                                          ST(4)€      2.5     € SUM_X
                           ’ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ „��������������…
                           �
           FMUL_ST, ST  \x11‘‘•                    FADDP_ST(2), ST
         ‚��������������ƒ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘\x10‚��������������ƒ
    ST(0)€      6.25    € X_ARRAY(19)²    ST(O)€      2.5     € X_ARRAY(19)
         †��������������‡                      †��������������‡
    ST(1)€      2.5     € X_ARRAY(19)     ST(1)€      6.25    € SUM_SQUARES
         †��������������‡                      †��������������‡
    ST(2)€      0.0     € SUM_SQUARES     ST(2)€      0.0     € SUM_INDEXES
         †��������������‡                      †��������������‡
    ST(3)€      0.0     € SUM_INDEXES     ST(3)€      2.5     € SUM_X
         †��������������‡                      „��������������…
    ST(4)€      2.5     € SUM_X                �
         „��������������…                      �
                           ’ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ •
           FIMUL N_OF_X \x11‘‘•                    FADDP_ST(2), ST
         ‚��������������ƒ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘\x10‚��������������ƒ
    ST(O)€      50.0    € X_ARRAY(19)*20  ST(O)€      6.25    € SUM_SQUARES
         †��������������‡                      †��������������‡
    ST(1)€      6.25    € SUM_SQUARES     ST(1)€      50.0    € SUM_INDEXES
         †��������������‡                      †��������������‡
    ST(2)€      0.0     € SUM_INDEXES     ST(2)€      2.5     € SUM_X
         †��������������‡                      „��������������…
    ST(3)€      2.5     € SUM_X
         „��������������…


5.1.6  80387 Emulation

The programming of applications to execute on both 80386 with an 80387 and
80386 systems without an 80387 is made much easier by the existence of an
80387 emulator for 80386 systems. The Intel EMUL387 emulator offers a
complete software counterpart to the 80387 hardware; NPX instructions can be
simply emulated in software rather than being executed in hardware. With
software emulation, the distinction between 80386 systems with or without an
80387 is reduced to a simple performance differential. Identical numeric
programs will simply execute more slowly (using software emulation of NPX
instructions) on 80386 systems without an 80387 than on an 80386/80387
system executing NPX instructions directly.

When incorporated into the systems software, the emulation of NPX
instructions on the 80386 systems is completely transparent to the
applications programmer. Applications software needs no special libraries,
linking, or other activity to allow it to run on an 80386 with 80387
emulation.

To the applications programmer, the development of programs for 80386
systems is the same whether the 80387 NPX hardware is available or not. The
full 80387 instruction set is available for use, with NPX instructions being
either emulated or executed directly. Applications programmers need not be
concerned with the hardware configuration of the computer systems on which
their applications will eventually run.

For systems programmers, details relating to 80387 emulators are described
in Chapter 6.

The EMUL387 software emulator for 80386 systems is available from Intel as
a separate program product.


5.2  Concurrent Processing With the 80387

Because the 80386 CPU and the 80387 NPX have separate execution units, it
is possible for the NPX to execute numeric instructions in parallel with
instructions executed by the CPU. This simultaneous execution of different
instructions is called concurrency.

No special programming techniques are required to gain the advantages of
concurrent execution; numeric instructions for the NPX are simply placed in
line with the instructions for the CPU. CPU and numeric instructions are
initiated in the same order as they are encountered by the CPU in its
instruction stream. However, because numeric operations performed by the NPX
generally require more time than operations performed by the CPU, the CPU
can often execute several of its instructions before the NPX completes a
numeric instruction previously initiated.

This concurrency offers obvious advantages in terms of execution
performance, but concurrency also imposes several rules that must be
observed in order to assure proper synchronization of the 80386 CPU and
80387 NPX.

All Intel high-level languages automatically provide for and manage
concurrency in the NPX. Assembly-language programmers, however, must
understand and manage some areas of concurrency in exchange for the
flexibility and performance of programming in assembly language. This
section is for the assembly-language programmer or well-informed
high-level-language programmer.


5.2.1  Managing Concurrency

Concurrent execution of the host and 80387 is easy to establish and
maintain. The activities of numeric programs can be split into two major
areas: program control and arithmetic. The program control part performs
activities such as deciding what functions to perform, calculating addresses
of numeric operands, and loop control. The arithmetic part simply adds,
subtracts, multiplies, and performs other operations on the numeric
operands. The NPX and host are designed to handle these two parts separately
and efficiently.

Concurrency management is required to check for an exception before letting
the 80386 change a value just used by the 80387. Almost any numeric
instruction can, under the wrong circumstances, produce a numeric exception.
For programmers in higher-level languages, all required synchronization is
automatically provided by the appropriate compiler. For assembly-language
programmers exception synchronization remains the responsibility of the
assembly-language programmer.

A complication is that a programmer may not expect his numeric program to
cause numeric exceptions, but in some systems, they may regularly happen. To
better understand these points, consider what can happen when the NPX
detects an exception.

Depending on options determined by the software system designer, the NPX
can perform one of two things when a numeric exception occurs:

  Ž  The NPX can provide a default fix-up for selected numeric exceptions.
     Programs can mask individual exception types to indicate that the NPX
     should generate a safe, reasonable result whenever that exception
     occurs. The default exception fix-up activity is treated by the NPX as
     part of the instruction causing the exception; no external indication
     of the exception is given. When exceptions are detected, a flag is set
     in the numeric status register, but no information regarding where or
     when is available. If the NPX performs its default action for all
     exceptions, then the need for exception synchronization is not
     manifest. However, as will be shown later, this is not sufficient
     reason to ignore exception synchronization when designing programs that
     use the 80387.

  Ž  As an alternative to the NPX default fix-up of numeric exceptions, the
     80386 CPU can be notified whenever an exception occurs. When a numeric
     exception is unmasked and the exception occurs, the NPX stops further
     execution of the numeric instruction and signals this event to the CPU.
     On the next occurrence of an ESC or WAIT instruction, the CPU traps to
     a software exception handler. The exception handler can then implement
     any sort of recovery procedures desired for any numeric exception
     detectable by the NPX. Some ESC instructions do not check for
     exceptions. These are the nonwaiting forms FNINIT, FNSTENV, FNSAVE,
     FNSTSW, FNSTCW, and FNCLEX.

When the NPX signals an unmasked exception condition, it is requesting
help. The fact that the exception was unmasked indicates that further
numeric program execution under the arithmetic and programming rules of the
NPX is unreasonable.

If concurrent execution is allowed, the state of the CPU when it recognizes
the exception is undefined. The CPU may have changed many of its internal
registers and be executing a totally different program by the time the
exception occurs. To handle this situation, the NPX has special registers
updated at the start of each numeric instruction to describe the state of
the numeric program when the failed instruction was attempted.

Exception synchronization ensures that the NPX is in a well-defined state
after an unmasked numeric exception occurs. Without a well-defined state, it
would be impossible for exception recovery routines to determine why the
numeric exception occurred, or to recover successfully from the exception.

The following two sections illustrate the need to always consider
exception synchronization when writing 80387 code, even when the code is
initially intended for execution with exceptions masked. If the code is
later moved to an environment where exceptions are unmasked, the same code
may not work correctly. An example of how some instructions written without
exception synchronization will work initially, but fail when moved into a
new environment is shown in Figure 5-8.


Figure 5-8.  Exception Synchronization Examples

INCORRECT ERROR SYNCHRONIZATION

FILD   COUNT  ; NPX instruction
INC    COUNT  ; CPU instruction alters operand
FSQRT  COUNT  ; subsequent NPX instruction -- error from
              ;    previous NPX instruction detected here

PROPER ERROR SYNCHRONIZATION

FILD   COUNT  ; NPX instruction
FSQRT         ; subsequent NPX instruction -- error from
              ;    previous NPX instruction detected here
INC    COUNT  ; CPU instruction alters operand


5.2.1.1  Incorrect Exception Synchronization

In Figure 5-8, three instructions are shown to load an integer, calculate
its square root, then increment the integer. The 80386-to-80387 interface
and synchronous execution of the NPX emulator will allow this program to
execute correctly when no exceptions occur on the FILD instruction.

This situation changes if the 80387 numeric register stack is extended to
memory. To extend the NPX stack to memory, the invalid exception is
unmasked. A push to a full register or pop from an empty register sets SF
and causes an invalid exception.

The recovery routine for the exception must recognize this situation, fix
up the stack, then perform the original operation.  The recovery routine
will not work correctly in the first example shown in the figure. The
problem is that the value of COUNT is incremented before the NPX can signal
the exception to the CPU. Because COUNT is incremented before the exception
handler is invoked, the recovery routine will load an incorrect value of
COUNT, causing the program to fail or behave unreliably.


5.2.1.2  Proper Exception Synchronization

Exception synchronization relies on the WAIT instruction and the BUSY# and
ERROR# signals of the 80387. When an unmasked exception occurs in the 80387,
it asserts the ERROR# signal, signaling to the CPU that a numeric exception
has occurred. The next time the CPU encounters a WAIT instruction or an
exception-checking ESC instruction, the CPU acknowledges the ERROR# signal
by trapping automatically to Interrupt #16, the processor-extension
exception vector. If the following ESC or WAIT instruction is properly
placed, the CPU will not yet have disturbed any information vital to
recovery from the exception.


Chapter 6  System-Level Numeric Programming

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

System programming for 80387 systems requires a more detailed understanding
of the 80387 NPX than does application programming. Such things as
emulation, initialization, exception handling, and data and error
synchronization are all the responsibility of the systems programmer. These
topics are covered in detail in the sections that follow.


6.1  80386/80387 Architecture

On a software level, the 80387 NPX appears as an extension of the 80386
CPU. On the hardware level, however, the mechanisms by which the 80386 and
80387 interact are more complex. This section describes how the 80387 NPX
and 80386 CPU interact and points out features of this interaction that are
of interest to systems programmers.


6.1.1  Instruction and Operand Transfer

All transfers of instructions and operands between the 80387 and system
memory are performed by the 80386 using I/O bus cycles. The 80387 appears to
the CPU as a special peripheral device. It is special in two respects: the
CPU initiates I/O automatically when it encounters ESC instructions, and the
CPU uses reserved I/O addresses to communicate with the 80387. These I/O
operations are completely transparent to software.

Because the 80386 actually performs all transfers between the 80387 and
memory, no additional bus drivers, controllers, or other components are
necessary to interface the 80387 NPX to the local bus. The 80387 can utilize
instructions and operands located in any memory accessible to the 80386 CPU.


6.1.2  Independent of CPU Addressing Modes

Unlike the 80287, the 80387 is not sensitive to the addressing and memory
management of the CPU. The 80387 operates the same regardless of whether the
80386 CPU is operating in real-address mode, in protected mode, or in
virtual 8086 mode.

The instruction FSETPM that was necessary in 80286/80287 systems to set the
80287 into protected mode is not needed for the 80387. The 80387 treats this
instruction as a no-op.

Because the 80386 actually performs all transfers between the 80387 and
memory, 80387 instructions can utilize any memory location accessible by the
task currently executing on the 80386. When operating in protected mode, all
references to memory operands are automatically verified by the 80386's
memory management and protection mechanisms as for any other memory
references by the currently-executing task. Protection violations associated
with NPX instructions automatically cause the 80386 to trap to an
appropriate exception handler.

To the numerics programmer, the operating modes of the 80386 affect only
the manner in which the NPX instruction and data pointers are represented in
memory following an FSAVE or FSTENV instruction. Each of these instructions
produces one of four formats depending on both the operating mode and on the
operand-size attribute in effect for the instruction. The differences are
detailed in the discussion of the FSAVE and FSTENV instructions in
Chapter 4.


6.1.3  Dedicated I/O Locations

The 80387 NPX does not require that any memory addresses be set aside for
special purposes. The 80387 does make use of I/O port addresses, but these
are 32-bit addresses with the high-order bit set (i.e. > 80000000H);
therefore, these I/O operations are completely transparent to the 80386
software. Because these addresses are beyond the 64 Kbyte I/O addressing
limit of I/O instructions, 80386 programs cannot reference these reserved
I/O addresses directly.


6.2  Processor Initialization and Control

One of the principal responsibilities of systems software is the
initialization, monitoring, and control of the hardware and software
resources of the system, including the 80387 NPX. In this section, issues
related to system initialization and control are described, including
recognition of the NPX, emulation of the 80387 NPX in software if the
hardware is not available, and the handling of exceptions that may occur
during the execution of the 80387.


6.2.1  System Initialization

During initialization of an 80386 system, systems software must

  Ž  Recognize the presence or absence of the NPX.

  Ž  Set flags in the 80386 MSW to reflect the state of the numeric
     environment.

If an 80387 NPX is present in the system, the NPX must be initialized. All
of these activities can be quickly and easily performed as part of the
overall system initialization.


6.2.2  Hardware Recognition of the NPX

The 80386 identifies the type of its coprocessor (80287 or 80387) by
sampling its ERROR# input some time after the falling edge of RESET and
before executing the first ESC instruction. The 80287 keeps its ERROR#
output in inactive state after hardware reset; the 80387 keeps its ERROR#
output in active state after hardware reset. The 80386 records this
difference in the ET bit of control register zero (CR0). The 80386
subsequently uses ET to control its interface with the coprocessor. If ET is
set, it employs the 32-bit protocol of the 80387; if ET is not set, it
employs the 16-bit protocol of the 80287.

Systems software can (if necessary) change the value of ET. There are three
reasons that ET may not be set:

  1.  An 80287 is actually present.

  2.  No coprocessor is present.

  3.  An 80387 is present but it is connected in a nonstandard manner that
      does not trigger the setting of ET.

An example of case three is the PC/AT-compatible design described in
Appendix F. In such cases, initialization software may need to change the
value of ET.


6.2.3  Software Recognition of the NPX

Figure 6-1 shows an example of a recognition routine that determines
whether an NPX is present, and distinguishes between the 80387 and the
8087/80287. This routine can be executed on any 80386, 80286, or 8086
hardware configuration that has an NPX socket.

The example guards against the possibility of accidentally reading an
expected value from a floating data bus when no NPX is present. Data read
from a floating bus is undefined. By expecting to read a specific bit
pattern from the NPX, the routine protects itself from the indeterminate
state of the bus. The example also avoids depending on any values in
reserved bits, thereby maintaining compatibility with future numerics
coprocessors.


Figure 6-1.  Software Routine to Recognize the 80287

8086/87/88/186 MACRO ASSEMBLER  Test for presence of a Numerics Chip, Revision 1.0


DOS 3.20 (033-N) 8086/87/88/186 MACRO ASSEMBLER V2.0 ASSEMBLY OF MODULE TEST_NPX
OBJECT MODULE PLACED IN FINDNPX.OBJ

LOC   OBJ         LINE    SOURCE

                     1 +1 $title('Test for presence of a Numerics Chip, Revision 1.0')
                     2
                     3            name    Test_NPX
                     4
----                 5    stack   segment stack 'stack'
0000  (100           6            dw      100 dup (?)
      ????
      )
00C8  ????           7    sst     dw              ?
----                 8    stack   ends
                     9
----                10    data    segment public 'data'
0000  0000          11    temp    dw      0h
----                12    data    ends
                    13
                    14    dgroup group    data, stack
                    15    cgroup group    code
                    16
----                17    code    segment public 'code'
                    18            assume cs:cgroup, ds:dgroup
                    19
0000                20    start:
                    21    ;
                    22    ;       Look for an 8087, 80287, or 80387 NPX.
                    23    ;       Note that we cannot execute WAIT on 8086/88 if no 8087 is present.
                    24    ;
0000                25    test npx:
0000  90DBE3        26            fninit                  ; Must use non-wait form
0003  BE0000     R  27            mov     [si],offset dgroup:temp
0006  C7045A5A      28            mov     word ptr [si],5A5AH ; Initialize temp to non-zero value
000A  90DD3C        29            fnstsw  [si]            ; Must use non-wait form of fstsw
                    30                                    ; It is not necessary to use a WAIT instruction
                    31                                    ;  after fnstsw or fnstcw.  Do not use one here.
000D  803C00        32            cmp     byte ptr [si],0 ; See if correct status with zeroes was read
0010  752A          33            jne     no_npx          ; Jump if not a valid status word, meaning no NPX
                    34    ;
                    35    ;       Now see if ones can be correctly written from the control word.
                    36    ;
0012  90D93C        37            fnstcw  [si]            ; Look at the control word; do not use WAIT form
                    38                                    ; Do not use a WAIT instruction here!
0015  8B04          39            mov     ax,[si]         ; See if ones can be written by NPX
0017  253F10        40            and     ax,103fh        ; See if selected parts of control word look OK
001A  3D3F00        41            cmp     ax,3fh          ; Check that ones and zeroes were correctly read
001D  7510          42            jne     no npx          ; Jump if no NPX is installed
                    43    ;
                    44    ;       Some numerics chip is installed.  NPX instructions and WAIT are now safe.
                    45    ;       See if the NPX is an 8087, 80287, or 80387.
                    46    ;       This code is necessary if a denormal exception handler is used or the
                    47    ;       new 80387 instructions will be used.
                    48    ;
001F 98D9E8         49            fld1                    ; Must use default control word from FNINIT
0022 9BD9EE         50            fldz                    ; Form infinity
0025 9BDEF9         51            fdiv                    ; 8087/287 says +inf = .inf
0028 9BD9C0         52            fld     st              ; Form negative infinity
002B 9BD9E0         53            fchs                    ; 80387 says +inf <> -inf
002E 9BDED9         54            fcompp                  ; See if they are the same and remove them
0031 9BDD3C         55            fstsw   [si]            ; Look at status from FCOMPP
0034 8B04           56            mov     ax,[si]
0036 9E             57            sahf                    ; See if the infinities matched
0037 7406           58            je      found_87_287    ; Jump if 8087/287 is present
                    59    ;
                    60    ;       An 80387 is present.  If denormal exceptions are used for an 8087/287,
                    61    ;       they must be masked.  The 80387 will automatically normalize denormal
                    62    ;       operands faster than an exception handler can.
                    63    ;
0039 EB0790         64            jmp     found_387
003C                65    no_npx:
                    66    ;       set up for no NPX
                    67    ;               ...
                    68    ;
003C EB0490         69            jmp exit
003F                70    found_87_287:
                    71    ;       set up for 87/287
                    72    ;               ...
                    73    ;
003F EB0190         74            jmp exit
0042                75    found_387:
                    76    ;       set up for 387
                    77    ;               ...
                    78    ;
0042                79    exit:
----                80    code    ends
                    81            end     start,ds:dgroup,ss:dgroup:sst

ASSEMBLY COMPLETE, NO ERRORS FOUND


6.2.4  Configuring the Numerics Environment

Once the 80386 CPU has determined the presence or absence of the 80387 or
80287 NPX, the 80386 must set either the MP or the EM bit in its own control
register zero (CR0) accordingly. The initialization routine can either

  Ž  Set the MP bit in CR0 to allow numeric instructions to be executed
     directly by the NPX.

  Ž  Set the EM bit in the CR0 to permit software emulation of the numeric
     instructions.

