* remove "\r" nonsense

[mascara-docs.git] / manuals / openwatcom / devel-docs / docs / intel 287 programmers reference manual.txt

INTEL 80287 PROGRAMMER'S REFERENCE MANUAL 1987

Intel Corporation makes no warranty for the use of its products and
assumes no responsibility for any errors which may appear in this document
nor does it make a commitment to update the information contained herein.

Intel retains the right to make changes to these specifications at any
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Contact your local sales office to obtain the latest specifications before
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iSXM, KEPROM, Library Manager, MAP-NET, MCS, Megachassis, MICROMAINFRAME,
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*MULTIBUS is a patented Intel bus.

Additional copies of this manual or other Intel literature may be obtained
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(c)INTEL CORPORATION 1987   CG-10/86


Preface

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An Introduction to the 80287
This supplement describes the 80287 Numeric Processor Extension (NPX) for
the 80286 microprocessor. Below is a brief overview of 80286 concepts, along
with some of the nomenclature used throughout this and other Intel
publications.


The 80286 Microsystem
The 80286 is a new VLSI microprocessor system with exceptional capabilities
for supporting large-system applications. Based on a new-generation CPU (the
Intel 80286), 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 80286 is a virtual-memory microprocessor with on-chip memory management
and protection. The 80286 microsystem offers a total-solution approach,
enabling you to develop high-speed, interactive, multiuser, multitasking‘‘
and 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 80286 includes a sophisticated and flexible
     four-level protection mechanism that isolates layers of operating
     system programs from application programs to maintain a high degree of
     system integrity.

  Ž  The 80286 provides 16 megabytes of physical address space to support
     today's application requirements. This large physical memory enables
     the 80286 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 80286 CPU provides on-chip memory
     management and virtual memory support. On an 80286-based system, each
     user can have up to a gigabyte (2^(30) bytes) of virtual-address space.
     This large address space virtually eliminates restrictions on the
     number or size of programs that may be part of the system.

  Ž  Large multiuser or real-time multitasking systems are easily supported
     by the 80286. High-performance features, such as a very high-speed
     task switch, fast interrupt-response time, inter-task protection, and a
     quick and direct operating system interface, make the 80286 highly
     suited to multiuser/multitasking applications.

  Ž  The 80286 has two operating modes: Real-Address mode and
     Protected-Address mode. In Real-Address mode, the 80286 is fully
     compatible with the 8086, 8088, 80186, and 80188 microprocessors; all
     of the extensive libraries of 8086 and 8088 software execute four to
     six times faster on the 80286, without any modification.

  Ž  In Protected-Address mode, the advanced memory management and
     protection features of the 80286 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). This compatibility between 80286 and 8086
     processor families reduces both the time and the cost of
     software development.


The Organization of This Manual
This manual describes the 80287 Numeric Processor Extension (NPX) for the
80286 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 One, "Overview of Numeric Processing," gives an overview of
     the 80287 NPX and reviews the concepts of numeric computation using the
     80287.

  Ž  Chapter Two, "Programming Numeric Applications," provides detailed
     information for software designers generating applications for systems
     containing an 80286 CPU with an 80287 NPX. The 80286/80287 instruction
     set mnemonics are explained in detail, along with a description of
     programming facilities for these systems. A comparative 80287
     programming example is given.

  Ž  Chapter Three, "System-Level Numeric Programming," provides
     information of interest to systems software writers, including details
     of the 80287 architecture and operational characteristics.

  Ž  Chapter Four, "Numeric Programming Examples," provides several
     detailed programming examples for the 80287, including conditional
     branching, the conversion between floating-point values and their ASCII
     representations, and the calculation of several trigonometric
     functions. These examples illustrate assembly-language programming on
     the 80287 NPX.

  Ž  Appendix A, "Machine Instruction Encoding and Decoding," gives
     reference information on the encoding of NPX instructions.

  Ž  Appendix B, "Compatability between the 80287 NPX and the 8087,"
     describes the differences between the 80287 and the 8087.

  Ž  Appendix C, "Implementing the IEEE P754 Standard," gives details of
     the IEEE P754 Standard.

  Ž  The Glossary defines 80287 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 80286 systems. The following manuals
contain information related to the content of this supplement and of
interest to programmers of 80287 systems:

  Ž  Introduction to the 80286, order number 210308

  Ž  ASM286 Assembly Language Reference Manual, order number 121924

  Ž  80286 Operating System Writer's Guide, order number 121960

  Ž  80286 Hardware Reference Manual, order number 210760

  Ž  Microprocessor and Peripheral Handbook, order number 210844

  Ž  PL/M-286 User's Guide, order number 121945

  Ž  80287 Support Library Reference Manual, order number 122129

  Ž  8086 Software Toolbox Manual, order number 122203 (includes
     information about 80287 Emulator Software)

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

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

Preface

Chapter 1  Overview of Numeric Processing

Introduction to the 80287 Numeric Processor Extension
    Performance
    Ease of Use
    Applications
    Upgradability
    Programming Interface
    Hardware Interface

80287 Numeric Processor Architecture
    The NPX Register Stack
    The NPX Status Word
    Control Word
    The NPX Tag Word
    The NPX Instruction and Data Pointers

Computation Fundamentals
    Number System
    Data Types and Formats
        Binary Integers
        Decimal Integers
        Real Numbers
    Rounding Control
    Precision Control
    Infinity Control

Special Computational Situations
    Special Numeric Values
        Nonnormal Real Numbers
            Denormals and Gradual Underflow
            Unnormals‘‘Descendents of Denormal Operands
        Zeros and Pseudo Zeros
        Infinity
        NaN (Not a Number)
        Indefinite
        Encoding of Data Types
    Numeric Exceptions
        Invalid Operation
        Zero Divisor
        Denormalized Operand
        Numeric Overflow and Underflow
        Inexact Result
        Handling Numeric Errors
            Automatic Exception Handling
            Software Exception Handling

Chapter 2  Programming Numeric Applications

The 80287 NPX Instruction Set
    Compatibility with the 8087 NPX
    Numeric Operands
    Data Transfer Instructions
    Arithmetic Instructions
    Comparison Instructions
    Transcendental Instructions
    Constant Instructions
    Processor Control Instructions
    Instruction Set Reference Information
        Instruction Execution Time
        Bus Transfers
        Instruction Length

Programming Facilities
    High-Level Languages
    PL/M-286
    ASM286
        Defining Data
        Records and Structures
        Addressing Modes
    Comparative Programming Example
    80287 Emulation

Concurrent Processing with the 80287
    Managing Concurrency
    Instruction Synchronization
    Data Synchronization
    Error Synchronization
        Incorrect Error Synchronization
        Proper Error Synchronization

Chapter 3  System-Level Numeric Programming

80287 Architecture
    Processor Extension Data Channel
    Real-Address Mode and Protected Virtual-Address Mode
    Dedicated and Reserved I/O Locations

Processor Initialization and Control
     System Initialization
     Recognizing the 80287 NPX
     Configuring the Numerics Environment
     Initializing the 80287
     80287 Emulation
     Handling Numeric Processing Exceptions
     Simultaneous Exception Response
     Exception Recovery Examples

Chapter 4  Numeric Programming Examples

Conditional Branching Examples

Exception Handling Examples

Floating-Point to ASCII Conversion Examples
    Function Partitioning
    Exception Considerations
    Special Instructions
    Description of Operation
    Scaling the Value
        Inaccuracy in Scaling
        Avoiding Underflow and Overflow
        Final Adjustments
    Output Format

Trigonometric Calculation Examples
    FPTAN and FPREM
    Cosine Uses Sine Code

Appendix A  Machine Instriction Encoding and Decoding

Appendix B  Compatibility Between the 80287 NPX and the 8087

Appendix C  Implementing The IEEE P754 Standard

    Options implemented in the 80287
    Areas of the Standard Implemented in Software
    Additional Software to Meet the Standard

Glossary of 80287 and Floating-Point Terminology

Index


Figures

1-1      Evolution and Performance of Numeric Processors
1-2      80287 NPX Block Diagram
1-3      80287 Register Set
1-4      80287 Status Word
1-5      80287 Control Word Format
1-6      80287 Tag Word Format
1-7      80287 Instruction and Data Pointer Image in Memory
1-8      80287 Number System
1-9      Data Formats
1-10     Projective versus Affine Closure
1-11     Arithmetic Example Using Infinity