The MP (monitor coprocessor) flag of CR0 indicates to the 80386 whether an
NPX is physically available in the system. The MP flag controls the function
of the WAIT instruction. When executing a WAIT instruction, the 80386 tests
the task switched (TS) bit only if MP is set; if it finds TS set under these
conditions, the CPU traps to exception #7.

The Emulation Mode (EM) bit of CR0 indicates to the 80386 whether NPX
functions are to be emulated. If the CPU finds EM set when it executes an
ESC instruction, program control is automatically trapped to exception #7,
giving the exception handler the opportunity to emulate the functions of an
80387.

For correct 80386 operation, the EM bit must never be set concurrently with
MP. The EM and MP bits of the 80386 are described in more detail in the
80386 Programmer's Reference Manual. More information on software
emulation for the 80387 NPX is described in the "80387 Emulation" section
later in this chapter. In any case, if ESC instructions are to be executed,
either the MP or EM bit must be set, but not both.


6.2.5  Initializing the 80387

Initializing the 80387 NPX simply means placing the NPX in a known state
unaffected by any activity performed earlier. A single FNINIT instruction
performs this initialization. All the error masks are set, all registers are
tagged empty, TOP is set to zero, and default rounding and precision
controls are set. Table 6-1 shows the state of the 80387 NPX following
FINIT or FNINIT. This state is compatible with that of the 80287 after
FINIT or after hardware RESET.

The FNINIT instruction does not leave the 80387 in the same state as that
which results from the hardware RESET signal. Following a hardware RESET
signal, such as after initial power-up, the state of the 80387 differs in
the following respects:

  1.  The mask bit for the invalid-operation exception is reset.

  2.  The invalid-operation exception flag is set.

  3.  The exception-summary bit is set (along with its mirror image, the
      B-bit).

These settings cause assertion of the ERROR# signal as described
previously. The FNINIT instruction must be used to change the 80387 state to
one compatible with the 80287.


Table 6-1.  NPX Processor State Following Initialization

Field                   Value               Interpretation

Control Word
   (Infinity Control)
The 80387 does not have infinity control. This value is listed to emphasize
that programs written for the 80287 may not behave the same on the 80387 if
they depend on this bit.    0                 Affine
   Rounding Control       00                Round to nearest
   Precision Control      11                64 bits
   Exception Masks        111111            All exceptions masked
Status Word
   (Busy)                 0                 ‘‘
   Condition Code         0000              ‘‘
   Stack Top              000               Register 0 is stack top
   Exception Summary      0                 No exceptions
   Stack Flag             0                 ‘‘
   Exception Flags        000000            No exceptions
Tag Word
   Tags                   11                Empty
   Registers              N.C.              Not changed
Exception Pointers
   Instruction Code       N.C.              Not changed
   Instruction Address    N.C.              Not changed
   Operand Address        N.C.              Not changed


6.2.6  80387 Emulation

If it is determined that no 80387 NPX is available in the system, systems
software may decide to emulate ESC instructions in software. This emulation
is easily supported by the 80386 hardware, because the 80386 can be
configured to trap to a software emulation routine whenever it encounters an
ESC instruction in its instruction stream.

Whenever the 80386 CPU encounters an ESC instruction, and its MP and EM
status bits are set appropriately (MP=0, EM=1), the 80386 automatically
traps to interrupt #7, the "processor extension not available" exception.
The return link stored on the stack points to the first byte of the ESC
instruction, including the prefix byte(s), if any. The exception handler can
use this return link to examine the ESC instruction and proceed to emulate
the numeric instruction in software.

The emulator must step the return pointer so that, upon return from the
exception handler, execution can resume at the first instruction following
the ESC instruction.

To an application program, execution on an 80386 system with 80387
emulation is almost indistinguishable from execution on a system with an
80387, except for the difference in execution speeds.

There are several important considerations when using emulation on an 80386
system:

  Ž  When operating in protected mode, numeric applications using the
     emulator must be executed in execute-readable code segments. Numeric
     software cannot be emulated if it is executed in execute-only code
     segments. This is because the emulator must be able to examine the
     particular numeric instruction that caused the emulation trap.

  Ž  Only privileged tasks can place the 80386 in emulation mode. The
     instructions necessary to place the 80386 in emulation mode are
     privileged instructions, and are not typically accessible to an
     application.

An emulator package (EMUL387) that runs on 80386 systems is available from
Intel. This emulation package operates in both real and protected mode as
well as in virtual 8086 mode, providing a complete functional equivalent for
the 80387 emulated in software.

When using the EMUL387 emulator, writers of numeric exception handlers
should be aware of one slight difference between the emulated 80387 and the
80387 hardware:

  Ž  On the 80387 hardware, exception handlers are invoked by the 80386 at
     the first WAIT or ESC instruction following the instruction causing the
     exception. The return link, stored on the 80386 stack, points to this
     second WAIT or ESC instruction where execution will resume following a
     return from the exception handler.

  Ž  Using the EMUL387 emulator, numeric exception handlers are invoked
     from within the emulator itself. The return link stored on the stack
     when the exception handler is invoked will therefore point back to the
     EMUL387 emulator, rather than to the program code actually being
     executed (emulated). An IRET return from the exception handler returns
     to the emulator, which then returns immediately to the emulated
     program. This added layer of indirection should not cause confusion,
     however, because the instruction causing the exception can always be
     identified from the 80387's instruction and data pointers.


6.2.7  Handling Numerics Exceptions

Once the 80387 has been initialized and normal execution of applications
has been commenced, the 80387 NPX may occasionally require attention in
order to recover from numeric processing exceptions. This section provides
details for writing software exception handlers for numeric exceptions.
Numeric processing exceptions have already been introduced in Chapter 3.

The 80387 NPX can take one of two actions when it recognizes a numeric
exception:

  Ž  If the exception is masked, the NPX will automatically perform its own
     masked exception response, correcting the exception condition according
     to fixed rules, and then continuing with its instruction execution.

  Ž  If the exception is unmasked, the NPX signals the exception to the
     80386 CPU using the ERROR# status line between the two processors. Each
     time the 80386 encounters an ESC or WAIT instruction in its instruction
     stream, the CPU checks the condition of this ERROR# status line. If
     ERROR# is active, the CPU automatically traps to Interrupt vector #16,
     the Processor Extension Error trap.

Interrupt vector #16 typically points to a software exception handler,
which may or may not be a part of systems software. This exception handler
takes the form of an 80386 interrupt procedure.

When handling numeric errors, the CPU has two responsibilities:

  Ž  The CPU must not disturb the numeric context when an error is
     detected.

  Ž  The CPU must clear the error and attempt recovery from the error.

Although the manner in which programmers may treat these responsibilities
varies from one implementation to the next, most exception handlers will
include these basic steps:

  Ž  Store the NPX environment (control, status, and tag words, operand and
     instruction pointers) as it existed at the time of the exception.

  Ž  Clear the exception bits in the status word.

  Ž  Enable interrupts on the CPU.

  Ž  Identify the exception by examining the status and control words in
     the saved environment.

  Ž  Take some system-dependent action to rectify the exception.

  Ž  Return to the interrupted program and resume normal execution.


6.2.8  Simultaneous Exception Response

In cases where multiple exceptions arise simultaneously, the 80387 signals
one exception according to the precedence shown at the end of Chapter 3.
This means, for example, that an SNaN divided by zero results in an invalid
operation, not in a zero divide exception.


6.2.9  Exception Recovery Examples

Recovery routines for NPX exceptions can take a variety of forms. They can
change the arithmetic and programming rules of the NPX. These changes may
redefine the default fix-up for an error, change the appearance of the NPX
to the programmer, or change how arithmetic is defined on the NPX.

A change to an exception response might be to automatically normalize all
denormals loaded from memory. A change in appearance might be extending the
register stack into memory to provide an "infinite" number of numeric
registers. The arithmetic of the NPX can be changed to automatically extend
the precision and range of variables when exceeded. All these functions can
be implemented on the NPX via numeric exceptions and associated recovery
routines in a manner transparent to the application programmer.

Some other possible application-dependent actions might include:

  Ž  Incrementing an exception counter for later display or printing

  Ž  Printing or displaying diagnostic information (e.g., the 80387
     environment andregisters)

  Ž  Aborting further execution

  Ž  Storing a diagnostic value (a NaN) in the result and continuing with
     the computation

Notice that an exception may or may not constitute an error, depending on
the application. Once the exception handler corrects the condition causing
the exception, the floating-point instruction that caused the exception can
be restarted, if appropriate. This cannot be accomplished using the IRET
instruction, however, because the trap occurs at the ESC or WAIT instruction
following the offending ESC instruction. The exception handler must obtain
(using FSAVE or FSTENV) the address of the offending instruction in the task
that initiated it, make a copy of it, execute the copy in the context of the
offending task, and then return via IRET to the current CPU instruction
stream.

In order to correct the condition causing the numeric exception, exception
handlers must recognize the precise state of the NPX at the time the
exception handler was invoked, and be able to reconstruct the state of the
NPX when the exception initially occurred. To reconstruct the state of the
NPX, programmers must understand when, during the execution of an NPX
instruction, exceptions are actually recognized.

Invalid operation, zero divide, and denormalized exceptions are detected
before an operation begins, whereas overflow, underflow, and precision
exceptions are not raised until a true result has been computed. When a
before exception is detected, the NPX register stack and memory have
not yet been updated, and appear as if the offending instructions has not
been executed.

When an after exception is detected, the register stack and memory appear
as if the instruction has run to completion; i.e., they may be updated.
(However, in a store or store-and-pop operation, unmasked over/underflow is
handled like a before exception; memory is not updated and the stack is not
popped.) The programming examples contained in Chapter 7 include an outline
of several exception handlers to process numeric exceptions for the 80387.


Chapter 7  Numeric Programming Examples

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

The following sections contain examples of numeric programs for the 80387
NPX written in ASM386. These examples are intended to illustrate some of the
techniques for programming the 80386/80387 computing system for numeric
applications.


7.1  Conditional Branching Example

As discussed in Chapter 2, several numeric instructions post their results
to the condition code bits of the 80387 status word. Although there are many
ways to implement conditional branching following a comparison, the basic
approach is as follows:

  Ž  Execute the comparison.

  Ž  Store the status word. (80387 allows storing status directly into AX
     register.)

  Ž  Inspect the condition code bits.

  Ž  Jump on the result.

Figure 7-1 is a code fragment that illustrates how two memory-resident
double-format real numbers might be compared (similar code could be used
with the FTST instruction). The numbers are called A and B, and the
comparison is A to B.

The comparison itself requires loading A onto the top of the 80387 register
stack and then comparing it to B, while popping the stack with the same
instruction. The status word is then written into the 80386 AX register.

A and B have four possible orderings, and bits C3, C2, and C0 of the
condition code indicate which ordering holds. These bits are positioned in
the upper byte of the NPX status word so as to correspond to the CPU's zero,
parity, and carry flags (ZF, PF, and CF), when the byte is written into the
flags. The code fragment sets ZF, PF, and CF of the CPU status word to the
values of C3, C2, and C0 of the NPX status word, and then uses the CPU
conditional jump instructions to test the flags. The resulting code is
extremely compact, requiring only seven instructions.

The FXAM instruction updates all four condition code bits. Figure 7-2 shows
how a jump table can be used to determine the characteristics of the value
examined. The jump table (FXAM_TBL) is initialized to contain the 32-bit
displacement of 16 labels, one for each possible condition code setting.
Note that four of the table entries contain the same value, "EMPTY." The
first two condition code settings correspond to "EMPTY." The two other table
entries that contain "EMPTY" will never be used on the 80387, but may be
used if the code is executed with an 80287.

The program fragment performs the FXAM and stores the status word. It then
manipulates the condition code bits to finally produce a number in register
BX that equals the condition code times 2. This involves zeroing the unused
bits in the byte that contains the code, shifting C3 to the right so that it
is adjacent to C2, and then shifting the code to multiply it by 2. The
resulting value is used as an index that selects one of the displacements
from FXAM_TBL (the multiplication of the condition code is required because
of the 2-byte length of each value in FXAM_TBL). The unconditional JMP
instruction effectively vectors through the jump table to the labeled
routine that contains code (not shown in the example) to process each
possible result of the FXAM instruction.


Figure 7-1.  Conditional Branching for Compares

    .
    .
    .
A   DQ  ?
B   DQ  ?
    .
    .
    .
    FLD     A  ; LOAD A ONTO TOP OF 387 STACK
    FCOMP   B  ; COMPARE A:B, POP A
    FSTSW   AX ; STORE RESULT TO CPU AX REGISTER
;
; CPU AX REGISTER CONTAINS CONDITION CODES
;   (RESULTS OF COMPARE)
; LOAD CONDITION CODES INTO CPU FLAGS
;
    SAHF
;
; USE CONDITIONAL JUMPS TO DETERMINE ORDERING OF A TO B
;
    JP A_B_UNORDERED            ; TEST C2 (PF)
    JB A_LESS          ; TEST C0 (CF)
    JE A_EQUAL         ; TEST C3 (ZF)
A_GREATER:             ; C0 (CF) = 0, C3 (ZF) = 0
    .
    .
A_EQUAL:                   ; C0 (CF) = 0, C3 (ZF) = 1
    .
    .
A_LESS:                    ; C0 (CF) = 1, C3 (ZF) = 0
    .
    .
A_B_UNORDERED:             ; C2 (PF) = 1
    .
    .


Figure 7-2.  Conditional Branching for FXAM

; JUMP TABLE FOR EXAMINE ROUTINE
;
FXAM_TBL  DD POS_UNNORM, POS NAN, NEG_UNNORM, NEG_NAN,
&         POS_NORM, POS_INFINITY, NEG_NORM,
&         NEG_INFINITY, POS_ZERO, EMPTY, NEG_ZERO,
&         EMPTY, POS_DENORM, EMPTY, NEG_DENORM, EMPTY
    .
    .
; EXAMINE ST AND STORE RESULT (CONDITION CODES)

    FXAM
    XOR EAX,EAX ; CLEAR EAX
    FSTSW AX

; CALCULATE OFFSET INTO JUMP TABLE

    AND AX,0100011100000000B ; CLEAR ALL BITS EXCEPT C3, C2-C0
    SHR EAX,6    ;  SHIFT C2-C0 INTO PLACE    (0000XXX0)
    SAL AH,5     ;  POSITION C3               (000X0000)
    OR  AL,AH    ;  DROP C3 IN ADJACENT TO C2 (000XXXX0)
    XOR AH,AH    ;  CLEAR OUT THE OLD COPY OF C3

; JUMP TO THE ROUTINE `ADDRESSED' BY CONDITION CODE

    JMP FXAM_TBL[EAX]

; HERE ARE THE JUMP TARGETS, ONE TO HANDLE
;    EACH POSSIBLE RESULT OF FXAM

POS_UNNORM:
    .
POS_NAN:
    .
NEG_UNNORM:
    .
NEG_NAN:
    .
POS_NORM:
    .
POS_INFINITY:
    .
NEG_NORM:
    .
NEG_INFINITY:
    .
POS_ZERO:
    .
EMPTY:
    .
NEG_ZERO:
    .
POS_DENORM:
    .
NEG_DENORM:


7.2  Exception Handling Examples

There are many approaches to writing exception handlers. One useful
technique is to consider the exception handler procedure as consisting of
"prologue," "body," and "epilogue" sections of code. This procedure is
invoked via interrupt number 16.

At the beginning of the prologue, CPU interrupts have been disabled. The
prologue performs all functions that must be protected from possible
interruption by higher-priority sources. Typically, this involves saving CPU
registers and transferring diagnostic information from the 80387 to memory.
When the critical processing has been completed, the prologue may enable CPU
interrupts to allow higher-priority interrupt handlers to preempt the
exception handler.

The body of the exception handler examines the diagnostic information and
makes a response that is necessarily application-dependent. This response
may range from halting execution, to displaying a message, to attempting to
repair the problem and proceed with normal execution.

The epilogue essentially reverses the actions of the prologue, restoring
the CPU and the NPX so that normal execution can be resumed. The epilogue
must not load an unmasked exception flag into the 80387 or another exception
will be requested immediately.

Figures 7-3 through 7-5 show the ASM386 coding of three skeleton
exception handlers. They show how prologues and epilogues can be written for
various situations, but provide comments indicating only where the
application dependent exception handling body should be placed.

Figures 7-3 and 7-4 are very similar; their only substantial difference is
their choice of instructions to save and restore the 80387. The tradeoff
here is between the increased diagnostic information provided by FNSAVE and
the faster execution of FNSTENV. For applications that are sensitive to
interrupt latency or that do not need to examine register contents, FNSTENV
reduces the duration of the "critical region," during which the CPU does not
recognize another interrupt request.

After the exception handler body, the epilogues prepare the CPU and the NPX
to resume execution from the point of interruption (i.e., the instruction
following the one that generated the unmasked exception). Notice that the
exception flags in the memory image that is loaded into the 80387 are
cleared to zero prior to reloading (in fact, in these examples, the entire
status word image is cleared).

The examples in Figures 7-3 and 7-4 assume that the exception handler
itself will not cause an unmasked exception. Where this is a possibility,
the general approach shown in Figure 7-5 can be employed. The basic
technique is to save the full 80387 state and then to load a new control
word in the prologue. Note that considerable care should be taken when
designing an exception handler of this type to prevent the handler from
being reentered endlessly.


Figure 7-3.  Full-State Exception Handler

SAVE_ALL         PROC
;
; SAVE CPU REGISTERS, ALLOCATE STACK SPACE
; FOR 80387 STATE IMAGE
    PUSH EBP
    MOV  EBP,ESP
    SUB  ESP,108
; SAVE FULL 80387 STATE, ENABLE CPU INTERRUPTS
    FNSAVE  [EBP-108]
    STI
;
; APPLICATION-DEPENDENT EXCEPTION HANDLING
; CODE GOES HERE
;
; CLEAR EXCEPTION FLAGS IN STATUS WORD
;  (WHICH IS IN MEMORY)
; RESTORE MODIFIED STATE IMAGE
    MOV BYTE PTR [EBP-104], 0H
    FRSTOR  [EBP-108]
; DEALLOCATE STACK SPACE, RESTORE CPU REGISTERS
    MOVE ESP,EBP
      .
      .
    POP EBP
;
; RETURN TO INTERRUPTED CALCULATION
    IRET
SAVE_ALL         ENDP


Figure 7-4.  Reduced-Latency Exception Handler

SAVE_ENVIRONMENT PROC
;
; SAVE CPU REGISTERS, ALLOCATE STACK SPACE
; FOR 80387 ENVIRONMENT
    PUSH    EBP
       .
    MOV     EBP,ESP
    SUB     ESP,28
; SAVE ENVIRONMENT, ENABLE CPU INTERRUPTS
    FNSTENV [EBP-28]
    STI
;
; APPLICATION EXCEPTION-HANDLING CODE GOES HERE
;
; CLEAR EXCEPTION FLAGS IN STATUS WORD
;  (WHICH IS IN MEMORY)
; RESTORE MODIFIED ENVIRONMENT IMAGE
    MOV     BYTE PTR [EBP-24], 0H
    FLDENV  [EBP-28]
; DE-ALLOCATE STACK SPACE, RESTORE CPU REGISTERS
    MOV     ESP,EBP
    POP     EBP
;
; RETURN TO INTERRUPTED CALCULATION
    IRET
SAVE_ENVIRONMENT ENDP


Figure 7-5.  Reentrant Exception Handler

        .
        .
        .
    LOCAL CONTROL  DW  ?  ; ASSUME INITIALIZED
        .
        .
        .
REENTRANT             PROC
;
; SAVE CPU REGISTERS, ALLOCATE STACK SPACE FOR
; 80387 STATE IMAGE
    PUSH    EBP
       .
       .
       .
    MOV     EBP,ESP
    SUB     ESP,108
; SAVE STATE, LOAD NEW CONTROL WORD,
; ENABLE CPU INTERRUPTS
    FNSAVE  [EBP-108]
    FLDCW   LOCAL_CONTROL
    STI
       .
       .
       .
; APPLICATION EXCEPTION HANDLING CODE GOES HERE.
; AN UNMASKED EXCEPTION GENERATED HERE WILL
; CAUSE THE EXCEPTION HANDLER TO BE REENTERED.
; IF LOCAL STORAGE IS NEEDED, IT MUST BE
; ALLOCATED ON THE CPU STACK.
       .
       .
       .
; CLEAR EXCEPTION FLAGS IN STATUS WORD
;  (WHICH IS IN MEMORY)
; RESTORE MODIFIED STATE IMAGE
    MOV     BYTE PTR [EBP-104], 0H
    FRSTOR  [EBP-108]
; DE-ALLOCATE STACK SPACE, RESTORE CPU REGISTERS
    MOV     ESP,EBP
       .
       .
       .
    POP     EBP
; RETURN TO POINT OF INTERRUPTION
    IRET
REENTRANT             ENDP


7.3  Flaoting-Point to ASCII Conversion Examples

Numeric programs must typically format their results at some point for
presentation and inspection by the program user. In many cases, numeric
results are formatted as ASCII strings for printing or display. This example
shows how floating-point values can be converted to decimal ASCII character
strings. The function shown in Figure 7-6 can be invoked from PL/M-386,
Pascal-386, FORTRAN-386, or ASM386 routines.

Shortness, speed, and accuracy were chosen rather than providing the
maximum number of significant digits possible. An attempt is made to keep
integers in their own domain to avoid unnecessary conversion errors.

Using the extended precision real number format, this routine achieves a
worst case accuracy of three units in the 16th decimal position for a
noninteger value or integers greater than 10^(18). This is double precision
accuracy. With values having decimal exponents less than 100 in magnitude,
the accuracy is one unit in the 17th decimal position.

Higher precision can be achieved with greater care in programming, larger
program size, and lower performance.


Figure 7-6.  Floating-Point to ASCII Conversion Routine

XENIX286 80380 MACRO ASSEMBLER V1.0, ASSEMBLY OF MODULE FLOATING_TO_ASCII
OBJECT MODULE PLACED IN fpasc.obj
ASSEMBLER INVOKED BY: asm386 fpasc.asm

LOC      OBJ                          LINE    SOURCE

                                1 +1  $title(`Convert a floating point number to ASCII')
                                2
                                3                    name    floating_to_ascii
                                4
00000000                        5                    public  floating_to_ascii
                                6                    extrn   get_power_10:near,tos_status:near
                                7    ;
                                8    ; This subroutine will convert the floating point
                                9    ; number in the top of the NPX stack to an ASCII
                               10    ; string and separate power of 10 scaling value
                               11    ; (in binary).  The maximum width of the ASCII string
                               12    ; formed is controlled by a parameter which must be
                               13    ; > 1.  Unnormal values, denormal values, and psuedo
                               14    ; zeroes will be correctly converted. However, unnormals
                               15    ; and pseudo zeros are no longer supported formats on the
                               16    ; 80387( in conformance with the IEEE floating point
                               17    ; standard) and hence not generated internally. A
                               18    ; returned value will indicate how many binary bits
                               19    ; of precision were lost in an unnormal or denormal
                               20    ; value.  The magnitude (in terms of binary power)
                               21    ; of a pseudo zero will also be indicated. Integers
                               22    ; less than 10**18 in magnitude are accurately converted
                               23    ; if the destination ASCII string field is wide enough
                               24    ; to hold all the digits. Otherwise the value is converted
                               25    ; to scientific notation.
                               26    ;
                               27    ; The status of the conversion is identified by the
                               28    ; return value, it can be:
                               29    ;
                               30    ;       0       conversion complete, string_size is defined
                               31    ;       1       invalid arguments
                               32    ;       2       exact integer conversion, string_size is defined
                               33    ;       3       indefinite
                               34    ;       4       + NAN (Not A Number)
                               35    ;       5       - NAN
                               36    ;       6       + Infinity
                               37    ;       7       - Infinity
                               38    ;       8       pseudo zero found, string_size is defined
                               39    ;
                               40    ;         The PLM/386 calling convention is:
                               41    ;
                               42    ; floating_to_ascii:
                               43    ;       procedure (number,denormal_ptr,string_ptr,size_ptr,
                               44    ;       field_size, power_ptr) word external;
                               45    ;       declare (denormal_ptr,string_ptr,power_ptr,size_ptr)
                               46    ;       pointer;
                               47    ;       declare field_size word,
                               48    ;       string_size based size ptr word;
                               49    ;       declare number real;
                               50    ;       declare denormal integer based denormal ptr;
                               51    ;       declare power integer based power_ptr;
                               52    ;       end floating_to_ascii:
                               53    ;
                               54    ;         The floating point value is expected to be
                               55    ;   on the top of the  NPX stack.  This subroutine
                               56    ;   expects 3 free entries on the NPX stack and
                               57    ;   will pop the passed value off when done.  The
                               58    ;   generated ASCII string will have a leading
                               59    ;   character either `-' or `+' indicating the sign
                               60    ;   of the value.  The ASCII decimal digits will
                               61    ;   immediately follow. The numeric value of the
                               62    ;   ASCII string is (ASCII STRING.)*10**POWER. If
                               63    ;   the given number was zero, the ASCII string will
                               64    ;   contain a sign and a single zero chacter.  The
                               65    ;   value string_size indicates the total length of
                               66    ;   the ASCII string including the sign character.
                               67    ;   String(0) will always hold the sign.  It is
                               68    ;   possible for string size to be less than
                               69    ;   field_size. This occurs for zeroes or integer
                               70    ;   values.  A pseudo zero will return a special
                               71    ;   return code.  The denormal count will indicate
                               72    ;      the power of two originally associated with the
                               73    ;   value.  The power of ten and ASCII string will
                               74    ;   be as if the value was an ordinary zero.
                               75    ;
                               76    ;   This subroutine is accurate up to a maximum of
                               77    ;   18 decimal digits for integers.  Integer values
                               78    ;   will have a decimal power of zero associated
                               79    ;   with them. For non integers, the result will be
                               80    ;   accurate to within 2 decimal digits of the 16th
                               81    ;   decimal place(double precision).  The exponentiate
                               82    ;   instruction is also used for scaling the value into
                               83    ;   the range acceptable for the BCD data type.  The
                               84    ;   roundirg mode in effect on entry to the
                               85    ;   subroutine is used for the conversion.
                               86    ;
                               87    ;         The following registers are not transparent:
                               88    ;
                               89    ;               eax ebx ecx edx esi edi eflags
                               90    ;
                               91    ;
                               92    ;         Define the stack layout.
                               93    ;
00000000[]                     94    ebp_save       equ     dword ptr [ebp]
00000004[]                     95    es_save        equ     ebp_save + size ebp_save
00000008[]                     96    return_ptr     equ     es_save + size es_save
0000000C[]                     97    power_ptr      equ     return_ptr + size return_ptr
00000010[]                     98    field_size     equ     power_ptr + size power_ptr
00000014[]                     99    size_ptr       equ     field_size + size field_size
00000018[]                    100    string_ptr     equ     size_ptr + size size_ptr
0000001C[]                    101    denormal_ptr   equ     string_ptr + size string_ptr
                              102
0014                          103    parms_size     equ     size power_ptr + size field_size +
                              104    &              size size_ptr + size string_ptr +
                              105    &              size denormal_ptr
                              106    ;
                              107    ;        Define constants used
                              108    ;
                              109    BCD_DIGITS     equ     18       ; Number of digits in bcd_value
                              110    WORD_SIZE      equ     4
                              111    BCD_SIZE       equ     10
                              112    MINUS          equ     1        ; Define return values
                              113    NAN            equ     4        ; The exact values chosen
                              114    INFINITY       equ     6        ; here are important.  They must
                              115    INDEFINITE     equ     3        ; correspond to the possible return
                              116    PSEUDO_ZERO    equ     8        ; values and be in the same numeric
                              117    INVALID        equ     -2       ; order as tested by the program.
                              118    ZERO           equ     -4
                              119    DENORMAL       equ     -6
                              120    UNNORMAL       equ     -8
                              121    NORMAL         equ     0
                              122    EXACT          equ     2
                              123    ;
                              124    ;        Define layout of temporary storage area.
                              125    ;
                              126    power_two      equ     word ptr [ebp - WORD_SIZE]
                              127    bcd_value      equ     tbyte ptr power two - BCD_SIZE
                              128    bcd_byte       equ     byte ptr bcd_value
                              129    fraction       equ     bcd_value
                              130
                              131    local_size     equ     size power_two + size bcd_value
                              132    ;
                              133    ;        Allocate stack space for the temporaries so
                              134    ;    the stack will be big enough
                              135    ;
                              136    stack  stackseg (local_size+6) ; Allocate stack
                              137                                   ; space for locals
                              138 +1 $eject
                              139    code           segment public er
                              140                   extrn   power_table:qword
                              141    ;
                              142    ;       Constants used by this function.
                              143    ;
                              144                   even                    ; Optimize for 16 bits
00000000 0A00                 145    const10        dw      10              ; Adjustment value for
                              140    ;                      ; too big BCD
                              147    ;
                              148    ; Convert the C3,C2,C1,C0 encoding from tos_status
                              149    ; into meaningful bit flags and values.
                              150    ;
00000002 F8                   151    status_table   db      UNNORMAL, NAN, UNNORMAL + MINUS,
00000003 04                   152    &       NAN + MINUS, NORMAL, INFINITY,
00000004 F9                   153    &       NORMAL + MINUS,  INFINITY + MINUS,
00000005 05                   154    &              ZERO, INVALID, ZERO + MINUS, INVALID,
00000006 00                   155    &              DENORMAL, INVALID, DENORMAL + MINUS, INVALID
00000007 06
00000008 01
00000009 07
0000000A FC
0000000B FE
0000000C FD
0000000D FE
0000000E FA
0000000F FE
00000010 FB
00000011 FE
                              156
00000012                      157    floating_to_ascii proc
                              158
00000012 E800000000        E  159            call   tos_status       ; Look at status of ST(0)
                              160
                              161    ; Get descriptor from table
00000017 2E0FB68002000000  R  162            movzx  eax, status_table[eax]
0000001F 3CFE                 163            cmp    al,INVALID              ; Look for empty ST(0)
00000021 7527                 164            jne    not_empty
                              165    ;
                              166    ;         ST(0) is empty!  Return the status value.
                              167    ;
00000023 C21400               168            ret    parms_size
                              169    ;
                              170    ;         Remove infinity from stack and exit.
                              171    ;
00000026                      172    found_infinity:
00000026 DDD8                 173            fstp   st(0)            ; OK to leave fstp running
00000028 EB02                 174            jmp    short exit_proc
                              175    ;
                              176    ;         String space is too small!
                              177    ;      Return invalid code.
                              178    ;
0000002A                      179    small_string:
0000002A B0FE                 180            mov    al,INVALID
0000002C                      181    exit_proc:
0000002C C9                   182            leave          ; Restore stack setup
0000002D 07                   183            pop     es
0000002E C21400               184            ret     parms_size
                              185    ;
                              186    ; ST(0) is NAN or indefinite.  Store the
                              187    ; value in memory and look at the fraction
                              188    ; field to separate indefinite from an ordinary NAN.
                              189    ;
00000031                      190    NAN_or_indefinite:
00000031 DB7DF2               191            fstp    fraction        ; Remove value from stack
                              192                                ; for examination
00000034 A801                 193            test    al,MINUS        ; Look at sign bit
00000036 9B                   194            fwait                           ; Insure store is done
00000037 74F3                 195            jz      exit_proc               ; Can't be indefinite if
                              196                                ; positive
                              197
00000039 BB000000C0           198            mov     ebx,0C0000000H  ; Match against upper 32
                              199                                ;bits of fraction
                              200
                              201    ; Compare bits 63-32
0000003E 2B5DF6               202            sub     ebx, dword ptr fraction + 4
                              203
                              204    ; Bits 31-0 must be zero
00000041 0B5DF2               205            or      ebx, dword ptr fraction
00000044 75E6                 206            jnz     exit_proc
                              207
                              208    ; Set return value for indefinite value
00000046 B003                 209        mov al,INDEFINITE
00000048 EBE2                 210            jmp     exit_proc
                              211    ;
                              212    ;         Allocate stack space for local variables
                              213    ;     and establish parameter addressibility.
                              214    ;
0000004A                      215    not_empty:
0000004A 06                   216            push    es              ; Save working register
0000004B C80C0000             217            enter local_size, 0     ; Setup stack addressing
                              218
                              219
                              220    ; Check for enough string space
0000004F 8B4D10               221            mov     ecx,field size
00000052 83F902               222            cmp     ecx,2
00000055 7CD3                 223            jl      small_string
                              224
00000057 49                   225            dec     ecx             ; Adjust for sign character
                              226
                              227    ; See if string is too large for BCD
00000058 83F912               228            cmp     ecx,BCD_DIGITS
0000005B 7605                 229            jbe     size_ok
                              230
                              231    ; Else set maximum string size
0000005D B912000000           232            mov     ecx,BCD_DIGITS
00000002                      233    size_ok:
00000062 3C06                 234            cmp     al,INFINITY     ; Look for infinity
                              235
                              236    ; Return status value for + or - inf
00000064 7DC0                 237            jge     found_infinity
                              238
00000066 3C04                 239             cmp     al,NAN          ; Look for NAN or INDEFINITE
00000068 7DC7                 240             jge     NAN_or_indefinite
                              241    ;
                              242    ;  Set default return values and check that
                              243    ;  the number is normalized.
                              244    ;
0000006A D9E1                 245            fabs    ; Use positive value only
                              246                            ; sign bit in al has true sign of value
0000006C 31D2                 247            xor     edx,edx                 ; Form 0 constant
0000006E 8B7D1C               248            mov     edi,denormal_ptr; Zero denormal count
00000071 668917               249            mov     [edi], dx
00000074 8B5D0C               250            mov     ebx,power_ptr   ; Zero power of ten value
00000077 668913               251            mov     [ebx], dx
0000007A 88C2                 252            mov dl, al
0000007C 80E201               253            and dl, 1
0000007F 80C202               254        add dl, EXACT
00000082 3CFC                 255            cmp     al,ZERO                 ; Test for zero
00000084 0F83BC000000         256            jae     convert_integer ; Ship power code if value
                              257                                                    ; is zero
0000008A DB7DF2               258        fstp   fraction
00000080 9B                   259        fwait
0000008E 8A45F9               260        mov    al, bcd_byte + 7
00000091 804DF980             261        or     byte ptr bcd_byte +  7, 80h
00000095 DB6DF2               262        fld    fraction
00000098 D9F4                 263        fxtract
0000009A A880                 264        test   al, 80h
0000009C 7524                 265        jnz    normal_value
                              266
0000009E D9E8                 267        fld1
000000A0 DEE9                 268        fsub
000000A2 D9E4                 269        ftat
000000A4 9BDFE0               270        fatsw  ax
000000A7 9E                   271        sahf
000000A8 7510                 272        jnz    set_unnormal_count
                              273    ;
                              274    ;   Found a pseudo zero
                              275    ;
000000AA D9EC                 276        fldlg2              ; Develop power of ten estimate
000000AC 80C206               277        add    dl, PSEUDO ZERO - EXACT
000000AF DECA                 278        fmulp  st(2), st
000000B1 D9C9                 279        fxch                  ; Get power of ten
000000B3 DF1B                 280        fistp  word ptr [ebx] ; Set power of ten
000000B5 E98C000000           281        jmp    convert_integer
                              282
000000BA                      283    set_unnonmal_count:
000000BA D9F4                 284        fxtract               ; Get original fraction,
                              285                              ; now normalized
000000BC D9C9                 286        fxch                  ; Get unnormal count
000000BE D9E0                 287        fchs
000000C0 DF1F                 288        fistp  word ptr [edi] ; Set unnormal count
                              289
                              290
                              291    ;  Calculate the decimal magnitude associated
                              292    ;  with this number to within one order.  This
                              293    ;  error will always be inevitable due to
                              294    ;  rounding and lost precision. As a result,
                              295    ;  we will deliberately fail to consider the
                              296    ;  LOG10 of the fraction value in calculating
                              297    ;  the order. Since the fraction will always
                              298    ;  be 1 <= F < 2, its  LOG10 will not change
                              299    ;  the basic accuracy of the function. To
                              300    ;  get the decimal order of magnitude, simply
                              301    ;  multiply the power of two by LOG10(2) and
                              302    ;  truncate the result to an integer.
                              303    ;
                              304    normal_value:
                              305            fstp   fraction         ; Save the fraction field
                              306                               ; for later use
                              307            fist   power_two        ; Save power of two
                              308            fldlg2                          ; Get LOG10(2)
                              309                                            ; Power_two is now safe to use
                              310            fmul            ; Form LOG10(of exponent of number)
                              311            fistp  word ptr [ebx]   ; Any rounding mode
                              312                                                            ; will work here
                              313    ;
                              314    ;         Check if the magnitude of the number rules
                              315    ;      out treating it as an integer.
                              316    ;
                              317    ;       CX has the maximum number of decimal digits
                              318    ;   allowed.
                              319    ;
                              320            fwait           ; Wait for power_ten to be valid
                              321
                              322    ; Get power of ten of value
                              323            movsx si, word ptr [ebx]
                              324            sub    esi,ecx                  ; Form scaling factor
                              325                                ; necessary in ax
                              326            ja     adjust result    ; Jump if number will not fit
                              327    ;
                              328    ;         The number is between 1 and 10**(field size).
                              329    ;       Test if it is an integer.
                              330    ;
                              331            fild   power_two        ; Restore original number
                              332            sub    dl,NORMAL-EXACT  ; Convert to exact return
                              333                                ; value
                              334            fld    fraction
                              335            fscale                          ; Form full value,  this
                              336                                ; is safe here
                              337            fst    st(1)                    ; Copy value for compare
                              338            frndint                         ; Test if its an integer
                              339            fcomp                           ; Compare values
                              340            fstsw  ax                       ; Save status
                              341            sahf                            ; C3=1 implies it was
                              342                                ; an integer
                              343            jnz    convert_integer
                              344
                              345            fstp   st(0)            ; Remove non integer value
                              346            add    dl,NORMAL-EXACT  ; Restore original return value
                              347    ;
                              348    ;      Scale the number to within the range allowed
                              349    ;  by the BCD format.The scaling operation should
                              350    ;  produce a number within one decimal order of
                              351    ;  magnitude of the largest decimal number
                              352    ;  representable within the given string width.
                              353    ;
                              354    ;        The scaling power of ten value is in si.
                              355    ;
000000F2                      356    adjust_result:
000000F2 8BC6                 357           mov     eax,esi                 ; Setup for pow10
000000F4 668903               358           mov     word ptr [ebx],ax       ; Set initial power
                              359                                    ; of ten return value
000000F7 F7D8                 360           neg     eax              ; Subtract one for each order of
                              361                                    ; magnitude the value is scaled by
000000F9 E800000000        E  362           call    get_power_10     ; Scaling factor is
                              363                                    ; returned as
                              364                                    ; exponent and fraction
000000FE DB6DF2               365           fld     fraction                         ; Get fraction
00000101 DEC9                 366           fmul                                     ; Combine fractions
00000103 8BF1                 367           mov     esi,ecx                 ; Form power of ten of
                              368                                    ; the maximum
00000105 C1E603               369           shl     esi,3                            ; BCD value to fit in
                              370                                    ; the strinq
00000108 DF45FC               371           fild    power_two               ; Combine powers of two
0000010B DEC2                 372           faddp   st(2),st
0000010D D9FD                 373           fscale                                   ; Form full value,
                              374                                    ; exponent was safe
0000010F DDD9                 375           fstp    st(1)                   ; Remove exponent
                              376    ;
                              377    ;        Test the adjusted value against a table
                              378    ;    of exact powers of ten. The combined errors
                              379    ;    of the magnitude estimate and power function
                              380    ;    can result in a value one order of magnitude
                              381    ;    too small or too large to fit correctly in
                              382    ;    the BCD field. To handle this problem, pretest
                              383    ;    the adjusted value, if it is too small or
                              384    ;    large, then adjust it by ten and adjust the
                              385    ;    power of ten value.
                              386    ;
00000111                      387    test_power:
                              388
                              389    ; Compare against exact power entry. Use the next
                              390    ; entry since cx has been decremented by one
00000111 2EDC9608000000    E  391            fcom    power_table[esi]+type power_table
00000118 9BDFE0               392            fstsw ax                        ; No wait is necessary
0000011B 9E                   393            sahf                ; If C3 = C0 = 0 then
0000011C 720F                 394            jb      test_for_small  ; too big
                              395
0000011E 2EDE3500000000    R  396            fidiv   const10          ; Else adjust value
00000125 80E2FD               397            and     dl,not EXACT     ; Remove exact flag
00000128 66FF03               398            inc     word ptr [ebx]   ; Adjust power of ten value
0000012B EB17                 399            jmp     short in range   ; Convert the value to a BCD
                              400                                ;  integer
0000012D                      401    test for small:
0000012D 2EDC9600000000    E  402            fcom    power table[esi]        ; Test relative size
0000134 9BDFE0                403            fstsw   ax                                      ; No wait is necessary
0000137 9E                    404            sahf                                            ; If CO = 0 then
                              405                                            ; st(O) >= lower bound
10000138 720A                 406            jc      in_range                                ; Convert the value to a
                              407                                            ; BCD integer
                              408
000013A 2EDE0D00000000     R  409            fimul   const10         ; Adjust value into range
0000141 66FF0B                410            dec     word ptr [ebx]  ; Adjust power of ten value
0000144                       411    in_range:
0000144 D9FC                  412            frndint                         ; Form integer value
                              413    ;
                              414    ;       Assert: 0 <= TOS <= 999,999,999,999,999,999
                              415    ;       The TOS number will be exactly representable
                              416    ;    in 18 digit BCD format.
                              417    ;
00000146                      418    convert_integer:
00000146 DF75F2               419            fbstp   bcd_value       ; Store as BCD format number
                              420    ;
                              421    ;         while the store BCD runs, setup registers
                              422    ;      for the conversion to ASCII.
                              423    ;
00000149 BE08000000           424            mov     esi,BCD_SIZE.2  ; Initial BCD index value
0000014E 66B9040F             425            mov     cx,0f04h                ; Set shift count and mask
00000152 BB01000000           426            mov     ebx,1                   ; Set initial size of ASCII
                              427                                ; field for sign
00000157 8B7D18               428            mov     edi,string_ptr  ; Get address of start of
                              429                                ; ASCII string
0000015A 8CD8                 430            mov     ax,ds                   ; Copy ds to es
0000015C 8EC0                 431            mov     es,ax
0000015E FC                   432            cld                                     ; Set autoincrement mode
0000015F B02B                 433            mov     al,'+'                  ; Clear sign field
00000161 F6C201               434            test    dl,MINUS        ; Look for negative value
00000164 7402                 435            jz      positive_result
                              436
00000166 B02D                 437            mov     al,`.'
00000168                      438    positive_result:
00000168 AA                   439            stosb                           ; Bump string pointer
                              440                                ; past sign
00000169 80E2FE               441            and     dl,not MINUS    ; Turn off sign bit
0000016C 9B                   442            fwait                           ; Hait for fbstp to finish
                              443    ;
                              444    ;         Register usage:
                              445    ;                               ah:     BCD byte value in use
                              446    ;                               al:     ASCII character value
                              447    ;                               dx:     Return value
                              448    ;                               ch:     BCD mask = 0fh
                              449    ;                               cl:     BCD shift count = 4
                              450    ;                               bx:     ASCII string field width
                              451    ;                               esi:    BCD field index
                              452    ;                               di:     ASCII string field pointer
                              453    ;                               ds,es:  ASCII string segment base
                              454    ;
                              455    ;         Remove leading zeroes from the number.
                              456    ;
0000016D                      457    skip_leading_zeroes:
0000016D 8A6435F2             458           mov     ah,bcd_byte[esi]                ; Get BCD byte
00000171 88E0                 459           mov     al,ah                   ; Copy value
00000173 D2E8                 460           shr     al,cl                   ; Get high order digit
00000175 240F                 461           and     al,0fh                  ; Set zero flag
00000177 7517                 462           jnz     enter_odd               ; Exit loop if leading
                              463                               ; non zero found
                              464
00000179 88E0                 465           mov     al,ah                   ; Get BCD byte again
0000017B 240F                 466           and     al,0fh                  ; Get low order digit
0000017D 7519                 467           jnz     enter_even              ; Exit loop if non zero
                              468                               ; digit found
                              469
0000017F 4E                   470           dec     esi                             ; Decrement BCD index
00000180 79EB                 471           jns     ship_leading_zeroes
                              472    ;
                              473    ;        The significand was all zeroes.
                              474    ;
00000182 B030                 475           mov     al,`O'                  ; Set initial zero
00000184 AA                   476           stosb
00000185 43                   477           inc     ebx                             ; Bump string length
00000186 EB17                 478           jmo     short exit_with_value
                              479    ;
                              480    ;        Now expand the BCD string into digit
                              481    ;     per byte values 0-9.
                              482    ;
00000188                      483    digit_loop:
00000188 8A6435F2             484           mov     ah,bcd_byte[esi]        ; Get BCD byte
0000018C 88E0                 485           mov     al,ah
0000018E D2E8                 486           shr     al,cl                   ; Get high order digit
00000190                      487    enter_odd:
00000190 0430                 488           add     al,`O'                  ; Convert to ASCII
00000192 AA                   489           stosb                           ; Put digit into ASCII
                              490                               ; string area
00000193 88E0                 491           mov     al,ah                   ; Get low order digit
00000195 240F                 492           and     al,0fh
00000197 43                   493           inc     ebx                     ; Bump field size counter
00000198                      494    enter_even:
00000198 0430                 495           add     al,`0'           ; Convert to ASCII
0000019A AA                   496           stosb                    ; Put digit into ASCII area
0000019B 43                   497           inc     ebx                     ; Bump field size counter
0000019C 4E                   498           dec     esi                     ; Go to next BCD byte
0000019D 79E9                 499           jns     digit_loop
                              500    ;
                              501    ;        Conversion complete.  Set the string
                              502    ;     size and remainder.
                              503    ;
0000019F                      504    exit_with_value:
0000019F 8B7D14               505           mov     edi,size_ptr
000001A2 66891F               506           mov     word ptr [edi],bx
000001A5 8BC2                 507           mov     eax,edx                 ; Set return value
000001A7 E980FEFFFF           508           jmp     exit_proc
                              509
000001AC                      510    floating_to_ascii      endp
                              511
--------                      512    code                   ends
                              513                           end