2-1      FSAVE/FRSTOR Memory Layout
2-2      FSTENV/FLDENV Memory Layout
2-3      Sample 80287 Constants
2-4      Status Word RECORD Definition
2-5      Structure Definition
2-6      Sample PL/M-286 Program
2-7      Sample ASM286 Program
2-8      Instructions and Register Stack
2-9      Synchronizing References to Shared Data
2-10     Documenting Data Synchronization
2-11     Nonconcurrent FIST Instruction Code Macro
2-12     Error Synchronization Examples

3-1      Software Routine to Recognize the 80287

4-1      Conditional Branching for Compares
4-2      Conditional Branching for FXAM
4-3      Full-State Exception Handler
4-4      Reduced-Latency Exception Handler
4-5      Reentrant Exception Handler
4-6      Floating-Point to ASCII Conversion Routine
4-7      Calculating Trigonometric Functions


Tables

1-1      Numeric Processing Speed Comparisons
1-2      Numeric Data Types
1-3      Principal NPX Instructions
1-4      Interpreting the NPX Condition Codes
1-5      Real Number Notation
1-6      Rounding Modes
1-7      Denormalization Process
1-8      Exceptions Due to Denormal Operands
1-9      Unnormal Operands and Results
1-10     Zero Operands and Results
1-11     Masked Overflow Response with Directed Rounding
1-12     Infinity Operands and Results
1-13     Binary Integer Encodings
1-14     Packed Decimal Encodings
1-15     Real and Long Real Encodings
1-16     Temporary Real Encodings
1-17     Exception Conditions and Masked Responses

2-1      Data Transfer Instructions
2-2      Arithmetic Instructions
2-3      Basic Arithmetic Instructions and Operands
2-4      Condition Code Interpretation after FPREM
2-5      Comparison Instructions
2-6      Condition Code Interpretation after FCOM
2-7      Condition Code Interpretation after FTST
2-8      FXAM Condition Code Settings
2-9      Transcendental Instructions
2-10     Constant Instructions
2-11     Processor Control Instructions
2-12     Key to Operand Types
2-13     Execution Penalties
2-14     Instruction Set Reference Data
2-15     PL/M-286 Built-In Procedures
2-16     80287 Storage Allocation Directives
2-17     Addressing Mode Examples

3-1      NPX Processor State Following Initialization
3-2      Precedence of NPX Exceptions

A-1      80287 Instruction Encoding
A-2      Machine Instruction Decoding Guide


Chapter 1  Overview of Numeric Processing

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The 80287 NPX is a high-performance numerics processing element that
extends the 80286 architecture by adding significant numeric capabilities
and direct support for floating-point, extended-integer, and BCD data types.
The 80286 CPU with 80287 NPX easily supports powerful and accurate numeric
applications through its implementation of the proposed IEEE 754 Standard
for Binary Floating-Point Arithmetic.


Introduction to the 80287 Numeric Processor Extension
The 80287 Numeric Processor Extension (NPX) is highly compatible with its
predecessor, the earlier Intel 8087 NPX.

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 the proposed IEEE 754 Floating-Point Standard.

With the 80287 Numeric Processor Extension, high-speed numeric computations
have been extended to 80286 high-performance multi-tasking and multi-user
systems. Multiple tasks using the numeric processor extension are afforded
the full protection of the 80286 memory management and protection features.

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


Figure 1-1.  Evolution and Performance of Numeric Processors

    DOUBLE-PRECISION       \x1e                ’‘‘‘‘‘‘‘‘‘‘‘‘‘“
       WHETSTONE           �                � 80286/80287 �
      PERFORMANCE          �                ”‘‘‘‘‘‘\a‘‘‘‘‘‘•
        (KOPS)        200 ‘š‘                  \a
                           �               \a
                           �     ’‘‘‘‘‘\a‘‘‘‘‘“
                           �     � 8086/8087 �
                           �     ”‘‘‘‘‘‘‘‘‘‘‘•
                           �
                      100 ‘š‘
                           �
                           �
                           �
                           �
                           ”‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘\x10
                                       1980         1983

                                       YEAR INTRODUCED


Performance
Table 1-1 compares the execution times of several 80287 instructions with
the equivalent operations executed in software on an 8-MHz 80286. The
software equivalents are highly-optimized assembly-language procedures from
the 80287 emulator. As indicated in the table, the 80287 NPX provides about
50 to 100 times the performance of software numeric routines on the 80286
CPU. An 8-MHz 80287 multiplies 32-bit and 64-bit real numbers in about 11.9
and 16.9 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 80287 also manipulates fixed-point binary
and decimal integers of up to 64 bits or 18 digits, respectively. The 80287
can improve the speed of multiple-precision software algorithms for integer
operations by 10 to 100 times.

Because the 80287 NPX is an extension of the 80286 CPU, no software
overhead is incurred in setting up the NPX for computation. The 80287 and
80286 processors coordinate their activities in a manner transparent to
software. Moreover, built-in coordination facilities allow the 80286 CPU to
proceed with other instructions while the 80287 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: 8 MHz 80287 to
                                                  8 MHz Protected Mode iAPX
’‘‘‘‘‘‘ Floating-Point Instruction ‘‘‘‘‘‘‘‘‘‘‘‘“  using E80287

FADD ST,ST (Temp Real)             Addition                1: 42
FDIV DWORD PTR (Single-Precision)  Division                1:266
FXAM (Stack(0) assumed)            Examine                 1:139
FYL2X (Stack(0),(1) assumed)       Logarithm               1: 99
FPATAN (Stack(0) assumed)          Arctangent              1:153
F2XM1 (Stack (0) assumed)          Exponentiation          1: 41


Ease of Use
The 80287 NPX offers more than raw execution speed for
computation-intensive tasks. The 80287 brings the functionality and power
of accurate numeric computation into the hands of the general user.

Like the 8087 NPX that preceded it, the 80287 is explicitly designed to
deliver stable, accurate results when programmed using straightforward
"pencil and paper" algorithms. The IEEE 754 standard 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 80287
delivers the correctly rounded result. Other typical examples of
undesirable machine behavior in straightforward calculations occur when
solving for the roots of a quadratic equation:

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

for computing financial rate of return, which involves the expression:
(1+i)^(n). 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 80287. This simple fact greatly reduces the software investment required
to develop safe, accurate computation-based products.

Beyond traditional numerics support for scientific applications, the 80287
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 two models of infinity,
directed rounding, gradual underflow, and either automatic or 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. By
default, on-chip exception handlers may be invoked to field 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 whenever various types of exceptions are
detected.


Applications
The NPX'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 80287:

  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 80287 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 80287 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. The advent of the
80287 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 80287's exponentiation and logarithmic
     instructions.

  Ž  Process control‘‘The 80287 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 80287's speed can be exploited in
     real-time operations.

  Ž  Computer numerical control (CNC)‘‘The 80287 can move and position
     machine tool heads with accuracy in real-time. Axis positioning also
     benefits from the hardware trigonometric support provided by the 80287.

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

  Ž  Navigation‘‘Very small, lightweight, and accurate inertial guidance
     systems can be implemented with the 80287. Its built-in trigonometric
     functions can speed and simplify the calculation of position from

  Ž  Graphics terminals‘‘The 80287 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 82720 Graphics Display Controller to perform high speed
     data transfers, very powerful and highly self-sufficient terminals can
     be built from a relatively small number of 80286 family parts.

  Ž  Data acquisition‘‘The 80287 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 80287 to
advantage. Indeed, the 80287 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. 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.


Upgradability
The architecture of the 80286 CPU is specifically adapted to allow easy
upgradability to use an 80287, simply by plugging in the 80287 NPX. For this
reason, designers of 80286 systems may wish to incorporate the 80287 NPX
into their designs in order to offer two levels of price and performance at
little additional cost.

Two features of the 80286 CPU make the design and support of upgradable
80286 systems particularly simple:

  Ž  The 80286 can be programmed to recognize the presence of an 80287 NPX;
     that is, software can recognize whether it is running on an 80286 or an
     80287 system.

  Ž  After determining whether the 80287 NPX is available, the 80286 CPU
     can be instructed to let the NPX execute all numeric instructions. If
     an 80287 NPX is not available, the 80286 CPU can emulate all 80287
     numeric instructions in software. This emulation is completely
     transparent to the application software‘‘the same object code may be
     used by both 80286 and 80287 systems. No relinking or recompiling of
     application software is necessary; the same code will simply execute
     faster on the 80287 than on the 80286 system.

To facilitate this design of upgradable 80286 systems, Intel provides a
software emulator for the 80287 that provides the functional equivalent of
the 80287 hardware, implemented in software on the 80286. Except for
timing, the operation of this 80287 emulator (E80287) is the same as for
the 80287 NPX hardware. When the emulator is combined as part of the
systems software, the 80286 system with 80287 emulation and the 80286 with
80287 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.