ASSEMBLY COMPLETE,   NO WARNINGS,   NO ERRORS.


XENIX286 80386 MACRO ASSEMBLER V1.0, ASSEMBLY OF MODULE_GET_POWER 10
OBJECT MODULE PLACED IN power10.obj
ASSEMBLER INVOKED BY: asm386 power10.asm

LOC      OBJ                         LINE     SOURCE

                             1 +1 $title(Calculate the value of 10**ax)
                             2    ;
                             3    ;       This subroutine will calculate the
                             4    ;    value of 10**eax.  For values of
                             5    ;    0 <= eax < 19, the result will exact.
                             6    ;    All 80386 registers are transparent
                             7    ;    and the value is returned on the TOS
                             8    ;    as two numbers, exponent in ST(1) and
                             9    ;    fraction in ST(0). The exponent value
                            10    ;    can be larger than the largest
                            11    ;    exponent of an extended real format
                            12    ;    number.  Three stack entries are used.
                            13    ;
                            14                    name    get_power_10
00000000                    15                    public  get_power_10,power_table
                            16
--------                    17    stack           stackseg    8
                            18
--------                    19    code            segment public er
                            20    ;
                            21    ;         Use exact values from 1.0 to 1e18.
                            22    ;
                            23                    even            ; Optimize 16 bit access
00000000 000000000000F03F   24    power_table     dq      1.0,1e1,1e2,1e3
00000008 00000000000D2440
00000010 0000000000005940
00000018 0000000000408F40
00000020 000000000088C340   25                    dq      1e4,1e5,1e6,1e7
00000028 00000000006AF840
00000030 0000000080842E41
00000038 00000000D0126341
00000040 0000000084D79741   26                    dq      1e8,1e9,1e10,1e11
00000048 0000000065CDCD41
00000050 000000205FA00242
00000058 000000E876483742
00000060 000000A2941A6D42   27                    dq      1e12,1e13,1e14,1e15
00000068 000040E59C30A242
00000070 0000901EC4BCD642
00000078 00003420F56B0C43
00000080 0080E03779C34143   28                    dq      1e16,1e17,1e18
00000088 00A0D88557347643
00000090 00C84E676DC1ABC3
                            29
00000098                    30    get_power_10    proc
                            31
00000098 3D12000000         32            cmp     eax,18          ; Test for 0 <= ax < 19
0000009D 770B               33            ja      out_of_range
                            34
0000009F 2EDD04C500000000 R 35            fld     power_table[eax*8]; Get exact value
000000A7 D9F4               36            fxtract                         ; Separate power


7.3.1  Function Partitioning

Three separate modules implement the conversion. Most of the work of the
conversion is done in the module FLOATING_TO_ASCII. The other modules are
provided separately, because they have a more general use. One of them,
GET_POWER_10, is also used by the ASCII to floating-point conversion
routine. The other small module, TOS_STATUS, identifies what, if anything,
is in the top of the numeric register stack.


7.3.2  Exception Considerations

Care is taken inside the function to avoid generating exceptions. Any
possible numeric value is accepted. The only possible exception is
insufficient space on the numeric register stack.

The value passed in the numeric stack is checked for existence, type (NaN
or infinity), and status (denormal, zero, sign). The string size is tested
for a minimum and maximum value. If the top of the register stack is empty,
or the string size is too small, the function returns with an error code.

Overflow and underflow is avoided inside the function for very large or
very small numbers.


7.3.3  Special Instructions

The functions demonstrate the operation of several numeric instructions,
different data types, and precision control. Shown are instructions for
automatic conversion to BCD, calculating the value of 10 raised to an
integer value, establishing and maintaining concurrency, data
synchronization, and use of directed rounding on the NPX.

Without the extended precision data type and built-in exponential function,
the double precision accuracy of this function could not be attained with
the size and speed of the shown example.

The function relies on the numeric BCD data type for conversion from binary
floating-point to decimal. It is not difficult to unpack the BCD digits into
separate ASCII decimal digits. The major work involves scaling the
floating-point value to the comparatively limited range of BCD values. To
print a 9-digit result requires accurately scaling the given value to an
integer between 10^(8) and 10^(9). For example, the number +0.123456789
requires a scaling factor of 10^(9) to produce the value +123456789.0, which
can be stored in 9 BCD digits. The scale factor must be an exact power of
10 to avoid changing any of the printed digit values.

These routines should exactly convert all values exactly representable in
decimal in the field size given. Integer values that fit in the given string
size are not be scaled, but directly stored into the BCD form. Noninteger
values exactly representable in decimal within the string size limits are
also exactly converted. For example, 0.125 is exactly representable in
binary or decimal. To convert this floating-point value to decimal, the
scaling factor is 1000, resulting in 125. When scaling a value, the function
must keep track of where the decimal point lies in the final decimal value.


7.3.4  Description of Operation

Converting a floating-point number to decimal ASCII takes three major
steps: identifying the magnitude of the number, scaling it for the BCD data
type, and converting the BCD data type to a decimal ASCII string.

Identifying the magnitude of the result requires finding the value X such
that the number is represented by I * 10^(X), where 1.0 ¾ I < 10.0. Scaling
the number requires multiplying it by a scaling factor 10^(S), so that the
result is an integer requiring no more decimal digits than provided for in
the ASCII string.

Once scaled, the numeric rounding modes and BCD conversion put the number
in a form easy to convert to decimal ASCII by host software.

Implementing each of these three steps requires attention to detail. To
begin with, not all floating-point values have a numeric meaning. Values
such as infinity, indefinite, or NaN may be encountered by the conversion
routine. The conversion routine should recognize these values and identify
them uniquely.

Special cases of numeric values also exist. Denormals have numeric values,
but should be recognized because they indicate that precision was lost
during some earlier calculations.

Once it has been determined that the number has a numeric value, and it is
normalized (setting appropriate denormal flags, if necessary, to indicate
this to the calling program), the value must be scaled to the BCD range.


7.3.5  Scaling the Value

To scale the number, its magnitude must be determined. It is sufficient to
calculate the magnitude to an accuracy of 1 unit, or within a factor of 10
of the required value. After scaling the number, a check is made to see if
the result falls in the range expected. If not, the result can be adjusted
one decimal order of magnitude up or down. The adjustment test after the
scaling is necessary due to inevitable inaccuracies in the scaling value.

Because the magnitude estimate for the scale factor need only be close, a
fast technique is used. The magnitude is estimated by multiplying the power
of 2, the unbiased floating-point exponent, associated with the number by
log{10}2. Rounding the result to an integer produces an estimate of
sufficient accuracy. Ignoring the fraction value can introduce a maximum
error of 0.32 in the result.

Using the magnitude of the value and size of the number string, the scaling
factor can be calculated. Calculating the scaling factor is the most
inaccurate operation of the conversion process. The relation
10^(X) = 2^(X * log{2}10) is used for this function. The exponentiate
instruction F2XM1 is used.

Due to restrictions on the range of values allowed by the F2XM1
instruction, the power of 2 value is split into integer and fraction
components. The relation 2^(I + F) = 2^(I) * 2^(F) allows using the FSCALE
instruction to recombine the 2^(F) value, calculated through F2XM1, and the
2^(I) part.


7.3.5.1  Inaccuracy in Scaling

The inaccuracy in calculating the scale factor arises because of the
trailing zeros placed into the fraction value of the power of two when
stripping off the integer valued bits. For each integer valued bit in the
power of 2 value separated from the fraction bits, one bit of precision is
lost in the fraction field due to the zero fill occurring in the least
significant bits.

Up to 14 bits may be lost in the fraction because the largest allowed
floating point exponent value is 2^(14) - 1. These bits directly reduce the
accuracy of the calculated scale factor, thereby reducing the accuracy of
the scaled value. For numbers in the range of 10^(±30), a maximum of 8 bits
of precision are lost in the scaling process.


7.3.5.2  Avoiding Underflow and Overflow

The fraction and exponent fields of the number are separated to avoid
underflow and overflow in calculating the scaling values. For example, to
scale 10^(-4932) to 10^(8) requires a scaling factor of 10^(4950), which
cannot be represented by the NPX.

By separating the exponent and fraction, the scaling operation involves
adding the exponents separate from multiplying the fractions. The exponent
arithmetic involves small integers, all easily represented by the NPX.


7.3.5.3  Final Adjustments 

It is possible that the power function (Get_Power_10) could produce a
scaling value such that it forms a scaled result larger than the ASCII field
could allow. For example, scaling 9.9999999999999999 * 10^(4900) by
1.00000000000000010 * 10^(-4883) produces 1.00000000000000009 * 10^(18). The
scale factor is within the accuracy of the NPX and the result is within the
conversion accuracy, but it cannot be represented in BCD format. This is why
there is a post-scaling test on the magnitude of the result. The result can
be multiplied or divided by 10, depending on whether the result was too
small or too large, respectively.


7.3.6  Output Format

For maximum flexibility in output formats, the position of the decimal
point is indicated by a binary integer called the power value. If the power
value is zero, then the decimal point is assumed to be at the right of the
rightmost digit. Power values greater than zero indicate how many trailing
zeros are not shown. For each unit below zero, move the decimal point to the
left in the string.

The last step of the conversion is storing the result in BCD and indicating
where the decimal point lies. The BCD string is then unpacked into ASCII
decimal characters. The ASCII sign is set corresponding to the sign of the
original value.


7.4  Trigonometric Calculation Examples (Not Tested)

In this example, the kinematics of a robot arm is modeled with the 4 * 4
homogeneous transformation matrices proposed by Denavit and Hartenberg
J. Denavit and R.S. Hartenberg, "A Kinematic Notation for Lower-Pair
Mechanisms Based on Matrices," J. Applied Mechanics, June 1955, pp. 215-221.

C.S. George Lee, "Robot Arm Kinematics, Dynamics, and Control," IEEE
Computer, Dec. 1982..
The translational and rotational relationships between adjacent links are
described with these matrices using the D-H matrix method. For each link,
there is a 4 * 4 homogeneous transformation matrix that represents the
link's coordinate system (L{i}) at the joint (J{i}) with respect to the
previous link's coordinate system (J{i-1}, L{i-1}). The following four
geometric quantities completely describe the motion of any rigid joint/link
pair (J{i}, L{i}), as Figure 7-7
See page 7-22 in the printed version of this manual. illustrates.

  Ú{i} =  The angular displacement of the x{i} axis from the x{i-1} axis by
          rotating around the z{i-1} axis (anticlockwise).

  d{i} =  The distance from the origin of the (i-1)^(th) coordinate system
          along the z{i-1} axis to the x{i} axis.

  a{i} =  The distance of the origin of the i^(th) coordinate system from
          the z{i-1} axis along the -x{i} axis.

  Ó{i} =  The angular displacement of the z{i} axis from the z{i-1} about
          the x{i} axis (anticlockwise).

The D-H transformation matrix A=^(i){i-1} for adjacent coordinate frames
(from joint{i-1} to joint{i}) is calculated as follows:

  A^(i){i-1}  =  T{z,d} * T{z,Ú} * T{x,a} * T{x,Ó}

...where...

  T{z,d}    represents a translation along the z=i-1 axis

  T{z,Ú}    represents a rotation of angle Ú about the z=i-1 axis

  T{x,a}    represents a translation along the x{i}axis

  T{x,Ó}    represents a rotation of angle Ó about the x{i}axis

              � COS Ú{i}   -COS Ó{i}SIN Ú{i}  SIN Ó{i}SIN Ú{i}    COS Ú{i} �
A^(i){i-1} =  � SIN Ú{i}   COS Ó{i}COS Ú{i}   -SIN Ó{i}COS Ú{i}   SIN Ú{i} �
              � 0          SIN Ó{i}           COS Ó{i}            d{i}     �
              � 0          0                  0                   1        �

The composite homogeneous matrix T which represents the position and
orientation of the joint/link pair with respect to the base system is
obtained by successively multiplying the D-H transformation matrices for
adjacent coordinate frames.

  T^(i){0} = A^(1){0}  *  A^(2){1}  * ... *  A^(i){i-1}

This example in Figure 7-8 illustrates how the transformation process can
be accomplished using the 80387. The program consists of two major
procedures. The first procedure TRANS_PROC is used to calculate the elements
in each D-H matrix, A^(i){i-1}.  The second procedure MATRIXMUL_PROC finds
the product of two successive D-H matrices.


Figure 7-8.  Robot Arm Kinematics Example

XENIX286 80386 MACRO ASSEMBLER V1.0, ASSEMBLY OF MODULE TOS_STATUS
OBJECT MODULE PLACED IN tos.obj
ASSEMBLER INVOKED BY: asm386 tos.asm

LOC      OBJ                LINE    SOURCE

                       1 +1 $title(Determine TOS register contents)
                       2    ;
                       3    ;         This subroutine will return a value
                       4    ;    from 0-15 in eax corresponding
                       5    ;         to the contents of NPX TOS.  All
                       6    ;    registers are transparent and no
                       7    ;         errors are possible.  The return
                       8    ;    value corresponds to c3,c2,c1,c0
                       9    ;         of FXAM instruction.
                      10    ;
                      11            name    tos_status
00000000              12            public  tos_status
                      13
--------              14    stack           stackseg      6
                      15
--------              16    code            segment public er
                      17
00000000              18    tos_status      proc
                      19
00000000 D9E5         20            fxam                    ; Get status of TOS register
00000002 9BDFE0       21            fstsw  ax       ; Get current status
00000D05 88E0         22            mov     al,ah           ; Put bit 10.8 into bits 2-0
00000007 2507400000   23            and     eax,4007h       ; Mask out bits c3,c2,c1,c0
0000000C C0EC03       24            shr     ah, 3           ; Put bit c3 into bit 11
0000000F 08E0         25            or      al,ah           ; Put c3 into bit 3
00000011 B400         26            mov     ah,0            ; Clear return value
00000013 C3           27            ret
                      28
00000014              29    tos_status      endp
                      30
--------              31    code            ends
                      32                    end

ASSEMBLY COMPLETE,   NO WARNINGS,    NO ERRORS.


LOC      OBJ                LINE    SOURCE

                    37                                ; and fraction
000000A9 C3         38            rat             ; OK to leave fxtract running
                    39    ;
                    40    ;         Calculate the value using the
                    41    ;     exponentiate instruction. The following
                    42    ;     relations are used:
                    43    ;               10**x = 2**(log2(10)*x)
                    44    ;               2**(I+F) = 2**I  * 2**F
                    45    ;       if st(1) = I and st(0)  = 2**F then
                    46    ;       fscale produces 2**(I+F)
                    47    ;
000000AA            48    out of range:
                    49
000000AA D9E9       50            fld12t                  ; TOS = LOG2(10)
000000AC C8040000   51            enter   4,0
                    52
                    53        ; save power of 10 value, P
000000B0 8945FC     54        mov     [ebp-4],eax
                    55
                    56        ; T0S,X = LOG2(10)*P =  LOG2(10**P)
000000B3 DA4DFC     57            fimul   dword ptr [ebp-4]
000000B6 D9E8       58            fld1            ; Set TOS = -1.0
000000B8 D9E0       59            fchs
000000BA D9C1       60            fld     st(1)   ; Copy power value
                    61                            ; in base two
000000BC D9FC       62            frndint         ; TOS = I: -inf < I <= X
                    63                            ; where I is an integer
                    64                            ; Rounding mode does
                    65                            ; not matter
0000003E D9CA       66            fxch    st(2)   ; TOS = X, ST(1) = -1.0
                    67                                ; ST(2) = I
000000C0 D8E2       68            fsub    st,st(2)    ; T0S,F = X-I:
                    69                                ; -1.0 < TOS <= 1.0
                    70
                    71            ; Restore orignal rounding control
000000C2 58         72            pop     eax
000000C3 D9F0       73            f2xm1                   ; TOS = 2**(F) - 1.0
000000C5 C9         74            leave                   ; Restore stack
000000C6 DEE1       75            fsubr                   ; Form 2**(F)
000000C8 C3         76            rat                     ; OK to leave fsubr running
                    77
000000C9            78    get_power_10    endp
                    79
--------            80    code            ends
                    81                    end

ASSEMBLY COMPLETE,   NO WARNINGS,   NO ERRORS.


XENIX286 80386 MACRO ASSEMBLER V1.0, ASSEMBLY OF MODULE ROT_MATRIX_CAL
OBJECT MODULE PLACED IN transx.obj
ASSEMBLER INVOKED BY: asm386 transx.asm

LOC      OBJ                LINE    SOURCE

                               1    Name ROT_MATRIX_CAL
                               2
                               3
                               4
                               5    ; This example illustrates the use
                               6    ; of the 80387 floating point
                               7    ; instructions, in particular, the
                               8    ; FSINCOS function which  gives both
                               9    ; the SIN and COS values.
                               10   ; The program calculates the
                               11   ; composite matrix for base to
                               12   ; end-effector transformation.
                               13   ;
                               14   ; Only the kinematics is considered in
                               15   ; this example.
                               16   ;
                               17   ; If the composite matrix mentioned above
                               18   ; is given by:
                               19   ; T1n = A1 x A2 x ... x An
                               20   ; T1n is found by successively calling
                               21   ; trans_proc and matrixmul_pro until
                               22   ; all matrices have been exhausted.
                               23   ;
                               24   ; trans_proc calculates entries in each
                               25   ; A(A1,...,An) while matrixmul_proc
                               26   ; performs the matrix multiplication for
                               27   ; Ai and Ai+1. matrixmul_proc in turn
                               28   ; calls matrix_row and matrix_elem to
                               29   ; do the multiplication.
                               30
                               31
                               32   ; Define stack space
                               33
--------                       34   trans_stack stackseg 400
                               35
                               36   ; Define the matrix structure for
                               37   ; 4X4 transformational matrices
                               38
--------                       39     a_matrix struc
00000000                       40             a11    dq    ?
00000008                       41             a12    dq    ?
00000010                       42             a13    dq    ?
00000018                       43             a14    dq    ?
00000020                       44             a21    dq    ?
00000028                       45             a22    dq    ?
00000030                       46             a23    dq    ?
00000038                       47             a24    dq    ?
00000040                       48             a31    dq    0h
00000048                       49             a32    dq    ?
00000050                       50             a33    dq    ?
00000058                       51             a34    dq    ?
00000060                       52             a41    dq    0h
00000068                       53             a42    dq    0h
00000070                       54             a43    dq    0h
00000078                       55             a44    dq    1h
--------                       56     a_matrix ends
                               57
                               58   ; Assume One joint in the storage
                               59   ; allocation and hence for
                               60   ; two sets of parameters; however,
                               61   ; more joints are possible
                               62   ;
                               63     alp_deg struc
00000000                       64             alpha_deg1 dd  ?
00000004                       65             alpha_deg2 dd
--------                       66     alp_deg ends
                               67
--------                       68     tht_deg struc
00000000                       69             theta_deg1 dd  ?
00000004                       70             theta_deg2 dd
--------                       71     tht_deg ends
                               72
--------                       73     A_array struc
00000000                       74             A1         dq  ?
00000008                       75             A2         dq  ?
--------                       76     A_array ends
                               77
--------                       78     D_array struc
00000000                       79             D1         dq  ?
00000008                       80             D2         dq  ?
--------                       81     D_array ends
                               82
                               83   ; trans_data is the data segment
                               84   ;
                               85
-------                        86   trans_data       segment rw public
                               87
                               88           Amx            a_matrix<>
00000000 ????????????????
00000008 ????????????????
00000010 ????????????????
00000018 ????????????????
00000020 ????????????????
00000028 ????????????????
00000030 ????????????????
00000038 ????????????????
00000040 0000000000000000
00000048 ????????????????
00000050 ????????????????
00000058 ????????????????
00000060 0000000000000000
00000068 0000000000000000
00000070 0000000000000000
00000078 0100000000000000
00000080 ????????????????      89           Bmx            a_matrix<>
00000088 ????????????????
00000090 ????????????????
00000098 ????????????????
000000A0 ????????????????
000000A8 ????????????????
000000B0 ????????????????
000000B8 ????????????????
000000C0 0000000000000000
000000C8 ????????????????
000000D0 ????????????????
000000D8 ????????????????
000000E0 0000000000000000
000000E8 0000000000000000
000000F0 0000000000000000
000000F8 0100000000000000
00000100 ????????????????      90           Tmx            a matrix<>
00000108 ????????????????
00000110 ????????????????
00000118 ????????????????
00000120 ????????????????
00000128 ????????????????
00000130 ????????????????
00000138 ????????????????
00000140 0000000000000000
00000148 ????????????????
00000150 ????????????????
00000158 ????????????????
00000160 0000000000000000
00000168 0000000000000000
00000170 0000000000000000
00000178 0100000000000000
00000180 ????????              91           ALPHA_DEG      alp_deg<>
00000184 ????????
00000188 ????????              92           THETA_DEG      tht_deg<>
0000018C ????????
00000190 ????????????????      93           A_VECT0R       A_array<>
00000198 ????????????????
000001A0 ????????????????      94           D_VECT0R       D_array<>
000001A8 ????????????????
000001B0 00000000              95           ZER0           dd            0
000001B4 B4000000              96           d180           dd          180
  0001                         97           NUM_JOIMT      equ           1
  0004                         98           NUM_ROW        equ           4
  0004                         99           NUM_CDL        equ           4
000001B8 01                   100           REVERSE        db            1h
--------                      101   trans_data ends
                              102
                              103   assume    ds:trans_data, es:trans_data
                              104
                              105
                              106   ; trans_code contains the procedures
                              107   ; for calculating matrix elements and
                              108   ; matrix multiplications
                              109
--------                      110   trans_code    segment    er public
                              111
                              112   ; create mnemonics for fsincos which is not
                              113   ; yet available from ASM386 as of now
                              114
   C MACRO                    115   codemacro fsincos
       #                      116   dw 0fbd9h
       #                      117   endm
                              118
00000000                      119   trans_proc proc far
                              120
                              121
                              122       ; Calculate alpha and theta in radians
                              123       ; from their values in degrees
                              124
00000000 D9EB                 125         fldpi
00000002 D835B4010000    R    126         fdiv    d180
                              127
                              128       ; Duplicate pi/180
00000008 D9C0                 129         fld     st
                              130
0000000A DC0CCD80010000  R    131         fmul    qword ptr ALPHA_DEG[ecx*8]
00000011 D9C9                 132         fxch    st(1)
00000013 DC0CCD88010000  R    133         fmul    qword ptr THETA_DEG[ecx*8]
                              134
                              135       ; theta(radians) in ST and
                              136       ; alpha(radians) in ST(1)
                              137
                              138       ; Calculate matrix elements
                              139       ; a11 = cos theta
                              140       ; a12 = - cos alpha * sin theta
                              141       ; a13 = sin alpha * sin theta
                              142       ; a14 = A * cos theta
                              143       ; a21 = sin theta
                              144       ; a22 = cos alpha * cos theta
                              145       ; a23 = -sin alpha * cos theta
                              146       ; a24 = A * sin theta
                              147       ; a32 = sin alpha
                              148       ; a33 = cos alpha
                              149       ; a34 = D
                              150       ; a31 = a41 = a42 = a43 = 0.0
                              151       ; a44 =1
                              152
                              153       ; ebx contains the offset for the matrix
                              154
0000001A D9FB                 155        fsincos           ;cos theta in ST
                              156                          ;sin theta in ST(1)
0000001C D9C0                 157        fld     st        ;duplicate cos theta
0000001E DD13                 158        fst     [ebx].a11 ;cos theta in a11
00000020 DC0CCD90010000  R    159        fmul    qword ptr A_VECTOR[ecx*8]
00000027 DD5B18               160        fstp    [ebx].a14 ;A * cos theta in a14
0000002A D9C9                 161        fxch    st(1)     ;sin theta in ST
0000002C DD5320               162        fst     [ebx].a21 ;sin theta in a21
0000002F D9C0                 163        fld     st        ;duplicate sin theta
00000031 DC0CCD90010000  R    164        fmul    qword ptr A_VECTOR[ecx*8]
00000038 DD5B38               165        fstp    [ebx].a24 ;A * sin theta in a24
0000003B D9C2                 166        fld     st(2)     ;alpha in ST
0000003D D9FB                 167        fsincos           ;cos alpha in ST
                              168                          ;sin alpha in ST(1)
                              169                          ;sin theta in ST(2)
                              170                          ;cos theta in ST(3)
0000003F DD5350               171        fst     [ebx].a33 ;cos alpha in a33
00000042 D9C9                 172        fxch    st(1)     ;sin alpha in ST
00000044 DD5348               173        fat     [ebx].a32 ;sin alpha in a32
00000047 D9C2                 174        fld     ST(2)     ;sin theta in ST
                              175                          ;sin alpha in ST(1)
00000049 D8C9                 176        fmul    st,st(1)  ;sin alpha * sin theta
0000004B DD5B10               177        fstp    [ebx].a13 ;stored in a13
0000004E D8CB                 178        fmul    st,st(3)  ;cos theta * sin alpha
00000050 D9E0                 179        fchs              ;-cos theta * sin alpha
00000052 DD5B30               180        fstp    [ebx].a23 ;stored in a23
00000055 D9C2                 181        fld     st(2)     ;cos theta in ST
                              182                          ;cos alpha in ST(1)
                              183                          ;sin theta in ST(2)
                              184                          ;cos theta in ST(3)
00000057 D8C9                 185        fmul    st,st(1)  ;cos theta * cos alpha
00000059 DD5B28               186        fstp    [ebx].a22 ;stored in a22
0000005C D8C9                 187        fmul    st,st(1)  ;cos alpha * sin theta
                              188       ;
                              189       ; To take advantage of parallel operations
                              190       ; between the CPU and NPX
                              191       ;
0000005E 50                   192        push    eax  ; save eax
                              193       ;
                              194       ; also move D into a34 in a faster way
0000005F 8B04CDA0010000  R    195        mov     eax, dword ptr D_VECTOR[ecx*8]
00000066 894358               196        mov     dword ptr [ebx + 88], eax
00000069 8B04CDA4010000  R    197        mov     eax, dword ptr D VECTOR[ecx*8 + 4]
00000070 89435C               198        mov     dword ptr [ebx + 92], eax
00000073 58                   199        pop     eax  ; restore eax
00000074 D9E0                 200        fchs              ;-cos alpha * sin theta
00000076 DD5B08               201        fstp    [ebx].a12 ;stored in a12
                              202                          ;and all nonzero elements
                              203                          ;have been calculated
00000079 CB                   204        rat
                              205
0000007A                      206   trans_proc endp
                              207
                              208
0000007A                      209   matrix_elem proc far
                              210
                              211       ; This procedure calculate the dot product
                              212       ; of the ith row of the first matrix and
                              213       ; the jth column of the second matrix:
                              214       ;
                              215       ; Tij where Tij = sum of Aik x Bkj over k
                              216       ;
                              217       ; parameters passed from the calling routine,
                              218       ; matrix_row:
                              219       ; ESI = (i-1)*8
                              220       ; EDI = (j-1)*8
                              221       ; local register, EBP = (k-1)*8
                              222       ;
0000007A 55                   223         push    ebp     ; save ebp
0000007B 51                   224         push    ecx     ; ecx to be used as a tmp reg
0000007C 8BCE                 225         mov     ecx, esi; save it for later indexing
                              226
                              227       ; locating the element in the first matrix, A
0000007E 6BC904               228         imul    ecx, NUM_COL   ; ecx contains offset due
                              229                                ; to preceding rows; the
                              230                                ; offset is from the
                              231                                ; beginning of the matrix
                              232
00000081 31ED                 233         xor     ebp, ebp; clear ebp, which will be
                              234                         ; used a temp reg to index( k)
                              235                         ; across the ith row of the first
                              236                         ; matrix as well as down the jth
                              237                         ; column of the second matrix
                              238
                              239       ; clear Tij for accumulating Aik*Bkj
00000083 892C39               240         mov     dword ptr [ecx][edi],ebp
00000086 896C3904             241         mov     dword ptr [ecx][edi+4], ebp
                              242
0000008A 51                   243         push    ecx     ; save on stack: esi * num_col =
                              244                         ; the offset of the beginning
                              245                         ; of the ith row from the
                              246                         ; beginning of the A matrix
                              247
0000008B                      248   NXT_k:
0000008B 01E9                 249         add     ecx, ebp ; get to the kth column entry
                              250                          ; of the ith row of the A matrix
                              251
                              252       ; load AiK into 80387
0000008D DD0408               253         fld     qword ptr [eax][ecx]
                              254
                              255       ; locating  Bkj
00000090 8BCD                 256         mov     ecx, ebp
00000092 6BC904               257         imul    ecx, NUM_ROW ; ecx contains the offset
                              258                              ; of the beginning of the
                              259                              ; kth row from the
                              260                              ; beginning of the B matrix
00000095 01F9                 261         add    ecx, edi      ; get to the jth column entry
                              262                              ; of the kth row of the B
                              263                              ; matrix
00000097 DC0C0B               264         fmul   qword ptr [ebx][ecx]; Aik * Bkj
0000009A 59                   265         pop    ecx           ; esi * num_col
                              266                              ; in ecx again
0000009B 51                   267         push   ecx           ; also at top of program
                              268                              ; stack
                              269
                              270       ; add to the result in the output matrix, Tij
0000009C 01F9                 271         add    ecx, edi
                              272
                              273       ; accumulating the sum of Aik * Bkj
0000009E DC040A               274         fadd   qword ptr [edx][ecx]
000000A1 DD1C0A               275         fstp   qword ptr [edx][ecx]
                              276       ; increment k by 1, i.e., ebp by 8
000000A4 83C508               277         add    ebp, 8
                              278
                              279       ; Has k reached the width of the matrix yet?
000000A7 83FD20               280         cmp    ebp, NUM_COL*8
000000AA 7CDF                 281         jl     NXT_k
                              282
                              283       ; Restore registers
000000AC 59                   284         pop    ecx      ; clear esi*num_col from stack
000000AD 59                   285         pop    ecx      ; restore ecx
000000AE 5D                   286         pop    ebp      ; restore ebp
000000AF CB                   287         ret
                              288
000000B0                      289   matrix_elem endp
                              290
                              291
000000B0                      292   matrix_row proc far
                              293
000000B0 31FF                 294         xor    edi, edi
                              295       ; scan across a row
                              296
000000B2                      297   NXT_COL:
000000B2 9A7A000000....  R    298         call   matrix_elem
000000B9 83C708               299         add    edi, 8
000000BC 83FF20               300         cmp    edi, NUM_COL*8
000000BF 7CF1                 301         jl     NXT_COL
000000C1 CB                   302         ret
                              303
000000C2                      304   matrix_row endp
                              305
                              306
000000C2                      307   matrixmul_proc proc far
                              308
                              309       ; This procedure does the matrix
                              310       ; multiplication by calling matrix_row
                              311       ; to calculate entries in each row
                              312       ;
                              313       ; The matrix multiplication is
                              314       ; performed in the following manner,
                              315       ;   Tij = Aik x Bkj
                              316       ; where i and j denote the row and column
                              317       ; respectively and k is the index for
                              318       ; scanning across the ith row of the
                              319       ; first matrix and the jth column of the
                              320       ; second matrix.
000000C2 5A                   321         pop     edx ; offset Tmx in edx
000000C3 5B                   322         pop     ebx ; offset Bmx in ebx
000000C4 58                   323         pop     eax ; offset Amx in eax
                              324
                              325       ; setup esi and edi
                              326       ; edi points to the column
                              327       ; eai points to the row
                              328
000000C5 31F6                 329         xor     esi, esi ; clear esi
                              330
000000C7                      331   NXT_ROW:
000000C7 9AB0000000----  R    332         call    matrix_row
000000CE 83C608               333         add     esi, 8
000000D1 83FE20               334         cmp     esi, NUM_ROW*8
000000D4 7CF1                 335         jl      NXT_ROW
000000D6 CB                   336         ret
                              337
000000D7                      338   matrixmul_proc endp
                              339
                              340
--------                      341   trans_code ends
                              342
                              343   ;***************************************
                              344   ;                                      ;
                              345   ;                                      ;
                              346   ;                                      ;
                              347   ;             Main program             ;
                              348   ;                                      ;
                              349   ;                                      ;
                              350   ;                                      ;
                              351   ;***************************************
                              352
--------                      353   main_code segment er
                              354
00000000                      355   START:
                              356
00000000 BC00000000      R    357         mov  esp,  stackstart trans_stack
                              358       ; save all registers
                              359
00000005 60                   360         pushed
                              361
                              362       ; ECX denotes the number of joints
                              363       ; where no of matrices = NUM_JOINT + 1
                              364       ; Find the first matrix( from the base
                              365       ; of the system to the first joint)
                              366       ; and call it Bmx
00000006 31C9                 367         xor  ecx, ecx          ; 1st matrix
00000008 BB80000000      R    368         mov  ebx, offset Bmx  ;
0000000D 9A00000000----  R    369         call trans_proc        ; is Bmx
00000014 41                   370         inc  ecx
                              371
00000015                      372   NXT MATRIX:
                              373       ; From the 2nd matrix and on, it
                              374       ; will be stored in Amx.
                              375       ; The result from the first matrix mult.
                              376       ; is stored in Tmx but will be accessed
                              377       ; as Bmx in the next multiplication.
                              378       ; As a matter of fact, the roles of Bmx
                              379       ; and Tmx alternate in successive
                              380       ; multiplications. This is achieved by
                              381       ; reversing the order of the Bmx and Tmx
                              382       ; pointers being passed onto the program
                              383       ; stack:  Thus, this is invisible to the
                              384       ; matrix multiplication procedure.
                              385       ; REVERSE serves as the indicator;
                              386       ; REVERSE = 0 means that the result
                              387       ;             is to placed in Tmx.
                              388
00000015 BB00000000      R    389         mov     ebx, offset Amx  ;find Amx
0000001A 9A00000000----  R    390         call    trans_proc
00000021 41                   391         inc     ecx
00000022 8035B801000001  R    392         xor     REVERSE, 1h
00000029 7511                 393         jnz     Bmx_as_Tmx
                              394
                              395       ; no reversing.  Bmx as the second input
                              396       ; matrix while Tmx as the output matrix.
0000002B 6800000000      R    397         push    offset Amx
00000030 6880000000      R    398         push    offset Bmx
00000035 6800010000      R    399         push    offset Tmx
0000003A EB0F                 400         jmp     CONTINUE
                              481
                              402       ; reversing. Tmx as the second input
                              403       ; matrix while Bmx as the output matrix.
0000003C                      404   Bmx_as_Tmx:
0000003C 6800000000      R    405         push    offset Amx
00000041 6800010000      R    406         push    offset Tmx  ;reversing the
00000046 6880000000      R    407         push    offset Bmx  ;pointers passed
                              408
UUUUUU4B                      409   CONTINUE:
0000004B 9AC2000000----  R    410         call    matrixmul_proc
00000052 83F901               411         cmp     ecx, NUM_JOINT
00000055 7EBE                 412         jle     NXT_MATRIX
                              413
                              414       ; if REVERSE = 1 then the final answer
                              415       ; will be in Bmx otherwise, in Tmx.
                              416
00000057 61                   417         popad
                              418
--------                      419   main_code  ends
                              420
                              421   end START, ds:trans data, ss:trans stack