Programming Interface
The 80286/80287 pair is programmed as a single processor; all of the 80287
registers appear to a programmer as extensions of the basic 80286 register
set. The 80286 has a class of instructions known as ESCAPE instructions, all
having a common format. These ESC instructions are numeric instructions for
the 80287 NPX. These numeric instructions for the 80287 are simply encoded
into the instruction stream along with 80286 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 are extended to the NPX as well.

Numeric processing in the 80287 centers around the NPX register stack.
Programmers can treat these eight 80-bit registers as either a fixed
register set, with instructions operating on explicitly-designated
registers, or a classical stack, with instructions operating on the top one
or two stack elements.

Internally, the 80287 holds all numbers in a uniform 80-bit temporary-real
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 temporary-real 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 80287,
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 even memory addresses.

Table 1-3 lists the 80287 instructions by class. No special programming
tools are necessary to use the 80287, because all of the NPX instructions
and data types are directly supported by the ASM286 Assembler and Intel's
appropriate high-level languages.

Software routines for the 80287 may be written in ASM286 Assembler or any
of the following higher-level languages:

  PL/M-286
  PASCAL-286
  FORTRAN-286
  C-286

In addition, all of the development tools supporting the 8086 and 8087 can
also be used to develop software for the 80286 and 80287 operating in
Real-Address mode.

All of these high-level languages provide programmers with access to the
computational power and speed of the 80287 without requiring an
understanding of the architecture of the 80286 and 80287 chips. Such
architectural considerations as concurrency and data synchronization are
handled automatically by these high-level languages. For the ASM286
programmer, specific rules for handling these issues are discussed in a
later section of this supplement.


Table 1-2. Numeric Data Types

                        Significant
Data Type        Bits     Digits     Approximate Range (Decimal)
                         (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)
Short real        32        6-7     8.43*10^(-37) ¾ �X� ¾ 3.37*10^(38)
Long real         64        15-16   4.19*10^(-307) ¾ �X� ¾ 1.67*10^(308)
Temporary real    80        19      3.4*10^(-4932) ¾ �X� ¾ 1.2*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, 2^(X) -1, Y*Log{2}(X+1), Y*Log{2}(X)

Constants        0, 1, Ò, Log{10}2, Log{e}2, Log{2}10, Log2{e}

Processor        Load Control Word, Store Control Word, Store Status Word,

Control          Load Environment, Store Environment, Save, Restore, Clear
                 Exceptions, Initialize, Set Protected Mode


Hardware Interface
As an extension of the 80286 processor, the 80287 is wired very much in
parallel with the 80286 CPU. Four special status signals, PEREQ, PEACK,
BUSY, and ERROR, permit the two processors to coordinate their
activities. The 80287 NPX also monitors the 80286 S1, S0,
COD/INTA, READY, HLDA, and CLK pins to monitor the execution of
ESC instructions (numeric instructions) by the 80286.

As shown in figure 1-2, the 80287 NPX is divided internally into two
processing elements; the Bus Interface Unit (BIU) and the Numeric Execution
Unit (NEU). The two units operate independently of one another: the BIU
receives and decodes instructions, requests operand transfers with memory,
and executes processor control instructions, whereas the NEU processes
individual numeric instructions.

The BIU handles all of the status and signal lines between the 80287 and
the 80286. The NEU executes all instructions that involve the register
stack. These instructions include arithmetic, logical, transcendental,
constant, and data transfer instructions. The data path in the NEU is 84
bits wide (68 fraction bits, 15 exponent bits, and a sign bit), allowing
internal operand transfers to be performed at very high speeds.

The 80287 executes a single numeric instruction at a time. Before executing
most ESC instructions, the 80286 tests the BUSY pin and, before initiating
the command, waits until the 80287 indicates that it is not busy. Once
initiated, the 80286 continues program execution, while the 80287 executes
the numeric instruction. Unlike the 8087, which required a WAIT instruction
to test the BUSY signal before each ESC opcode, these WAIT instructions are
permissible, but not necessary, in 80287 programs.

In all cases, a WAIT or ESC instruction should be inserted after any 80287
store to memory (except FSTSW or FSTCW) or load from memory (except FLDENV,
FLDCW, or FRSTOR) before the 80286 reads or changes the memory value.

When needed, all data transfers between memory and the 80287 NPX are
performed by the 80286 CPU, using its Processor Extension Data Channel.
Numeric data transfers performed by the 80286 use the same timing as any
other bus cycle, and all such transfers come under the supervision of the
80286 memory management and protection mechanisms. The 80286 Processor
Extension Data Channel and the hardware interface between the 80286 and
80287 processors are described in Chapter Six of the 80286 Hardware
Reference Manual.

From the programmer's perspective, the 80287 can be considered just an
extension of the 80286 processor. All interaction between the 80286 and the
80287 processors on the hardware level is handled automatically by the 80286
and is transparent to the software.

To communicate with the 80287, the 80286 uses the reserved I/O port
addresses 00F8H, 00FAH, and 00FCH (I/O ports numbered 00F8H through 00FFH
are reserved for the 80286/80287 interface). These I/O operations are
performed automatically by the 80286 and are distinct from I/O operations
that result from program I/O instructions. I/O operations resulting from
the execution of ESC instructions are completely transparent to software.
Any program may execute ESCAPE (numeric) instructions, without regard to its
current I/O Privilege Level (IOPL).

To guarantee correct operation of the 80287, programs must not perform any
explicit I/O operations to any of the eight ports reserved for the 80287.
The IOPL of the 80286 can be used to protect the integrity of 80287
computations in multiuser reprogrammable applications, preventing any
accidental or other tampering with the 80287 (see Chapter Eight of the 80286
Operating System Writer's Guide).


Figure 1-2.  80287 NPX Block Diagram

       ’ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘˜‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ “
          BUS INTERFACE UNIT                  NUMERIC EXECUTION UNIT
       �                      �                                                    �
          ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘“             EXPONENT BUS \x1e      \x1e FRACTION BUS
       �  � CONTROL WORD �    �     ’‘‘‘‘‘‘‘‘‘‘“    €      €     ’‘‘‘‘‘‘‘‘‘‘‘‘‘“   �
          –‘‘‘‘‘‘‘‘‘‘‘‘‘‘—          � EXPONENT �\x11��\x10€INTER-€\x11���\x10//PROGRAMMABLE//
       �  � STATUS WORD  �    �     �  MODULE  �    € FACE €     //   SHIFTER  //  �
          ”‘‘‘‘‘\x1e‘‘‘‘‘‘‘‘•          ”‘‘‘‘‘‘‘‘‘‘•    €\x11�/��\x10€     ”‘‘‘‘‘‘‘\x1e‘‘‘‘‘•
       �        €             �  ’‘‘‘‘‘‘‘‘‘‘‘‘“     €  16  €\x11������������…         �
                €       NEU      � MICROCODE  �  16 /      €\x11�����ƒ ’‘‘‘‘‘‘‘‘‘‘‘‘“
       �    ’‘‘‘\x1f‘‘‘‘“INSTRUCTION�CONTROL UNIT�     €      / 68   „\x10� ARITHMETIC � �
            �        ã����������\x10”‘‘‘‘‘‘‘‘‘‘‘‘•‚����‡      €\x11������ƒ�   MODULE   �
       �    �  DATA  �        �        ‚�������… 16 /      / 64    €”‘‘‘‘‘\x1e‘‘‘‘‘‘• �
Data\x11������\x10� BUFFER �                 € ’‘‘“       €      €\x11����ƒ „������…
       �    �        �    ’‘‘‘�‘‘‘‘‘‘“ € � ’\x1f“’‘‘‘‘‘\x1f‘‘‘‘‘‘\x1f“    €   ’‘‘‘‘‘‘‘‘‘‘‘“ �
            �        �\x11��\x10� OPERANDS �\x11… � �T��             �(7) „��\x10� TEMPORARY �
       �    ”‘‘‘\x1e‘‘‘‘•    �  QUEUE   �   � �A��             � ·      � REGISTERS � �
                €\x11‘‘‘‘‘“  ”‘‘‘�‘‘‘‘‘‘•   � �G��             � ·      ”‘‘‘‘‘‘‘‘‘‘‘•
       �  ’‘‘‘‘‘\x1f‘‘‘“  �                 � � ��  REGISTER   � ·                    �
Status\x11���\x10 CONTROL �  ”‘‘‘‘‘‘�‘‘‘‘‘‘‘‘‘‘• �W��   STACK     �
       �  �         �                      �O��             � ·                    �
Address\x11��\x10  UNIT   �         �            �R��             �
       �  ”‘‘‘‘‘‘‘‘‘•                      �D��\x11‘ 80 BITS ‘\x10�(0)                   �
                              �            ”‘•”‘‘‘‘‘‘‘‘‘‘‘‘‘•
       ” ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘™‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ ‘ •