ASSEMBLY COMPLETE,   NO WARNINGS,   NO ERRORS.


Appendix A  Machine Instruction Encoding and Decoding

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

’‘‘1st  Byte‘‘“
Hex  Binary      2nd Byte      Bytes 3-7    ASM386 Instruction Format

D8   1101 1000   MOD 000 R/M   SIB, displ   FADD       single-real
D8   1101 1000   MOD 001 R/M   SIB, displ   FMUL       single-real
D8   1101 1000   MOD 010 R/M   SIB, displ   FCOM       single-real
D8   1101 1000   MOD 011 R/M   SIB, displ   FCOMP      single-real
D8   1101 1000   MOD 100 R/M   SIB, displ   FSUB       single-real
D8   1101 1000   MOD 101 R/M   SIB, displ   FSUBR      single-real
D8   1101 1000   MOD 110 R/M   SIB, displ   FDIV       single-real
D8   1101 1000   MOD 111 R/M   SIB, displ   FDIVR      single-real
D8   1101 1000   1100 0 REG                 FADD       ST,ST(i)
D8   1101 1000   1100 1 REG                 FMUL       ST,ST(i)
D8   1101 1000   1101 0 REG                 FCOM       ST(i)
D8   1101 1000   1101 1 REG                 FCOMP      ST(i)
D8   1101 1000   1110 0 REG                 FSUB       ST,ST(i)
D8   1101 1000   1110 1 REG                 FSUBR      ST,ST(i)
D8   1101 1000   1111 0 REG                 FDIV       ST,ST(i)
D8   1101 1000   1111 1 REG                 FDIVR      ST,ST(i)
D9   1101 1001   MOD 000 R/M   SIB, displ   FLD        single-real
D9   1101 1001   MOD 001 R/M                reserved
D9   1101 1001   MOD 010 R/M   SIB, displ   FST        single-real
D9   1101 1001   MOD 011 R/M   SIB, displ   FSTP       single-real
D9   1101 1001   MOD 100 R/M   SIB, displ   FLDENV     14 or 28 bytes
The size of operand transferred depends on the 80386 operand-size
attribute in effect for the instruction.





D9   1101 1001   MOD 101 R/M   SIB, displ   FLDCW      2 bytes
D9   1101 1001   MOD 110 R/M   SIB, displ   FSTENV     14 or 28 bytes
The size of operand transferred depends on the 80386 operand-size
attribute in effect for the instruction.





D9   1101 1001   MOD 111 R/M   SIB, displ   FSTCW      2 bytes
D9   1101 1001   1100 0 REG                 FLD        ST(i)
D9   1101 1001   1100 1 REG                 FXCH       ST(i)
D9   1101 1001   1101 0000                  FNOP
D9   1101 1001   1101 0001                  reserved
D9   1101 1001   1101 001-                  reserved
D9   1101 1001   1101 01--                  reserved
D9   1101 1001   1101 1 REG                 reserved
D9   1101 1001   1110 0000                  FCHS
D9   1101 1001   1110 0001                  FABS
D9   1101 1001   1110 001-                  reserved
D9   1101 1001   1110 0100                  FTST
D9   1101 1001   1110 0101                  FXAM
D9   1101 1001   1110 011-                  reserved
D9   1101 1001   1110 1000                  FLD1
D9   1101 1001   1110 1001                  FLDL2T
D9   1101 1001   1110 1010                  FLDL2E
D9   1101 1001   1110 1011                  FLDPI
D9   1101 1001   1110 1100                  FLDLG2
D9   1101 1001   1110 1101                  FLDLN2
D9   1101 1001   1110 1110                  FLDZ
D9   1101 1001   1110 1111                  reserved
D9   1101 1001   1111 0000                  F2XM1
D9   1101 1001   1111 0001                  FYL2X
D9   1101 1001   1111 0010                  FPTAN
D9   1101 1001   1111 0011                  FPATAN
D9   1101 1001   1111 0100                  FXTRACT
D9   1101 1001   1111 0101                  FPREM1
D9   1101 1001   1111 0110                  FDECSTP
D9   1101 1001   1111 0111                  FINCSTP
D9   1101 1001   1111 1000                  FPREM
D9   1101 1001   1111 1001                  FYL2XP1
D9   1101 1001   1111 1010                  FSQRT
D9   1101 1001   1111 1011                  FSINCOS
D9   1101 1001   1111 1100                  FRNDINT
D9   1101 1001   1111 1101                  FSCALE
D9   1101 1001   1111 1110                  FSIN
D9   1101 1001   1111 1111                  FCOS
DA   1101 1010   MOD 000 R/M   SIB, displ   FIADD      short-integer
DA   1101 1010   MOD 001 R/M   SIB, displ   FIMUL      short-integer
DA   1101 1010   MOD 010 R/M   SIB, displ   FICOM      short-integer
DA   1101 1010   MOD 011 R/M   SIB, displ   FICOMP     short-integer
DA   1101 1010   MOD 100 R/M   SIB, displ   FISUB      short-integer
DA   1101 1010   MOD 101 R/M   SIB, displ   FISUBR     short-integer
DA   1101 1010   MOD 110 R/M   SIB, displ   FIDIV      short-integer
DA   1101 1010   MOD 111 R/M   SIB, displ   FIDIVR     short-integer
DA   1101 1010   110- ----                  reserved
DA   1101 1010   1110 0---                  reserved
DA   1101 1010   1110 1000                  reserved
DA   1010 1010   1110 1001                  FUCOMPP
DA   1101 1010   1110 101-                  reserved
DA   1101 1010   1110 11--                  reserved
DA   1101 1010   1111 ----                  reserved
DB   1101 1011   MOD 000 R/M   SIB, displ   FILD       short-integer
DB   1101 1011   MOD 001 R/M   SIB, displ   reserved
DB   1101 1011   MOD 010 R/M   SIB, displ   FIST       short-integer
DB   1101 1011   MOD 011 R/M   SIB, displ   FISTP      short-integer
DB   1101 1011   MOD 100 R/M   SIB, displ   reserved
DB   1101 1011   MOD 101 R/M   SIB, displ   FLD        extended-real
DB   1101 1011   MOD 110 R/M   SIB, displ   reserved
DB   1101 1011   MOD 111 R/M   SIB, displ   FSTP       extended-real
DB   1101 1011   110- ----                  reserved
DB   1101 1011   1110 0000                  
This encoding can be generated by the language translators;
however, the 80387 treats it as FNOP. It corresponds to the following
8087 or 80287 instructions: FENI.





DB   1101 1011   1110 0001                  
This encoding can be generated by the language translators;
however, the 80387 treats it as FNOP. It corresponds to the following
8087 or 80287 instructions: FEDISI.





DB   1101 1011   1110 0010                  FCLEX
DB   1101 1011   1110 0011                  FINIT
DB   1101 1011   1110 0100                  
This encoding can be generated by the language translators;
however, the 80387 treats it as FNOP. It corresponds to the following
8087 or 80287 instructions: FSETPM.





DB   1101 1011   1110 0101                  reserved
DB   1101 1011   1110 011-                  reserved
DB   1101 1011   1110 1---                  reserved
DB   1101 1011   1111 ----                  reserved
DC   1101 1100   MOD 000 R/M   SIB, displ   FADD       double-real
DC   1101 1100   MOD 001 R/M   SIB, displ   FMUL       double-real
DC   1101 1100   MOD 010 R/M   SIB, displ   FCOM       double-real
DC   1101 1100   MOD 011 R/M   SIB, displ   FCOMP      double-real
DC   1101 1100   MOD 100 R/M   SIB, displ   FSUB       double-real
DC   1101 1100   MOD 101 R/M   SIB, displ   FSUBR      double-real
DC   1101 1100   MOD 110 R/M   SIB, displ   FDIV       double-real
DC   1101 1100   MOD 111 R/M   SIB, displ   FDIVR      double-real
DC   1101 1100   1100 0 REG                 FADD       ST(i),ST
DC   1101 1100   1100 1 REG                 FMUL       ST(i),ST
DC   1101 1100   1101 0 REG                 reserved
DC   1101 100    1101 1 REG                 reserved
DC   1101 1100   1110 0 REG                 FSUBR      ST(i),ST
DC   1101 1100   1110 1 REG                 FSUB       ST(i),ST
DC   1101 1100   1111 0 REG                 FDIVR      ST(i),ST
DC   1101 1100   1111 1 REG                 FDIV       ST(i),ST
DD   1101 1101   MOD 000 R/M   SIB, displ   FLD        double-real
DD   1101 1101   MOD 001 R/M                reserved
DD   1101 1101   MOD 010 R/M   SIB, displ   FST        double-real
DD   1101 1101   MOD 011 R/M   SIB, displ   FSTP       double-real
DD   1101 1101   MOD 100 R/M   SIB, displ   FRSTOR     94 or 108 bytes
The size of operand transferred depends on the 80386 operand-size
attribute in effect for the instruction.





DD   1101 1101   MOD 101 R/M   SIB, displ   reserved
DD   1101 1101   MOD 110 R/M   SIB, displ   FSAVE      94 or 108 bytes
The size of operand transferred depends on the 80386 operand-size
attribute in effect for the instruction.





DD   1101 1101   MOD 111 R/M   SIB, displ   FSTSW      2 bytes
DD   1101 1101   1100 0 REG                 FFREE      ST(i)
DD   1101 1101   1100 1 REG                 reserved
DD   1101 1101   1101 0 REG                 FST        ST(i)
DD   1101 1101   1101 1 REG                 FSTP       ST(i)
DD   1101 1101   1110 0 REG                 FUCOM      ST(i)
DD   1101 1101   1110 1 REG                 FUCOMP     ST(i)
DD   1101 1101   1111 ----                  reserved
DE   1101 1110   MOD 000 R/M   SIB, displ   FIADD      word-integer
DE   1101 1110   MOD 001 R/M   SIB, displ   FIMUL      word-integer
DE   1101 1110   MOD 010 R/M   SIB, displ   FICOM      word-integer
DE   1101 1110   MOD 011 R/M   SIB, displ   FICOMP     word-integer
DE   1101 1110   MOD 100 R/M   SIB, displ   FISUB      word-integer
DE   1101 1110   MOD 101 R/M   SIB, displ   FISUBR     word-integer
DE   1101 1110   MOD 110 R/M   SIB, displ   FIDIV      word-integer
DE   1101 1110   MOD 111 R/M   SIB, displ   FIDIVR     word-integer
DE   1101 1110   1100 0 REG                 FADDP      ST(i),ST
DE   1101 1110   1100 1 REG                 FMULP      ST(i),ST
DE   1101 1110   1101 0---                  reserved
DE   1101 1110   1101 1000                  reserved
DE   1101 1110   1101 1001                  FCOMPP
DE   1101 1110   1101 101-                  reserved
DE   1101 1110   1101 11--                  reserved
DE   1101 1110   1110 0 REG                 FSUBRP     ST(i),ST
DE   1101 1110   1110 1 REG                 FSUBP      ST(i),ST
DE   1101 1110   1111 0 REG                 FDIVRP     ST(i),ST
DE   1101 1110   1111 1 REG                 FDIVP      ST(i),ST
DF   1101 1111   MOD 000 R/M   SIB, displ   FILD       word-integer
DF   1101 1111   MOD 001 R/M   SIB, displ   reserved
DF   1101 1111   MOD 010 R/M   SIB, displ   FIST       word-integer
DF   1101 1111   MOD 011 R/M   SIB, displ   FISTP      word-integer
DF   1101 1111   MOD 100 R/M   SIB, displ   FBLD       packed-decimal
DF   1101 1111   MOD 101 R/M   SIB, displ   FILD       long-integer
DF   1101 1111   MOD 110 R/M   SIB, displ   FBSTP      packed-decimal
DF   1101 1111   MOD 111 R/M   SIB, displ   FISTP      long-integer
DF   1101 1111   1100 0 REG                 reserved
DF   1101 1111   1100 1 REG                 reserved
DF   1101 1111   1101 0 REG                 reserved
DF   1101 1111   1101 1 REG                 reserved
DF   1101 1111   1110 0000                  FSTSW AX
DF   1101 1111   1110 0001                  reserved
DF   1101 1111   1110 001-                  reserved
DF   1101 1111   1110 01--                  reserved
DF   1101 1111   1110 1---                  reserved
DF   1101 1111   1111 ----                  reserved


Appendix B  Exception Summary

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

The following table lists the instruction mnemonics in alphabetical order.
For each mnemonic, it summarizes the exceptions that the instruction may
cause. When writing 80387 programs that may be used in an environment that
employs numerics exception handlers, assembly-language programmers should be
aware of the possible exceptions for each instruction in order to determine
the need for exception synchronization. Chapter 4 explains the need for
exception synchronization.