80287 Numeric Processor Architecture
To the programmer, the 80287 NPX appears as a set of additional registers
complementing those of the 80286. These additional registers consist of

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

  Ž  Three sixteen-bit registers containing:
       an NPX status word
       an NPX control word
       a tag word

  Ž  Four 16-bit registers containing the NPX instruction and data pointers

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


The NPX Register Stack
The 80287 register stack is shown in figure 1-3. Each of the eight numeric
registers in the 80287's register stack is 80 bits wide and is divided into
fields corresponding to the NPX's temporary-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 ST (Stack Top) field in the NPX status word. Load or push
operations decrement ST by one and load a value into the new top register.
A store-and-pop operation stores the value from the current ST register and
then increments ST by one. Like 80286 stacks in memory, the 80287 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 ST. The ASM286 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 ST in the stack (0 ¾ i ¾ 7). For example, if ST contains 011B
(register 3 is the top of the stack), the following statement would add the
contents of the top two registers on 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 ST may, for example, refer to physical
register 3 in one invocation and physical register 5 in another.


Figure 1-3.  80287 Register Set

                             80287 STACK:                          TAG FIELD
      79  78    64 63                                           0   1    0
 R1 ‚����Ð��������Ð����������������������������������������������ƒ ‚������ƒ
    €SIGN�EXPONENT�                  SIGNIFICAND                 € €      €
 R2 Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘Â
 R3 Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘Â
 R4 Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘Â
 R5 Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘Â
 R6 Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘Â
 R7 Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘Â
 R8 Ñ‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ Ñ‘‘‘‘‘Â
    „����¤��������¤����������������������������������������������… „������…

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


The NPX Status Word
The 16-bit status word shown in figure 1-4 reflects the overall state of
the 80287. This status word may be stored into memory using the
FSTSW/FNSTSW, FSTENV/FNSTENV, and FSAVE/FNSAVE instructions, and can be
transferred into the 80286 AX register with the FSTSW AX/FNSTSW AX
instructions, allowing the NPX status to be inspected by the CPU.

The Busy bit (bit 15) and the BUSY pin indicate whether the 80287's
execution unit is idle (B = 0) or is executing a numeric instruction or
signalling an exception (B = 1). (The instructions FNSTSW, FNSTSW AX,
FNSTENV, and FNSAVE do not set the Busy bit themselves, nor do they require
the Busy bit to be clear in order to execute.)

The four NPX condition code bits (C{0}-C{3}) are similar to the flags in a
CPU: the 80287 updates these bits to reflect the outcome of arithmetic
operations. The effect of these instructions on the condition code bits is
summarized in table 1-4. These condition code bits are used principally for
conditional branching. The FSTSWAX instruction stores the NPX status word
directly into the CPU AX register, allowing these condition codes to be
inspected efficiently by 80286 code.

Bits 12-14 of the status word point to the 80287 register that is the
current Stack Top (ST). The significance of the stack top has been described
in the section on the Register Stack.

Figure 1-4 shows the six error flags in bits 0-5 of the status word. Bit 7
is the error 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.


Table 1-4.  Interpreting the NPX Condition Codes


Instruction
Type            C{3}  C{2}  C{1}  C{0}   Interpretation

Compare, Test     0     0     X
X = value is not affected by instruction     0    ST
ST = Top of stack > Source or 0 (FTST)
                  0     0     X
X = value is not affected by instruction     1    ST
ST = Top of stack < Source or 0 (FTST)
                  1     0     X
X = value is not affected by instruction     0    ST
ST = Top of stack = Source or 0 (FTST)
                  1     1     X
X = value is not affected by instruction     1    ST
ST = Top of stack is not comparable

Remainder         Q{1}
Q{n} = Quotient bit n following complete reduction (C{2}=0) 0   Q{0}
Q{n} = Quotient bit n following complete reduction (C{2}=0) Q{2}
Q{n} = Quotient bit n following complete reduction (C{2}=0)  Complete reduction with three
                                         low bits of quotient in C{0},
                                         C{3}, and C{1}
                  U
U = value is undefined following instruction    1     U
U = value is undefined following instruction    U
U = value is undefined following instruction   Incomplete Reduction

Examine           0     0     0     0    Valid, positive unnormalized
                  0     0     0     1    Invalid, positive, exponent = 0
                  0     0     1     0    Valid, negative, unnormalized
                  0     0     1     1    Invalid, negative, exponent = 0
                  0     1     0     0    Valid, positive, normalized
                  0     1     0     1    Infinity, positive
                  0     1     1     0    Valid, negative, normalized
                  0     1     1     1    Infinity, negative
                  1     0     0     0    Zero, positive
                  1     0     0     1    Empty Register
                  1     0     1     0    Zero, negative
                  1     0     1     1    Empty Register
                  1     1     0     0    Invalid, positive, exponent = 0
                  1     1     0     1    Empty Register
                  1     1     1     0    Invalid, negative, exponent = 0
                  1     1     1     1    Empty Register



Figure 1-4.  80287 Status Word

  15                                     0
 ‚�Ð��Ð���Ð��Ð��Ð��Ð��Ð�Ð��Ð��Ð��Ð��Ð��Ð��ƒ            EXCEPTION FLAGS
 €B�C3�S T�C2�C1�C0�ES�X�PE�UE�OE�ZE�DE�IE€                (1 = EXCEPTION
 „Ф�ФÐÐФ�Ф�Ф�Ф�ФФ�Ф�Ф�Ф�Ф�Ф�Ð…                 HAS OCCURRED)
  �  � ���  �  �  �  � �  �  �  �  �  �  ”‘‘‘‘‘‘‘‘‘‘‘  INVALID OPERATION
For definitions, see the section on exception handling.
  �  � ���  �  �  �  � �  �  �  �  �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘  DENORMALIZED OPERAND
For definitions, see the section on exception handling.
  �  � ���  �  �  �  � �  �  �  �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  ZERO DIVIDE
For definitions, see the section on exception handling.
  �  � ���  �  �  �  � �  �  �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  OVERFLOW
For definitions, see the section on exception handling.
  �  � ���  �  �  �  � �  �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  UNDERFLOW
For definitions, see the section on exception handling.
  �  � ���  �  �  �  � �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  PRECISION
For definitions, see the section on exception handling.
  �  � ���  �  �  �  � ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  (RESERVED)
  �  � ���  �  �  �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  ERROR SUMMARY STATUS
ES is set if any unmasked exception bit is set, cleared or otherwise.
  �  ”‘���‘‘™‘‘™‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  CONDITION CODE
See Table 1-4 for condition code interpretation.
  �    ”™™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘  STACK TOP POINTER
S T VALUES:
      000 = REGISTER 0 IS TOP OF STACK
      001 = REGISTER 1 IS TOP OF STACK
       ¨             ¨
       ¨             ¨
      111 = REGISTER 7 IS TOP OF STACK
  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ NEU BUSY


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 1-5
shows the format and encoding of the fields in the control word.

The low-order byte of this control word configures the 80287 error and
exception masking. Bits 0-5 of the control word contain individual masks for
each of the six exception conditions recognized by the 80287. The high-order
byte of the control word configures the 80287 processing options, including

  Ž  Precision control
  Ž  Rounding control
  Ž  Infinity control

The Precision control bits (bits 8-9) can be used to set the 80287 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
80287, as required by the IEEE 754 standard. Setting a lower precision,
however, will not affect the execution time of numeric calculations.

The rounding control bits (bits 10-11) provide for directed rounding and
true chop as well as the unbiased round-to-nearest-even mode specified in
the IEEE 754 standard.

The infinity control bit (bit 12) determines the manner in which the 80287
treats the special values of infinity. Either affine closure (where positive
infinity is distinct from negative infinity) or projective closure (infinity
is treated as a single unsigned quantity) may be specified. These two
alternative views of infinity are discussed in the section on Computation
Fundamentals.


Figure 1-5.  80287 Control Word Format

 ‚�����Ð���Ð���Ð���Ð�Ð�Ð��Ð��Ð��Ð��Ð��Ð��ƒ
 €X X X�I C�R C�P C�X�X�PM�UM�OM�ZM�DM�IM€    EXCEPTION MASKS
 „Ð�Ð�Ф�Ð�¤Ð�ФÐ�ФФФ�Ф�Ф�Ф�Ф�Ф�Ð…      (1 = EXCEPTION IS MASKED)
  � � �  �  � � � � � �  �  �  �  �  �  ”‘‘‘‘     INVALID OPERATION
  � � �  �  � � � � � �  �  �  �  �  ”‘‘‘‘‘‘‘     DENORMALIZED OPERAND
  � � �  �  � � � � � �  �  �  �  ”‘‘‘‘‘‘‘‘‘‘     ZERO DIVIDE
  � � �  �  � � � � � �  �  �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘     OVERFLOW
  � � �  �  � � � � � �  �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘     UNDERFLOW
  � � �  �  � � � � � �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘     PRECISION
  � � �  �  � � � � � ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ (RESERVED)
  � � �  �  � � � � ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ (RESERVED)
  � � �  �  � � ”‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ PRECISION CONTROL
PRECISION CONTROL
00 = 24-BIT SIGNIFICAND
01 = RESERVED
10 = 53-BIT SIGNIFICAND
11 = 64-BIT SIGNIFICAND
  � � �  �  ”‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ ROUNDING CONTROL
ROUNDING CONTROL
00 = ROUND TO NEAREST OR EVEN
01 = ROUND DOWN (TOWARD -ý)
10 = ROUND UP (TOWARD +ý)
11 = CHOP (TRUNCATE TOWARD ZERO)
  � � �  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ INFINITY CONTROL
  � � �                                         (0 = PROJECTIVE, 1 = AFFINE)
  ”‘™‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘ (RESERVED)


The NPX Tag Word
The tag word indicates the contents of each register in the register stack,
as shown in figure 1-6. The tag word is used by the NPX itself in order to
track its numeric registers and optimize performance. Programmers may use
this tag information to interpret the contents of the numeric registers.
The tag values are stored in the tag word corresponding to the physical
registers 0-7. Programmers must use the current Stack Top (ST) pointer
stored in the NPX status word to associate these tag values with the
relative stack registers ST(0) through ST(7).


Figure 1-6.  80287 Tag Word Format

  15                                                                    0
 ‚��������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


The NPX Instruction and Data Pointers
The NPX instruction and data registers provide support for programmed
exception-handlers. Whenever the 80287 executes a math instruction, the NPX
internally saves the instruction address, the operand address (if present),
and the instruction opcode. The 80287 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.

When stored in memory, the instruction and data pointers appear in one of
two formats, depending on the operating mode of the 80287. Figure 1-7 shows
these pointers as they are stored following an FSTENV instruction. In
Real-Address mode, these values are the 20-bit physical address and 11-bit
opcode formatted like the 8087. In Protected mode, these values are the
32-bit virtual addresses used by the program that executed the ESC
instruction.

The instruction address saved in the 80287 will point to any prefixes that
preceded the instruction. This is different from the 8087, for which the
instruction address pointed only to the ESC instruction opcode.


Figure 1-7.  80287 Instruction and Data Pointer Image in Memory

                                                       MEMORY OFFSET
                      ‚�������������������������������ƒ
   REAL MODE          €         CONTROL WORD          € +0
                      Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €          STATUS WORD          € +2
                      Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €           TAG WORD            € +4
                      Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €   INSTRUCTION POINTER(15-0)   € +6
                      Ñ‘‘‘‘‘‘‘‘‘‘˜‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €INSTRUCTION� �   INSTRUCTION   €
                      €  POINTER  �0�     OPCODE      € +8
                      €  (19-16)  � �     (10-0)      €
                      Ñ‘‘‘‘‘‘‘‘‘‘™‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €      DATA POINTER(15-0)       € +10
                      Ñ‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €   DATA    �                   €
                      €  POINTER  �         0         € +12
                      €  (19-16)  �                   €
                      „�����������¤�������������������…
                       15       12 11                0

                                                       MEMORY OFFSET
                      ‚�������������������������������ƒ
   PROTECTED MODE     €         CONTROL WORD          € +0
                      Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €          STATUS WORD          € +2
                      Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €           TAG WORD            € +4
                      Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €           IP OFFSET           € +6
                      Ñ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘Â
                      €          CS SELECTOR          € +8
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                      €      DATA OPERAND OFFSET      € +10
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Computation Fundamentals
This section covers 80287 programming concepts that are common to all
applications. It describes the 80287's internal number system and the
various types of numbers that can be employed in NPX programs. The most
commonly used options for rounding, precision, and infinity (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.


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 is always an infinity of
numbers both larger and smaller. There is also an infinity of 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 1-8 superimposes the basic 80287 real number system on a real number
line (decimal numbers are shown for clarity, although the 80287 actually
represents numbers in binary). The dots indicate the subset of real numbers
the 80287 can represent as data and final results of calculations. The
80287's range is approximately ±4.19*10^(-307) to ±1.67*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 370 is about
±0.54*10^(-78) to ±0.72*10^(76).

The finite spacing in figure 1-8 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 80287 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 80287 can accommodate (e.g., a 20-digit
number) is represented with some loss of accuracy. Notice also that the
80287'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 80287 actually employs a number system that
is a substantial superset of that shown in figure 1-8. The internal format
(called temporary real) extends the 80287's range to about ±3.4*10^(-4932)
to ±1.2*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 80287'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 80287 does perform exact arithmetic
on integer operands. That is, an operation on two integers returns an exact
integral result, provided that the true result is an integer and is in
range. 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 1-8.  80287 Number System

|\x11‘‘‘NEGATIVE RANGE (NORMALIZED)‘‘\x10|
|                                  |
|             -5  -4  -3  -2  -1   |
’‘‘‘˜‘‘‘˜‘“’‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘“
�   �   � ��›››�›››�œœœ�œœœ���������
”‘‘‘™‘‘‘™‘•”‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘•
\x1e                                  \x1e
�                                  �   ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
”-1.67*10^(308)     -4.19*10^(-307)•   � ‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘       �
                                       � �������������œœœœœœœœœœœœœœ       �
                                       � �������������œœœœœœœœœœœœœœ       �
|\x11‘‘‘POSITIVE RANGE (NORMALIZED)‘‘\x10|   � ‘‘‘‘\a‘‘‘‘‘‘‘\a‘‘‘‘‘‘‘\a‘‘‘‘‘‘       �
|                                  |   �     \x1e       \x1e                     �
|   1   2   3   4   5              |   �     �\x11‘‘˜‘‘\x10�                     �
’‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘˜‘‘‘“’‘˜‘‘‘˜‘‘‘“   �     �   �   ” 2.00000000000000000 �
���������œœœ�œœœ�›››�›››�� �   �   �   �     �   �                         �
”‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘•”‘™‘‘‘™‘‘‘•   �     �   ”‘‘‘‘ (NOT REPRESENTABLE) �
\x1e     ”‘˜‘•                        \x1e   �     �                             �
�       ”‘‘‘‘‘‘‘‘“                 �   �     ”‘‘‘‘‘‘‘‘ 1.99999999999999999 �
�                �    1.67*10^(308)•   �   PRECISION:  �\x11‘‘ 18 DIGITS ‘‘\x10� �
”4.19*10^(-307)  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•


Data Types and Formats
The 80287 recognizes seven numeric data types, 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 1-9 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. Table 1-5 provides the
range and number of signficant (decimal) digits that each format can
accommodate.


Table 1-5.  Real Number Notation

’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
�     Notation      �                         Value                        �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Ordinary Decimal   �  178.125                                             �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Scientific Decimal �  1{\x1e}78125E2                                         �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Scientific Binary  �  1{\x1e)0110010001E111                                  �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Scientific Binary  �  1{\x1e}0110010001E10000110                             �
�(Biased Exponent)  �                                                      �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�                   �  Sign     Biased Exponent    Significand             �
�                   –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�80287 Short Real   �                                                      �
�(Normalized)       �  0        10000110        \x1e  01100100010000000000000 �
�                   �                           ”‘‘‘‘1{\x1e}(implicit)        �
”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•