Mnemonic            Instruction             IS
IS‘‘Invalid operand due to stack overflow/underflow  I
I‘‘Invalid operand due to other cause  D
D‘‘Denormal operand  Z
Z‘‘Zero-divide  O
O‘‘Overflow  U
U‘‘Underflow  P
P‘‘Inexact result (precision)






F2XM1               2^(X) - 1                Y   Y   Y           Y   Y
FABS                Absolute value           Y
FADD(P)             Add real                 Y   Y   Y       Y   Y   Y
FBLD                BCD load                 Y
FBSTP               BCD store and pop        Y   Y                   Y
FCHS                Change sign              Y
FCLEX               Clear exceptions
FCOM(P)(P)          Compare real             Y   Y   Y
FCOS                Cosine                   Y   Y   Y           Y   Y
FDECSTP             Decrement stack pointer
FDIV(R)(P)          Divide real              Y   Y   Y   Y   Y   Y   Y
FFREE               Free register
FIADD               Integer add              Y   Y   Y       Y   Y   Y
FICOM(P)            Integer compare          Y   Y   Y
FIDIV               Integer divide           Y   Y   Y   Y       Y   Y
FIDIVR              Integer divide reversed  Y   Y   Y   Y   Y   Y   Y
FILD                Integer load             Y
FIMUL               Integer multiply         Y   Y   Y   Y   Y   Y
FINCSTP             Increment stack pointer
FINIT               Initialize processor
FIST(P)             Integer store            Y   Y                   Y
FISUB(R)            Integer subtract         Y   Y   Y       Y   Y   Y
FLD extended
or stack            Load real                Y
FLD single
or double           Load real                Y   Y   Y
FLD1                Load + 1.0               Y
FLDCW               Load Control word        Y   Y   Y   Y   Y   Y   Y
FLDENV              Load environment         Y   Y   Y   Y   Y   Y   Y
FLDL2E              Load log{2}e             Y
FLDL2T              Load log{2}10            Y
FLDLG2              Load log{10}2            Y
FLDLN2              Load log{e}2             Y
FLDPI               Load Ò                   Y
FLDZ                Load + 0.0               Y
FMUL(P)             Multiply real            Y   Y   Y       Y   Y   Y
FNOP                No operation
FPATAN              Partial arctangent       Y   Y   Y           Y   Y
FPREM               Partial remainder        Y   Y   Y           Y
FPREM1              IEEE partial remainder   Y   Y   Y           Y
FPTAN               Partial tangent          Y   Y   Y           Y   Y
FRNDINT             Round to integer         Y   Y   Y               Y
FRSTOR              Restore state            Y   Y   Y   Y   Y   Y   Y
FSAVE               Save state
FSCALE              Scale                    Y   Y   Y       Y   Y   Y
FSIN                Sine                     Y   Y   Y           Y   Y
FSINCOS             Sine and cosine          Y   Y   Y           Y   Y
FSQRT               Square root              Y   Y   Y               Y
FST(P) stack
or extended         Store real               Y
FST(P) single
or double           Store real               Y   Y   Y       Y   Y   Y
FSTCW               Store control word
FSTENV              Store Environment
FSTSW (AX)          Store status word
FSUB(R)(P)          Subtract real            Y   Y   Y       Y   Y   Y
FTST                Test                     Y   Y   Y
FUCOM(P)(P)         Unordered compare real   Y   Y   Y
FWAIT               CPU Wait
FXAM                Examine
FXCH                Exchange registers       Y
FXTRACT             Extract                  Y   Y   Y   Y
FYL2X               Y * log{2}X              Y   Y   Y   Y   Y   Y   Y
FYL2XP1             Y * log{2}(X + 1)        Y   Y   Y           Y   Y


Appendix C  Compatibility Between the 80387 and the 80287/8087

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

This appendix summarizes the differences between the 80387 and its
predecessors the 80287 and the 8087, and analyzes the impact of these
differences on software that must be transported from the 80287 or 8087 to
the 80387. Any migration from the 8087 directly to the 80387 must also take
into account the additional differences between the 8087 and the 80387 as
listed in Appendix D of this manual.


                ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Difference Description‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
Issue            80387 Behavior                  8087/80287 Behavior          Impact on Software              Reason for the Difference

C.1  INITIALIZATION SEQUENCE

RESET,           After a hardware RESET,         No difference between        80387 initialization            Permits the 80386 to
FINIT,           the ERROR# output is            RESET and FINIT.             software must execute an        differentiate between the 80287
and              asserted to indicate that an                                 FNINIT instruction to clear     and the 80387.
ERROR#           80387 is present. To                                         ERROR#. The FNINIT is
PIN              accomplish this, the IE and                                  not required for 80287/8087
                 ES bits of the status word                                   software, though Intel
                 are set, and the IM bit in                                   documentation
                 the control word is reset.                                   recommends its use (refer to
                 After FINIT, the status                                      the Numerics Supplement to
                 word and the control word                                    the iAPX 286 Programmer's
                 have the same values as in                                   Reference Manual.)
                 an 80287/8087 after
                 RESET.

C.2  DATA TYPES AND EXCEPTION HANDLING

NaN              The 80387 distinguishes         The 80287/8087 only          Uninitialized memory            IEEE Standard 754
                 between signaling NaNs          generates one kind of NaN    locations that contain          compatibility.
                 and quiet NaNs. The 80387       (the equivalent of a quiet   QNaNs should be changed
                 only generates quiet NaNs.      NaN) but raises an           to SNaNs to cause the
                 An invalid-operation            invalid-operation exception  80387 to fault when
                 exception is raised only        upon encountering any kind   uninitialized memory locations
                 upon encountering a             of NaN.                      are referenced.
                 signaling NaN (except for
                 FCOM, FIST, and FBSTP
                 which also raise IE for
                 quiet NaNs).

Pseudozero,      The 80387 neither               The 80287/8087 defines       None. The 80387 does not        IEEE Standard 754
Pseudo-NaN,      generates not supports these    and supports special         generate these formats,         compatibility.
Pseudoinfinity,  formats; it raises an           handling for these formats.  and therefore will not
and Unnormal     invalid-operation exception                                  encounter them unless a
Formats          whenever it encounters                                       programmer deliberately
                 them in an arithmetic                                        enters them.
                 operation.

Tag Word         The encoding in the tag         The encoding for pseudo-     The exception handler may       IEEE Standard 754
Bits for         word for the unsupported        zero and unnormal is         need to be changed if           compatibility.
Unsupported      data formats mentioned in       "valid" (type 00); the       programmers use such
Data             Section C.2.2 is "special       others are"special data"     data types.
Formats          data" (type 10).                (type 10).

Invalid-         No invalid-operation            Upon encountering a          None. Software on the           Upgrade, to eliminate
Operation        exception is raised upon        denormal in FSQRT, FDIV,     80387 will continue to          exception.
Exception        encountering a denormal in      or FPREM or upon             execute in cases where the
                 FSQRT, FDIV, or FPREM           conversion to BCD or to      80287/8087 would trap.
                 or upon conversion to           integer, the invalid-
                 BCD or to integer. The          operation exception is
                 operation proceeds by first     raised.
                 normalizing the value.

Denormal         The denormal exception is       The denormal exception is    The exception handler           Performance enhancement
Exception        raised in transcendental        not raised in transcendental needs to be changed only        for normal case.
                 instructions and FXTRACT.       instructions and FXTRACT.    if it gives special treatment
                                                                              to different opcodes.

Overflow         Overflow exception              Overflow exception           Overflow exception              IEEE Standard 754
Exception        masked.                         masked.                      masked.                         compatibility.
                 If the rounding mode is set     The 80287/8087 does not      Under the most common
                 to chop (toward zero), the      signal the overflow          rounding modes, no
                 result is the most positive     exception when the masked    impact. If rounding is
                 or most negative number.        response is not infinity;    toward zero (chop), a
                                                 i.e., it signals overflow    program on the 80387
                                                 only when the rounding       produces under overflow
                                                 control is not set to round  conditions a result that is
                                                 to zero .If rounding is set  different in the least
                                                 to chop (toward zero), the   significant bit of the
                                                 result is positive or        significand, compared to
                                                 negative infinity.           the result on the 80287.

                 Overflow exception not          Overflow exception not       Overflow exception not
                 masked.                         masked.                      masked.
                 The precision exception is      The precision exception is   If the result is stored on
                 flagged. When the result is     not flagged and the          the stack, a program on
                 stored in the stack, the        significand is not rounded.  the 80387 produces a
                 significand is rounded                                       different result under
                 according to the precision                                   overflow conditions than
                 control (PC) bit of the                                      on the 80287/8087. The
                 control word of according                                    difference is apparent only
                 to the opcode.                                               to the exception handler.

Underflow        Conditions for underflow.       Conditions for underflow.    Underflow exception             IEEE Standard 754
Exception        When the underflow              When the underflow           masked.                         compatibility.
                 exception is masked, the        exception is masked and      No impact. The underflow
Two related      underflow exception is          rounding is toward zero, the exception occurs less
events           signaled when both the          underflow exception flag is  often when rounding is
contribute to    result is tiny and              raised on tininess,          toward zero.
underflow:       denormalization results         regardless of loss of
                 in a loss of accuracy.          accuracy.
1. The creation
   tiny result.  Response to underflow.          Response to underflow.       Underflow exception not
   A tiny        When the underflow              When the underflow           masked.
   number,       exception is unmasked           exception is not masked and  A program on the 80387
   because it    and the instruction is          the destination is the       produces a different result
   is so small,  supposed to store the           stack, the significand is    during underflow
   may cause     result on the stack, the        not rounded but rather is    conditions than on the 80287/
   some other    significand is rounded to       left as is.                  8087 if the result is
   exception     the appropriate precision                                    stored on the stack. The
   later (such   (according to the precision                                  difference is only in the
   as overflow   control (PC) bit of the                                      least significant bit of the
   upon          control word, for those                                      significand and is apparent
   division).    instructions controlled by                                   only to the exception handler.
                 PC, otherwise to extended
 2. Loss of      precision).
   accuracy
   during the
   denormaliza-
   tion of a
   tiny number.
   Which of
   these events
   triggers the
   underflow
   exception
   depends on
   whether the
   underflow
   exception
   is masked.

Which of these
events triggers
the underflow
exception
depends on
whether the
underflow
exception is
masked.

Exception       There is no difference in       When the denormal             None, but some unneeded         Operational improvement.
Precedence      the precedence of the           exception is not masked,      normalization of denormal
                denormal exception,             it takes precedence over      operands is prevented on
                whether it be masked or         all other exceptions.         the 80387.
                not.

C.3  TAG, STATUS, AND CONTROL WORDS

Bits C3-C0 of   After FINIT, incomplete         After FINIT, incomplete       None.                           Upgrade, to provide
Status Word     FPREM, and hardware             FPREM, and hardware                                           consistent state after reset.
                reset, the 80387 sets these     reset, the 80287/8087
                bits to zero.                   leaves these bits intact
                                                (they contain the prior
                                                value).

Bit C2 of       Bit 10 (C2) serves as an        This bit is undefined for     None. Programs don't            Upgrade to allow fast
Status Word     incomplete bit for FPTAN.       FPTAN.                        check C2 after FPTAN.           checking of operand range.


Infinity        Only affine closure is          Both affine and projective    Software that requires          IEEE Standard 754
Control         supported. Bit 12 remains       closures are supported.       projective infinity             compatibility.
                programmable but has no         After RESET, the default      arithmetic may give
                effect on 80387 operation.      value in the control word is  different results.
                                                projective.

Status Word     When an invalid-operation       When an invalid-operation     None. Existing exception        Upgrade and performance
Bit 6 for       exception occurs due to         exception occurs due to       handlers need not change,       improvement.
Stack Fault     stack overflow or               stack overflow or underflow,  but may be upgraded to
                underflow, not only is bit 0    only bit 0 (IE) of the        take advantage of the
                (IE) the status word set, but   status word is set. Bit 6 is  additional information.
                also bit 6 is set to indicate   RESERVED.                     Newly written handlers will
                a stack fault and bit 9 (C1)                                  be more effective.
                specifies overflow or
                underflow. Bit 6 is called
                SF and serves to distinguish
                invalid exceptions caused by
                stack overflow/underflow from
                those caused by numeric
                operations.

Tag Word        When loading the tag word       The corresponding tag is      Software may not operate        Performance improvement.
                with an FLDENV or               checked before each           correctly if it uses FLDENV
                FRSTOR instruction, the         register access to determine  or FRSTOR to change tags
                only interpretations of tag     the class of operand in the   to values (other than
                values used by the 80387        register; the tag is updated  empty) that are different
                are empty (value 11) and        after every change to a       from actual register
                Nonempty (values 00, 01,        register so that the tag      contents.
                and 10). Subsequent             always reflects the most
                operations on a nonempty        recent status of the
                register always examine         register. Programmers can
                the value in the register,      load a tag with a value that
                not the value in its tag.       disagrees with the contents
                The FSTENV and FSAVE            of a register (for example,
                instructions examine the        the register contains valid
                nonempty registers and          contents, but the tag says
                put the correct values in       special; the 80287/8087, in
                the tags before storing the     this case, honors the tag
                tag word.                       and does not examine the
                                                register).

C.4  INSTRUCTION SET

FBSTP, FDIV,    Operation on denormal           Operation on denormal         The exception handler for       IEEE Standard 754
FIST(P), FPREM, operand is supported. An        operand raises                underflow may require           compatibility.
FSQRT           underflow exception can         invalid-operation exception.  change only if it gives
                occur.                          Underflow is not possible.    different treatment to
                                                                              different opcodes.  Possibly
                                                                              fewer invalid-operation
                                                                              exceptions will occur.

FSCALE          The range of the scaling        The range of the scaling      Different result when           Upgrade.
                operand is not restricted.      operand is retricted. If 0 <  0 < �ST(1)� < 1.
                If 0 < �ST(1)� < 1, the         �ST(1)� < 1, the result is
                scaling factor is zero;         undefined and no exception
                therefore, ST(0) remains        is signaled.
                unchanged. If the rounded
                result is not exact or if
                there was a loss of
                accuracy (masked underflow),
                the precision exception
                is signaled.

FPREM1          Performs partial remainder      Does not exist.               None.                           IEEE Standard 754
                according to IEEE                                                                             compatibility and upgrade.
                Standard 754 standard.

FPREM           Bits C0, C3, C1 of the          The quotient bits are         None. Software that works       Upgrade.
                status word, correctly          incorrect when performing a   around the bug should not
                reflect the three low-order     reduction of 64^(N) + M when  be affected.
                bits of the quotient.           N � 1 and M=1 or M=2.


FUCOM, FUCOMP,  Perform unordered               Do not exist.                 None.                           IEEE Standard 754
FUCOMPP         compare according to                                                                          compatibility.
                IEEE Standard 754
                standard.

FPTAN           Range of operand is much        Range of operand is           None.                           Upgrade.
                less restricted (�ST(0)� <      restricted (�ST(0)� < Ò/4);
                2^(63)); reduces operand        operand must be reduced
                internally using an internal    to range using FPREM.
                Ò/4 constant that is more
                accurate.

                After a stack overflow          After a stack overflow                                        IEEE Standard 754
                when the invalid-operation      when the invalid-operation                                    compatibility.
                exception is masked, both       exception is masked, the
                ST and ST(1) contain quiet      original operand remains
                NaNs.                           unchanged, but is pushed
                                                to ST(1).

FSIN, FCOS,     Perform three common            Do not exist.                 None.                           Upgrade.
FSINCOS         trigonometric functions.

FPATAN          Range of operands is            �ST(0)� must be smaller       None.                           Upgrade.
                unrestricted.                   than �ST(1)�.

F2XM1           Wider range of operand          The supported operand         None.                           Upgrade.
                (-1 ¾ ST(0) ¾ +1).              range is 0 ¾ ST(0) ¾ 0.5.

FLD             Does not report denormal        Reports denormal exception.   None.                           Upgrade.
extended-real   exception because the
                instruction is not arithmetic.

FXTRACT         If the operand is zero, the     If the operand is zero,       None. Software usually          IEEE 754 recommendation
                zero-divide exception is        ST(1) is zero and no          bypasses zero and ý.            to fully support the logb
                reported and ST(1) is -ý.       exception is reported. If                                     function.
                If the operand is +ý, no        the operand is +ý, the
                exception is reported.          invalid-operation exception
                                                is reported.

FLD constant    Rounding control is in          Rounding control is not in    Results are the same as         IEEE 754 recommendation.
                effect.                         effect.                       for the 8087/80287 when
                                                                              rounding control is set to
                                                                              round to zero, round to
                                                                              -ý, and (in the case of
                                                                              FLDL2T) round to nearest.
                                                                              Results are different by
                                                                              one in the least significant
                                                                              bit of the significand in
                                                                              round to +ý and round to
                                                                              nearest (excluding
                                                                              FLDL2T). FLD1 and FLDZ
                                                                              are always the same.

FLD             Loading a denormal              Loading a denormal causes     If the next instruction is      IEEE Standard 754
single/double   causes the number to be         the number to be converted    FXTRACT or FXAM, the            compatibility.
precision       converted to extended           to an unnormal.               80387 will give a different
                precision (because it is put                                  result than the 80287/8087.
                on the stack).

FLD             When loading a signaling        Does not raise an             The exception handler           IEEE Standard 754
single/double   NaN, raises invalid exception.  exception when loading a      need to be updated to           compatibility.
precision                                       signaling NaN.                handle this condition.

FSETPM          Treated as FNOP (no             Informs the 80287 that the    None.                           The 80386 handles all
                operation).                     system is in protected                                        addressing and
                                                mode.                                                         exception-pointer information,
                                                                                                              whether in protected mode
                                                                                                              or not.

FXAM            When encountering an            May generate these            None.                           Upgrade, to provide
                empty register, the 80387       combinations, among others.                                   repeatable results.
                will not generate
                combinations of C3-C0 equal to
                1101 or 1111.

All             May generate different          Round-up bit of status        None.                           Upgrade, to signal
Transcendental  results in round-up bit of      word is undefined for these                                   rounding status.
Instructions    status word.                    instructions.


Appendix D  Compatibility Between the 80387 and the 8087

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The 80386/80387 operating in real-address mode will execute 8087 programs
without major modification. However, because of differences in the handling
of numeric exceptions between the 80387 NPX and the 8087 NPX,
exception-handling routines may need to be changed.

This appendix summarizes the additional differences between the 80387 NPX
and the 8087 NPX (other than those already included in Appendix B), and
provides details showing how 8087 programs can be ported to the 80387.

  1.  The 80387 signals exceptions through a dedicated ERROR# line to 
      the 80386; no interrupt controller is needed for this purpose. The
      8087 requires an interrupt controller (8259A) to interrupt the CPU 
      when an unmasked exception occurs. Therefore, any 
      interrupt-controller-oriented instructions in numeric exception
      handlers for the 8087 should be deleted.

  2.  The 8087 instructions FENI/FNENI and FDISI/FNDISI perform no useful
      function in the 80387. If the 80387 encounters one of these opcodes in
      its instruction stream, the instruction will effectively be
      ignored‘‘none of the 80387 internal states will be updated. While 8087
      code containing these instructions may be executed on the 80387, it
      is unlikely that the exception-handling routines containing these
      instructions will be completely portable to the 80387.

  3.  In real mode and protected mode (not including virtual 8086 mode),
      interrupt vector 16 must point to the numeric exception handling
      routine. In virtual 8086 mode, the V86 monitor can be programmed to
      accommodate a different location of the interrupt vector for numeric
      exceptions.

  4.  The ESC instruction address saved in the 80386/80387 or 80386/80287
      includes any leading prefixes before the ESC opcode. The corresponding
      address saved in the 8086/8087 does not include leading prefixes.

  5.  In protected mode (not including virtual 8086 mode), the format of
      the 80387's saved instruction and address pointers is different than
      for the 8087. The instruction opcode is not saved in protected
      mode‘‘exception handlers will have to retrieve the opcode from memory
      if needed.

  6.  Interrupt 7 will occur in the 80386 when executing ESC instructions
      with either TS (task switched) or EM (emulation) of the 80386 MSW set
      (TS=1 or EM=1). If TS is set, then a WAIT instruction will also cause
      interrupt 7. An exception handler should be included in 80387 code to
      handle these situations.