Figure 1-9.  Data Formats

                 \x11‘‘‘‘‘‘‘‘ INCREASING SIGNIFICANCE
           ‚�Ð���������ƒ
    WORD   €S�MAGNITUDE€ (TWO'S
 INTEGER   „�¤���������… COMPLEMENT)
            15        0
           ‚�Ð����������������������ƒ
   SHORT   €S�      MAGNITUDE       € (TWO'S
 INTEGER   „�¤����������������������… COMPLEMENT)
            31                     0
           ‚�Ð������������������������������������������������ƒ
    LONG   €S�                   MAGNITUDE                    € (TWO'S
 INTEGER   „�¤������������������������������������������������… COMPLEMENT)
            63                                               0
           ‚�Ð���Ð������������������������MAGNITUDE������������������������ƒ
  PACKED   €S� X �d17 d16   \a  \a  \a   d11 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0€
 DECIMAL   „�¤���¤���¤���¤�����������¤���¤���¤��¤��¤��¤��¤��¤��¤��¤��¤��¤��…
            79   72                                                       0
           ‚�Ð��������Ð�������������ƒ
   SHORT   €S� BIASED � SIGNIFICAND €
    REAL   € �EXPONENT�             €
           „�¤������� 
Position of implicit binary point. Integer bit of significand; stored in
temporary real, implicit (always 1) in short and long real ������������…
           31        22             0
           ‚�Ð������������Ð����������������������������������ƒ
    LONG   €S�   BIASED   �           SIGNIFICAND            €
    REAL   € �  EXPONENT  �                                  €
           „�¤����������� 
Position of implicit binary point. Integer bit of significand; stored in
temporary real, implicit (always 1) in short and long real ���������������������������������…
           63             51                                  0
           ‚�Ð�������������Ð�Ð���������������������������������������������ƒ
 TEMPORARY €S�   BIASED    �1�                  SIGNIFICAND                €
 REAL      € �  EXPONENT   � �                                             €
           „�¤�������������¤ 
Position of implicit binary point ��������������������������������������������…
           79            64 63                                             0

 NOTES:
    S  = Sign bit (0 = positive, 1 = negative)
    dn = Decimal digit (two per byte)
    X  = Bits have no significance;
         80287 ignores when loading, zeros when storing.
    Exponent Bias (normal values):
         Short Real:  127  (7FH)
         Long Real:  1023  (3FFH)
         Temporary Real:  16383  (3FFFH)


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 80287 format to use two's complement). The quantity
zero is represented with a positive sign (all bits are 0). The 80287 word
integer format is identical to the 16-bit signed integer data type of the
80286.


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 0H-9H.


Real Numbers
The 80287 stores real numbers in a three-field binary format that resembles
scientific, or exponential, notation. The number's significant digits are
held in the significand field, the exponent field locates the binary point
within the significant digits (and therefore determines the number's
magnitude), and the sign field indicates whether the number is positive or
negative. (The exponent and significand are analogous to the terms
"characteristic" and "mantissa" used to describe floating point numbers on
some computers.) Negative numbers differ from positive numbers only in the
sign bits of their significands.

Table 1-5 shows how the real number 178.125 (decimal) is stored in the
80287 short 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 ASM286 and PL/M-286 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 is an integer
and a fraction 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 short, 52 for long, and 63 for
temporary real. By normalizing real numbers so that their integer bit is
always a 1, the 80287 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 short and
long real formats, the integer bit is implicit and is not actually stored;
the integer bit is physically present in the temporary real format only.

If one were to examine only the signficand with its assumed binary point,
all normalized real numbers would have values between 1 and 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 80287
stores exponents in a biased form. This means that a constant is added to
the true exponent described above. The value of this bias is different for
each real format (see figure 1-9). 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 short and long real formats exist in memory only. If a number in one of
these formats is loaded into an 80287 register, it is automatically
converted to temporary real, the format used for all internal operations.
Likewise, data in registers can be converted to short or long real for
storage in memory. The temporary real format may be used in memory also,
typically to store intermediate results that cannot be held in registers.

Most applications should use the long real form to store real number data
and results; it provides sufficient range and precision to return correct
results with a minimum of programmer attention. The short 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 debugging algorithms, because roundoff problems will
manifest themselves more quickly in this format. The temporary 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 long real form are
adequate for most microcomputer applications.


Rounding Control
Internally, the 80287 employs three extra bits (guard, round, and sticky
bits) that enable it to represent 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 80287
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 1-5). Given a true result b that cannot be represented by
the target data type, the 80287 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 1-6. Round introduces an error in a result that is
less than one unit in the last place to which the result is rounded. "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. The chop mode is provided for integer arithmetic applications.

"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.


Table 1-6. Rounding Modes

’‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
�RC Field� Rounding Mode          �  Rounding Action                       �
–‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�   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 representable, b is not.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


Precision Control
The 80287 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
temporary-real format. The other settings are required by the proposed IEEE
standard, and are provided to obtain compatibility with the specifications
of certain existing programming languages. Specifying less precision
nullifies the advantages of the temporary real format's extended fraction
length, and does not increase execution speed. When reduced precision is
specified, the rounding of the fractional value clears the unused bits on
the right to zeros.


Infinity Control
The 80287's system of real numbers may be closed by either of two models of
infinity. These two means of closing the number system, projective and
affine closure, are illustrated schematically in figure 1-10. The setting
of the IC field in the control word selects one model or the other. The
default means of closure is projective, and this is recommended for most
computations. When projective closure is selected, the NPX treats the
special values +ý and -ý as a single unsigned infinity (similar to its
treatment of signed zeros). In the affine mode the NPX respects the signs
of +ý and -ý.

While affine mode may provide more information than projective, there are
occasions when the sign may in fact represent misinformation. For example,
consider an algorithm that yields an intermediate result x of +0 and -0 (the
same numeric value) in different executions. If 1/x were then computed in
affine mode, two entirely different values (+ý and -ý) would result from
numerically identical values of x. Projective mode, on the other hand,
provides less information but never returns misinformation. In general,
then, projective mode should be used globally, with affine mode reserved
for local computations where the programmer can take advantage of the sign
and knows for certain that the nature of the computations will not produce a
misleading result.


Figure 1-10.  Projective versus Affine Closure

             PROJECTIVE CLOSURE                AFFINE CLOSURE

                     ý
               ’‘‘‘‘\x10\a\x11‘‘‘‘“
               �           �
             - �           � +            -ý      -       +     +ý
               �           �              \a\x11‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘\x10\a
               ”‘‘‘‘‘š‘‘‘‘‘•                          0
                     0


Special Computational Situations
Besides being able to represent positive and negative numbers, the 80287
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 80287 successfully. This section describes the special
values that may occur in certain cases and the significance of each. The
80286 exceptions are also described, for writers of exception handlers and
for those interested in probing the limits of computation using the 80287.

The material presented in this section is mainly of interest to programmers
concerned with writing exception handlers. For many readers, this section
can be browsed lightly.


Special Numeric Values
The 80287 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  Non-normal real numbers, including
       denormals
       unnormals

  Ž  Zeros and pseudo zeros

  Ž  Positive and negative infinity

  Ž  NaN (Not-a-Number)

  Ž  Indefinite

The following description explains the origins and significance of each of
these special values. Tables 1-12 through 1-15 at the end of this
section show how each of these special values is encoded for each of the
numeric data types.


Nonnormal Real Numbers
As described previously, the 80287 generally stores nonzero real numbers in
normalized floating-point form; that is, the integer (leading) bit of the
significand is always a 1. This bit is explicitly stored in the temporary
real format, and is implicitly assumed to be a one (1{\x1e}) in the short- and
long-real 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 floating-point numeric value becomes very close to zero, normalized
storage cannot be used to express the value accurately. To accommodate these
instances, the 80287 can store and operate on reals that are not normalized,
i.e., whose significands contain one or more leading zeros. Nonnormals
typically arise when the result of a calculation yields a value that is too
small to be represented in normal form.

Nonnormal values can exist in one of two forms:

  Ž  The floating-point exponent may be stored at its most negative value
     (a Denormal),

  Ž  The integer bit (and perhaps other leading bits) of the significand
     may be zero (an Unnormal).

The leading zeros of nonnormals permit smaller numbers to be represented,
at the 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 temporary real format for holding intermediate, values as
small as ±3.4*10^(-4932) can be represented; this makes the occurrence of
nonnormal numbers a rare phenomenon in 80287 applications. Nevertheless, the
NPX can load, store, and operate on nonnormalized real numbers when they do
occur.


Denormals and Gradual Underflow
A denormal is the result of the NPX's response to an underflow exception
when that exception has been masked by the programmer (see the 80287 control
word, figure 1-5). Underflow occurs when the absolute value of a real
number becomes too small to be represented in the destination format, that
is, when the exponent of the true result is too negative to be represented
in the destination format. For example, a true exponent of -130 will cause
underflow if the destination is short real, because -126 is the smallest
exponent this format can accommodate. No underflow would occur if the
destination were long real or temporary real, since these formats can handle
exponents down to -1023 and -16,383, respectively.

Most computers underflow "abruptly:" they simply return a zero result,
which is likely to produce an unacceptable final result if computation
continues. The 80287, on the other hand, underflows "gradually" when the
underflow exception is masked. Gradual underflow is accomplished by
denormalizing the result until it is just within the exponent range of the
destination format. 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 short-real, long-real, or temporary-real
formats. Table 1-7 illustrates how a result might be denormalized to fit a
short-real destination.