  7.  Interrupt 9 will occur if the second or subsequent words of a
      floating-point operand fall outside a segment's size. Interrupt 13
      will occur if the starting address of a numeric operand falls outside
      a segment's size. An exception handler should be included to report
      these programming errors.

  8.  Except for the processor control instructions, all of the 80387
      numeric instructions are automatically synchronized by the 80386
      CPU‘‘the 80386 automatically waits until all operands have been
      transferred between the 80386 and the 80387 before executing the
      next ESC instruction. No explicit WAIT instructions are required to
      assure this synchronization. For the 8087 used with 8086 and 8088
      processors, explicit WAITs are required before each numeric
      instruction to ensure synchronization. Although 8087 programs having
      explicit WAIT instructions will execute perfectly on the 80387
      without reassembly, these WAIT instructions are unnecessary.

  9.  Since the 80387 does not require WAIT instructions before each
      numeric instruction, the ASM386 assembler does not automatically
      generate these WAIT instructions. The ASM86 assembler, however,
      automatically precedes every ESC instruction with a WAIT
      instruction. Although numeric routines generated using the ASM86
      assembler will generally execute correctly on the 80386/20,
      reassembly using ASM386 may result in a more compact code image and
      faster execution.

      The processor control instructions for the 80387 may be coded using
      either a WAIT or No-WAIT form of mnemonic. The WAIT forms of these
      instructions cause ASM386 to precede the ESC instruction with a CPU
      WAIT instruction, in the identical manner as does ASM86.

  10. The address of a memory operand stored by FSAVE or FSTENV is
      undefined if the previous ESC instruction did not refer to memory.

  11. Because the 80387 automatically normalizes denormal numbers when
      possible, an 8087 program that uses the denormal exception solely to
      normalize denormal operands can run on an 80387 by masking the
      denormal exception. The 8087 denormal exception handler would not be
      used by the 80387 in this case. A numerics program runs faster when
      the 80387 performs normalization of denormal operands. A program can
      detect at run-time whether it is running on an 80387 or 8087/80287 and
      disable the denormal exception when an 80387 is used.


Appendix E  80387 80-Bit CHMOS III Numeric Processor Extension

For Advance Information on the Intel 80387 please consult Appendix E of the
printed version of this book or the 80387 Data Sheet, order number 231920.


Appendix F  PC/AT-Compatible 80387 Connection

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The PC/AT uses a nonstandard scheme to report 80287 exceptions to the
80286. When replicating the PC/AT coprocessor interface in 80386-based
systems, the PC/AT interface cannot be used in exactly the same way;
however, this appendix outlines a similar interface that works on
80386/80387 systems and maintains compatibility with the nonstandard PC/AT
scheme.

Note that the interface outlined here does not represent a new interface
standard; it needs to be incorporated in AT-compatible designs only because
the 80286 and 80287 in the PC/AT are not connected according to the
standards defined by Intel. The standard 80386/80387 connection recommended
by Intel in the 80387 Data Sheet functions properly; the 80386
implementation has not been and will not be altered.


F.1  The PC/AT Interface

In the PC/AT, the ERROR# input to the 80286 is tied inactive (high)
permanently. The ERROR# output of the 80287 is tied to an interrupt port
(IRQ13). This interrupt replaces exception signaling via the 80286's ERROR#
input. To guarantee (in the case of an 80287 exception) that INTR 13 will be
serviced prior to the execution of any further 80287 instructions, an
edge-triggered flip-flop latches BUSY# using ERROR# as a clock. The output
of this latch is ORed with the BUSY# output of the 80287 and drives the
BUSY# input of the 80286. This PC/AT scheme effectively delays deactivation
of BUSY# at the 80286 whenever an 80287 ERROR# is signaled.

Since the 80286 BUSY# input remains active after an exception, the 80286
interrupt 13 handler is guaranteed to execute before any other 80287
instructions may begin. The interrupt 13 handler clears the BUSY# latch (via
a write to a special I/O port), thus allowing execution of 80287
instructions to proceed. The interrupt 13 handler then branches to the NMI
handler, where the user-defined numerics exception handler resides in
PC-compatible systems.

The use of an interrupt guarantees that an exception from a coprocessor
instruction will be detected. Latching BUSY# guarantees that any coprocessor
instruction (except FINIT, FSETPM, and FCLEX) following the instruction that
raised the exception will not be executed before the NMI handler is
executed.

This PC/AT scheme approximates the exception reporting scheme between the
8087 and 8088 in the original PC.


F.2  How to Achieve the Same Effect in an 80386 System

The 80386 can use a PC/AT-compatible interface to communicate with an 80387
provided that, when an NPX exception occurs, BUSY# active time is extended
and PEREQ is reactivated only after 80387 BUSY# has gone inactive. The 80387
is left active (tying STEN high) at all times. Also, the 80386 and 80387
must be reset by the same RESET signal.

The reactivation of PEREQ for the 80386 is needed for store instructions
(for example, FST mem) because the 80387 drops PEREQ once it signals an
exception. While the 80386 has not yet recognized the occurrence of the
exception, it still expects the data transfers to complete via PEREQ
reactivation. It is permissible for the 80386 to receive undefined data
during such I/O read cycles. Disabling the 80387 is not necessary, because
the dummy data-transfer cycles directed to the 80387 when PEREQ is
externally reactivated for the 80386 will not disturb the operation of the
80387. The interrupt 13 handler should remove the extension of BUSY# and
reactivation of PEREQ via a write to PC/AT-compatible hardware at I/O port
F0H.


Glossary of 80387 and Floating-Point Terminology

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This glossary defines many terms that have precise technical meanings as
specified in the IEEE 754 Standard or as specified in this manual. Where
these terms are used, they have been italicized to emphasize the precision
of their meanings. 

Base
  (1) a term used in logarithms and exponentials. In both contexts, it is a
  number that is being raised to a power. The two equations (y = log base b
  of x) and (b^(y) = x) are the same.

Base
  (2) a number that defines the representation being used for a string of
  digits. Base 2 is the binary representation; base 10 is the decimal
  representation; base 16 is the hexadecimal representation. In each case,
  the base is the factor of increased significance for each succeeding
  digit (working up from the bottom).

Bias
  a constant that is added to the true exponent of a real number to obtain
  the exponent field of that number's floating-point representation in the
  80387. To obtain the true exponent, you must subtract the bias from the
  given exponent. For example, the single real format has a bias of 127
  whenever the given exponent is nonzero. If the 8-bit exponent field
  contains 10000011, which is 131, the true exponent is 131-127, or +4.

Biased Exponent
  the exponent as it appears in a floating-point representation of a number.
  The biased exponent is interpreted as an unsigned, positive number. In the
  above example, 131 is the biased exponent.

Binary Coded Decimal
  a method of storing numbers that retains a base 10 representation. Each
  decimal digit occupies 4 full bits (one hexadecimal digit). The
  hexadecimal values A through F (1010 through 1111) are not used. The
  80387 supports a packed decimal format that consists of 9 bytes of binary
  coded decimal (18 decimal digits) and one sign byte.

Binary Point
  an entity just like a decimal point, except that it exists in binary
  numbers. Each binary digit to the right of the binary point is multiplied
  by an increasing negative power of two.

C3‘‘C0
  the four "condition code" bits of the 80387 status word. These bits are
  set to certain values by the compare, test, examine, and remainder 
  functions of the 80387.

Characteristic
  a term used for some non-Intel computers, meaning the exponent field of a
  floating-point number.

Chop
  to set one or more low-order bits of a real number to zero, yielding the
  nearest representable number in the direction of zero.

Condition Code
  the four bits of the 80387 status word that indicate the results of the
  compare, test, examine, and remainder functions of the 80387.

Control Word
  a 16-bit 80387 register that the user can set, to determine the modes of
  computation the 80387 will use and the exception interrupts that will be
  enabled.

Denormal
  a special form of floating-point number. On the 80387, a denormal is
  defined as a number that has a biased exponent of zero. By providing a
  significand with leading zeros, the range of possible negative
  exponents can be extended by the number of bits in the significand.
  Each leading zero is a bit of lost accuracy, so the extended exponent
  range is obtained by reducing significance.

Double Extended
  the Standard's term for the 80387's extended format, with more exponent
  and significand bits than the double format and an explicit integer bit
  in the significand.

Double Format
  a floating-point format supported by the 80387 that consists of a sign, an
  11-bit biased exponent, an implicit integer bit, and a 52-bit
  significand‘‘a total of 64 explicit bits.

Environment
  the 14 or 28 (depending on addressing mode) bytes of 80387 registers
  affected by the FSTENV and FLDENV instructions. It encompasses the entire
  state of the 80387, except for the 8 registers of the 80387 stack.
  Included are the control word, status word, tag word, and the instruction,
  opcode, and operand information provided by interrupts.

Exception
  any of the six conditions (invalid operand, denormal, numeric overflow,
  numeric underflow, zero-divide, and precision) detected by the 80387 that
  may be signaled by status flags or by traps.

Exception Pointers
  The data maintained by the 80386 to help exception handlers identify
  the cause of an exception. This data consists of a pointer to the most
  recently executed ESC instruction and a pointer to the memory operand of
  this instruction, if it had a memory operand. An exception handler can use
  the FSTENV and FSAVE instructions to access these pointers.

Exponent
  (1) any number that indicates the power to which another number is raised.

Exponent
  (2) the field of a floating-point number that indicates the magnitude of
  the number. This would fall under the above more general definition (1),
  except that a bias sometimes needs to be subtracted to obtain the correct
  power.

Extended Format
  the 80387's implementation of the Standard's double extended format.
  Extended format is the main floating-point format used by the 80387.
  It consists of a sign, a 15-bit biased exponent, and a significand with an
  explicit integer bit and 63 fractional-part bits.

Floating-Point
  of or pertaining to a number that is expressed as base, a sign, a
  significand, and a signed exponent. The value of the number is the signed
  product of its significand and the base raised to the power of the
  exponent. Floating-point representations are more versatile than integer
  representations in two ways. First, they include fractions. Second, their
  exponent parts allow a much wider range of magnitude than possible with
  fixed-length integer representations.

Gradual Underflow
  a method of handling the underflow error condition that minimizes the loss
  of accuracy in the result. If there is a denormal number that represents
  the correct result, that denormal is returned. Thus, digits are lost only
  to the extent of denormalization. Most computers return zero when
  underflow occurs, losing all significant digits.

Implicit Integer Bit
  a part of the significand in the single real and double real formats
  that is not explicitly given. In these formats, the entire given
  significand is considered to be to the right of the binary point. A single
  implicit integer bit to the left of the binary point is always one, except
  in one case. When the exponent is the minimum (biased exponent is zero),
  the implicit integer bit is zero.

Indefinite
  a special value that is returned by functions when the inputs are such
  that no other sensible answer is possible. For each floating-point format
  there exists one quiet NaN that is designated as the indefinite value. For
  binary integer formats, the negative number furthest from zero is often
  considered the indefinite value. For the 80387 packed decimal format, the
  indefinite value contains all 1's in the sign byte and the uppermost
  digits byte.

Inexact
  The Standard's term for the 80387's precision exception.

Infinity
  a value that has greater magnitude than any integer or any real number. It
  is often useful to consider infinity as another number, subject to special
  rules of arithmetic. All three Intel floating-point formats provide
  representations for +ý and -ý.

Integer
  a number (positive, negative, or zero) that is finite and has no
  fractional part. Integer can also mean the computer representation for
  such a number: a sequence of data bytes, interpreted in a standard way. It
  is perfectly reasonable for integers to be represented in a floating-point
  format; this is what the 80387 does whenever an integer is pushed onto the
  80387 stack.

Integer Bit
  a part of the significand in floating-point formats. In these formats, the
  integer bit is the only part of the significand considered to be to the
  left of the binary point. The integer bit is always one, except in one
  case: when the exponent is the minimum (biased exponent is zero), the
  integer bit is zero. In the extended format the integer bit is explicit;
  in the single format and double format the integer bit is implicit; i.e.,
  it is not actually stored in memory.

Invalid Operation
  the exception condition for the 80387 that covers all cases not covered by
  other exceptions. Included are 80387 stack overflow and underflow, NaN
  inputs, illegal infinite inputs, out-of-range inputs, and inputs in
  unsupported formats.

Long Integer
  an integer format supported by the 80387 that consists of a 64-bit two's
  complement quantity.

Long Real
  an older term for the 80387's 64-bit double format.

Mantissa
  a term used with some non-Intel computers for the significand of a
  floating-point number.

Masked
  a term that applies to each of the six 80387 exceptions I,D,Z,O,U,P. An
  exception is masked if a corresponding bit in the 80387 control word is
  set to one. If an exception is masked, the 80387 will not generate an
  interrupt when the exception condition occurs; it will instead provide its
  own exception recovery.

Mode
  One of the status word fields "rounding control" and "precision control"
  which programs can set, sense, save, and restore to control the execution
  of subsequent arithmetic operations.

NaN
  an abbreviation for "Not a Number"; a floating-point quantity that does
  not represent any numeric or infinite quantity. NaNs should be returned
  by functions that encounter serious errors. If created during a sequence
  of calculations, they are transmitted to the final answer and can contain
  information about where the error occurred.

Normal
  the representation of a number in a floating-point format in which the
  significand has an integer bit one (either explicit or implicit).

Normalize
  convert a denormal representation of a number to a normal representation.

NPX
  Numeric Processor Extension. This is the 80387, 80287, or 8087.

Overflow
  an exception condition in which the correct answer is finite, but has
  magnitude too great to be represented in the destination format. This kind
  of overflow (also called numeric overflow) is not to be confused with 
  stack overflow.

Packed Decimal
  an integer format supported by the 80387. A packed decimal number is a
  10-byte quantity, with nine bytes of 18 binary coded decimal digits and 
  one byte for the sign.

Pop
  to remove from a stack the last item that was placed on the stack.

Precision
  The effective number of bits in the significand of the floating-point
  representation of a number.

Precision Control
  an option, programmed through the 80387 control word, that allows all 
  80387 arithmetic to be performed with reduced precision. Because no 
  speed advantage results from this option, its only use is for strict 
  compatibility with the standard and with other computer systems.

Precision Exception
  an 80387 exception condition that results when a calculation does not 
  return an exact answer. This exception is usually masked and ignored; it 
  is used only in extremely critical applications, when the user must know 
  if the results are exact. The precision exception is called inexact 
  in the standard.

Pseudozero
  one of a set of special values of the extended real format. The set
  consists of numbers with a zero significand and an exponent that is 
  neither all zeros nor all ones. Pseudozeros are not created by the 80387 
  but are handled correctly when encountered as operands.

Quiet NaN
  a NaN in which the most significant bit of the fractional part of the
  significand is one. By convention, these NaNs can undergo certain
  operations without causing anexception.

Real
  any finite value (negative, positive, or zero) that can be represented by
  a (possibly infinite) decimal expansion. Reals can be represented as the
  points of a line marked off like a ruler. The term real can also refer
  to a floating-point number that represents a real value.

Short Integer
  an integer format supported by the 80387 that consists of a 32-bit two's
  complement quantity. short integer is not the shortest 80387 integer
  format‘‘the 16-bit word integer is.

Short Real
  an older term for the 80387's 32-bit single format.

Signaling NaN
  a NaN that causes an invalid-operation exception whenever it enters into
  a calculation or comparison, even a nonordered comparison.

Significand
  the part of a floating-point number that consists of the most significant
  nonzero bits of the number, if the number were written out in an unlimited
  binary format. The significand is composed of an integer bit and a
  fraction. The integer bit is implicit in the single format and double
  format. The significand is considered to have a binary point after the
  integer bit; the binary point is then moved according to the value of the
  exponent.

Single Extended
  a floating-point format, required by the standard, that provides greater
  precision than single; it also provides an explicit integer bit in the
  significand. The 80387's extended format meets the single extended
  requirement as well as the double extended requirement.

Single Format
  a floating-point format supported by the 80387, which consists of a sign,
  an 8-bit biased exponent, an implicit integer bit, and a 23-bit
  significand‘‘a total of 32 explicit bits.

Stack Fault
  a special case of the invalid-operation exception which is indicated by a
  one in the SF bit of the status word. This condition usually results from
  stack underflow or overflow.

Standard
  "IEEE Standard for Binary Floating-Point Arithmetic," ANSI/IEEE
  Std 754-1985.

Status Word
  A 16-bit 80387 register that can be manually set, but which is usually
  controlled by side effects to 80387 instructions. It contains condition
  codes, the 80387 stack pointer, busy and interrupt bits, and exception
  flags.

Tag Word
  a 16-bit 80387 register that is automatically maintained by the 80387. For
  each space in the 80387 stack, it tells if the space is occupied by a
  number; if so, it gives information about what kind of number.

Temporary Real
  an older term for the 80387's 80-bit extended format.

Tiny
  of or pertaining to a floating-point number that is so close to zero that
  its exponent is smaller than smallest exponent that can be represented in
  the destination format.

TOP
  The three-bit field of the status word that indicates which 80387 register
  is the current top of stack.

Transcendental
  one of a class of functions for which polynomial formulas are always
  approximate, never exact for more than isolated values. The 80387 supports
  trigonometric, exponential, and logarithmic functions; all are
  transcendental.

Two's Complement
  a method of representing integers. If the uppermost bit is zero, the
  number is considered positive, with the value given by the rest of the
  bits. If the uppermost bit is one, the number is negative, with the value
  obtained by subtracting (2^(bit count)) from all the given bits. For
  example, the 8-bit number 11111100 is -4, obtained by subtracting 2^(8)
  from 252.

Unbiased Exponent
  the true value that tells how far and in which direction to move the
  binary point of the significand of a floating-point number. For example,
  if a single-format exponent is 131, we subtract the Bias 127 to obtain the
  unbiased exponent +4. Thus, the real number being represented is the
  significand with the binary point shifted 4 bits to the right.

Underflow
  an exception condition in which the correct answer is nonzero, but has a
  magnitude too small to be represented as a normal number in the
  destination floating-point format. The Standard specifies that an attempt
  be made to represent the number as a denormal. This denormalization may
  result in a loss of significant bits from the significand. This kind of
  underflow (also called numeric overflow) is not to be confused with stack
  underflow.

Unmasked
  a term that applies to each of the six 80387 exceptions: I,D,Z,O,U,P. An
  exception is unmasked if a corresponding bit in the 80387 control word is
  set to zero. If an exception is unmasked, the 80387 will generate an
  interrupt when the exception condition occurs. You can provide an 
  interrupt routine that customizes your exception recovery.

Unnormal
  a extended real representation in which the explicit integer bit of the
  significand is zero and the exponent is nonzero. Unnormal values are
  not supported by the 80387; they cause the invalid-operation exception
  when encountered as operands.

Unsupported Format
  Any number representation that is not recognized by the 80387. This
  includes several formats that are recognized by the 8087 and 80287;
  namely: pseudo-NaN, pseudoinfinity, and unnormal.

Word Integer
  an integer format supported by both the 80386 and the 80387 that consists
  of a 16-bit two's complement quantity.

Zero divide
  an exception condition in which the inputs are finite, but the correct
  answer, even with an unlimited exponent, has infinite magnitude.