The intent of the 80287's masked response to underflow is to allow
computation to continue without program intervention, while introducing an
error that carries about the same risk of contaminating the final result as
roundoff error. Roundoff (precision) errors occur frequently in real number
calculations; sometimes they spoil the result of computation, but often they
do not. Recognizing that roundoff errors are often nonfatal, computation
usually proceeds, and the programmer inspects the final results to see if
these errors have had a significant effect. The 80287's masked underflow
response allows programmers to treat underflows in a similar manner; the
computation continues and the programmer can examine the final result to
determine if an underflow has had important consequences. (If the underflow
has had a significant effect, an invalid operation will probably be
signalled later in the computation.)

Denormalization produces 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 1-14 and
1-15 later in this chapter show how denormal values are encoded in each of
the real data formats.

The denormalization process may cause the loss of low-order significand
bits as they are shifted off the right. 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 such. However, this is a
comparatively rare occurrence and, in any case, is no worse than "abrupt"
underflow.

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 short or long 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 temporary real memory variables.
If storage limitations force the use of short or long reals for
intermediates, and small values are produced, underflow may occur, and, if
masked, may generate denormals.

Accessing a denormal may produce an exception as shown in table 1-8. (The
denormalized exception signals that a denormal has been fetched.) Denormals
may have reduced significance due to lost low-order bits, and an option of
the proposed IEEE standard precludes operations on nonnormalized operands.
This option may be implemented in the form of an exception handler that
responds to unmasked denormalized 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.

As table 1-8 shows, the division and remainder operations do not accept
denormal divisors and raise the invalid operation exception. Recall also
that the transcendental instructions require normalized operands and do not
check for exceptions. In all other cases, the NPX converts denormals to
unnormals, and the rules governing unnormal arithmetic then apply
(unnormals are described in the following section).


Table 1-7. Denormalization Process

’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
�Operation         �  Sign  �  Exponent
Expressed as unbiased, decimal number �    Significand            �
�‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�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
Before storing, significand is rounded to 24 bits, integer bit is
dropped, and exponent is biased by adding 126  �    0   �    -126    �    0{\x1e}00101011100...00   �
”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•



Table 1-8. Exceptions Due to Denormal Operands

’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
�Operation                    � Exception �   Masked Response            �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�FLD (short/long real)        �   D       �  Load as equivalent unnormal �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Arithmetic (except following)�   D       �  Convert (in a work area)    �
�                             �           �  denormal to equivalent      �
�                             �           �  unnormal and proceed        �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Compare and test             �   D       �  Convert (in a work area)    �
�                             �           �  denormal to equivalent      �
�                             �           �  unnormal and proceed        �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Division or FPREM with       �   I       �  Return real indefinite      �
�denormal divisor             �           �                              �
”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•


Unnormals‘‘Descendents of Denormal Operands
An unnormal is the result of a computation using denormal operands and is
therefore the descendent of the 80287's masked underflow response. An
unnormal may exist only in the temporary real format; it may have any
exponent that a normal value may have (that is, in biased form any nonzero
value), but it is distinguished from a normal by the integer bit of its
significand, which is always 0. An unnormal in a register is tagged valid.
Unnormals are distinct from denormals, which have an exponent of 00...00 in
biased form.

Unnormals allows arithmetic to continue following an underflow while still
retaining their identity as numbers that may have reduced significance. That
is, unnormal operands generate unnormal results, so long as their
unnormality has a significant effect on the result. Unnormals are thus
prevented from "masquerading" as normals, numbers that have full
significance. On the other hand, if an unnormal has an insignificant effect
on a calculation with a normal, the result will be normal. For example,
adding a small unnormal to a large normal yields a normal result. The
converse situation yields an unnormal.

Table 1-9 shows how the instruction set deals with unnormal operands. Note
that the unnormal may be the original operand or a temporary created by the
80287 from a denormal.


Table 1-9. Unnormal Operands and Results


Operation                          Result

Addition/subtraction               Normalization of operand with larger
                                   abosolute value determines normalization
                                   of result.

Multiplication                     If either operand is unnormal, result
                                   is unormal.

Division (unnormal dividend only)  Result is unnormal.

FPREM (unnormal dividend only)     Result if normalized.

Division/FPREM (unnormal           Signal invalid operation.
                                   divisor)

Compare/FTST                       Normalize as much as possible before
                                   making comparison.

FRNDINT                            Normalize as much as possible before
                                   rounding.

FSQRT                              Signal invalid operation.

FST, FSTP (short/long real         If value is above destination's underflow
destination)                       boundary, then signal invalid operation;
                                   else signal underflow.

FSTP (temporary real destination)  Store as usual.

FIST, FISTP, FBSTP                 Signal invalid operation.

FLD                                Load as usual.

FXCH                               Exchange as usual.

Transcendental instructions        Undefined; operands must be normal and
                                   are not checked.


Zeros and Pseudo 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.

The zeros discussed above are called true zeros; if one of them is loaded
or generated in a register, the register is tagged zero. Table 1-10 lists
the results of instructions executed with zero operands and also shows how a
true zero may be created from nonzero operands.

Only the temporary real format may contain a special class of values called
pseudo zeros. A pseudo zero is an unnormal whose significand is all zeros,
but whose (biased) exponent is nonzero (true zeros have a zero exponent).
Neither is a pseudo zero's exponent all ones, since this encoding is
reserved for infinities and NANs. A pseudo zero result will be produced if
two unnormals, containing a total of more than 64 leading zero bits in their
significands, are multiplied together. This is a remote possibility in most
applications, but it can happen.

Pseudo zero operands behave like unnormals, except in the following cases
where they produce the same results as true zeros:

  Ž  Compare and test instructions

  Ž  FRNDINT (round to integer)

  Ž  Division, where the dividend is either a true zero or a pseudo zero
     (the divisor is a pseudo zero)

In addition and subtraction of a pseudo zero and a true zero or another
pseudo zero, the pseudo zero(s) behaves like unnormals, except for the
determination of the result's sign. The sign is determined as shown in table
1-10 for two true zero operands.


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. The
significand distinguishes infinities from NANs, including real indefinite.

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 when
rounding is up or down, the masked response may create the largest valid
value representable in the destination rather than infinity. See table 1-11
for details. As operands, infinities behave somewhat differently depending
on how the infinity control field in the control word is set (see table
1-12). When the projective model of infinity is selected, the infinities
behave as a single unsigned representation; because of this, infinity
cannot be compared with any value except infinity. In affine mode, the signs
of the infinities are observed, and comparisons are possible.


Table 1-10. Zero Operands and Results


Operation/Operands          Result              Operation/Operands      Result

FLD, FBLD
Arithmetic and compare operations with binary integers interpret the
integer sign in the same manner.                                       Division
 +0                         +0                   ±0 ÷ ±0                Invalid operation
 -0                         -0                   ±X ÷ ±0                Zerodivide
FILD
Arithmetic and compare operations with binary integers interpret the
integer sign in the same manner.                                            +0 ÷ +X, -0 ÷ -X       +0
 +0                         +0                   +0 ÷ -X, -0 ÷ +X       -0
FST, FSTP                                        -X ÷ -Y, +X ÷ +Y       +0, underflow
Very small X and very large Y may yield zero, after rounding of true
result. NPX signals underflow to warn that zero has been yielded from
nonzero operands.




 +0                         +0                   -X ÷ +Y, +X ÷ -Y       -0, underflow
Very small X and very large Y may yield zero, after rounding of true
result. NPX signals underflow to warn that zero has been yielded from
nonzero operands.




 -0                         -0
 +X
Severe underflows in storing to short or long real may generate zeros                        +0                  FPREM
 -X
Severe underflows in storing to short or long real may generate zeros                        -0                   ±0 rem ±0              Invalid operation
FBSTP                                            ±X rem ±0              Invalid operation
 +0                         +0                   +0 rem +X, +0 rem -X   +0
 -0                         -0                   -0 rem +X, -0 rem -X   -0
FIST, FISTP                                      +X rem +Y, +X rem -Y   +0
When Y divides into X exactly




 +0                         +0                   -X rem -Y, -X rem +Y   -0
When Y divides into X exactly




 -0                         +0
 +X
Small values (�X� < 1) stored into integers may round to zero                        +0                  FSQRT
 -X
Small values (�X� < 1) stored into integers may round to zero                        +0                   -0                     -0
                                                 +0                     +0
Addition
 +0 plus +0                 +0                  Compare
 -0 plus -0                 -0                   ±0: +X                 A < B
 +0 plus -0, -0 plus +0     *0
Sign is determined by round mode:
   * = + for nearest, up, or chop
   * = - for down                  ±0: ±0                 A = B
 -X plus +X, +X plus -X     *0
Sign is determined by round mode:
   * = + for nearest, up, or chop
   * = - for down                  ±0: -X                 A > B
 ±0 plus ±X, ±X plus ±0     šX
š = sign of X




                                                FTST
Subtraction                                      ±0                     Zero
 +0 minus -0                +0                  FCHS
 -0 minus +0                -0                   +0                     -0
 +0 minus +0, -0 minus -0   *0
Sign is determined by round mode:
   * = + for nearest, up, or chop
   * = - for down                  -0                     +0
 +X minus +X, -X minus -X   *0
Sign is determined by round mode:
   * = + for nearest, up, or chop
   * = - for down                 FABS
 ±0 minus ±X, ±X minus ±0   šX
š = sign of X                  ±0                     +0
                                                F2XM1
Multiplication                                   +0                     +0
 +0 * +0, -0 * -0           +0                   -0                     -0
 +0 * -0, -0 * +0           -0                  FRNDINT
 +0 * +X, +X * +0           +0                   +0                     +0
 +0 * -X, -X * +0           -0                   -0                     -0
 -0 * +X, +X * -0           -0                  FXTRACT
 -0 * -X, -X * -0           +0                   +0                     Both +0
 +X * +Y, -X * -Y           +0, underflow
Very small values of X and Y may yield zeros, after rounding of true
result. NPX signals underflow to warn that zero has been yielded by
nonzero operands.       -0                     Both -0
 +X * -Y, -X * +Y           -0, underflow
Very small values of X and Y may yield zeros, after rounding of true
result. NPX signals underflow to warn that zero has been yielded by
nonzero operands.





NaN (Not a Number)
A NaN (Not a Number) is a member of a class of special values that exist 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.

The 80287 will generate the special NaN, 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
programmer-created values.

Whenever the NPX uses an operand that is a NaN, it signals an invalid
operation exception in its status word. If this exception is masked in the
80287 control word, the 80287's masked exception response is to return the
NaN as the operation result. If both operands of an instruction are NaNs,
the result is the NaN with the larger absolute value. In this way, a NaN
that enters a computation propagates through the computation and will
eventually be delivered as the final result. Note, however, that the
transcendental instructions do not check their operands, and a NaN will
produce an undefined result.

By unmasking the invalid operation exception, the programmer can use 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 NaNs as references to uninitialized
(real) array elements. The compiler could preinitialize each array element
with a 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.

NaNs could also be used 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 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 1-11. Masked Overflow Response with Directed Rounding

’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
�    True Result    �          �                                           �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘— Rounding �         Result Delivered                  �
�Normalization �Sign�   Mode   �                                           �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘š‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Normal        � +  �  Up      � +ý                                        �
�Normal        � +  �  Down    � Largest finite positive number
The largest valid representable reals are encoded:
  exponent: 11...10B
  significand: (1){\x1e}11...10B           �
�Normal        � -  �  Up      � Largest finite negative number
The largest valid representable reals are encoded:
  exponent: 11...10B
  significand: (1){\x1e}11...10B           �
�Normal        � -  �  Down    � -ý                                        �
�Unnormal      � +  �  Up      � +ý                                        �
�Unnormal      � -  �  Down    � Largest exponent, result's significand
The significand retains its identity as an unnormal; the true
result is rounded as usual (effectively chopped toward 0 in this
case). The exponent is encoded 11...10B.   �
�Unnormal      � +  �  Up      � Largest exponent, result's significand
The significand retains its identity as an unnormal; the true
result is rounded as usual (effectively chopped toward 0 in this
case). The exponent is encoded 11...10B.   �
�Unnormal      � -  �  Down    � -ý                                        �
”‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘™‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•


Table 1-12. Infinity Operands and Results


Key to symbols used in this table
X = zero or nonzero operand
Y = nonzero operand
* = sign of original operand
š = sign is complement of original operand's sign
Þ = sign is "exclusive or" original operand signs (+ if operands had
    same sign, - if operands had different signs)


Operation           Projective Result               Affine Result

Addition
 +ý plus +ý         Invalid operation               +ý
 -ý plus -ý         Invalid operation               -ý
 +ý plus -ý         Invalid operation               Invalid operation
 -ý plus +ý         Invalid operation               Invalid operation
 ±ý plus ±X         *ý                              *ý
 ±X plus ±ý         *ý                              *ý

Subtraction
 +ý minus -ý        Invalid operation               +ý
 -ý minus +ý        Invalid operation               -ý
 +ý minus +ý        Invalid operation               Invalid operation
 -ý minus -ý        Invalid operation               Invalid operation
 ±ý minus ±X        *ý                              *ý
 ±X minus ±ý        šý                              šý

Multiplication
 ±ý * ±ý            Þ                               Þ
 ±ý * ±Y            Þ                               Þ
 ±0 * ±ý, ±ý * ±0   Invalid operation               Invalid operation

Division
 ±ý ÷ ±ý            Invalid operation               Invalid operation
 ±ý ÷ ±X            Þ                               Þ
 ±X ÷ ±ý            Þ                               Þ

FSQRT
 -ý                 Invalid operation               Invalid operation
 +ý                 Invalid operation               +ý

FPREM
 ±ý rem ±ý          Invalid operation               Invalid operation
 ±ý rem ±X          Invalid operation               Invalid operation
 ±Y rem ±ý          *Y                              *Y
 ±0 rem ±ý          *0                              *0

FRNDINT
 ±ý                 *ý                              *ý

FSCALE
 ±ý scaled by ±ý    Invalid operation               Invalid operation
 ±ý scaled by ±X    *ý                              *ý
 ±0 scaled by ±ý    *0                              *0
 ±Y scaled by ±     Invalid operation               Invalid operation

FXTRACT
 ±ý                 Invalid operation               Invalid operation

Compare
 ±ý: ±ý             A = B                           -ý < +ý
 ±ý: ±Y             A ? B (and) invalid operation   -ý < Y < +ý
 ±ý: ±0             A ? B (and) invalid operation   -ý < 0 < +ý

FTST
 ±ý                 A ? B (and) invalid operation   *ý


Indefinite
For every 80287 numeric data type, one unique encoding is reserved for
representing the special value indefinite. The 80287 produces this encoding
as its response to a masked invalid-operation exception. In the case of
reals, the indefinite value can be stored and loaded like any NaN, and it
always retains its special identity; programmers are advised not to use this
encoding for any other purpose. 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. 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
80287 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 packed decimal or binary integer.


Encoding of Data Types
Tables 1-13 through 1-16 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.


Table 1-13. 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 80287 interprets it as the largest
negative number representable in the format: -2^(15), -2^(31), or -2^(63).
The 80287 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 1-14. 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 1-15. Real and Long Real Encodings


                                          Biased      Significand
Integer bit is implied and not stored


            Class                Sign     Exponent    {\x1e}ff...ff
   ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �                  NaNs        0       11...11     11...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
   �      �                       ¨          ¨           ¨
   �    NaNs                      ¨          ¨           ¨
   �      �                       ¨          ¨           ¨
   �      �         ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �      �           Indefinite  1       11...11     10...00
   �      ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
   �                              ¨          ¨           ¨
   �                              ¨          ¨           ¨
   �                              ¨          ¨           ¨
   �                              1       11...11     11...11
   ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                              Short:  � ‘‘‘8 bits‘‘ � ‘‘23 bits‘‘ �
                               Long:  � ‘‘11 bits‘‘ � ‘‘52 bits‘‘ �


Table 1-16. Temporary Real Encodings


                                        Biased     Significand
Integer bit is implied and not stored


            Class              Sign     Exponent   1{\x1e}ff...ff
    ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �         NaNs              0       11...11    111...11
    �                           ¨          ¨          ¨
    �                           ¨          ¨          ¨
    �                           ¨          ¨          ¨
    �                           0       11...11    100...01
    �      ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �         ý                 0       11...11    100...00
    �      ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    0       11...10    Normals
    �      �                    ¨          ¨       111...11
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨       100...00
    �      �                    ¨          ¨     ‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Positives  �                    ¨          ¨       Unnormals
    �      �                    ¨          ¨       011...11
    �    Reals                  ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    0       00...01    000...00
    �      �                                     ‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                                       Denormals
    �      �                    0       00...00    011...11
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    0       00...00    000...01
    �      –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �        Zero        0       00...00    000...00
    ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �        Zero        1       00...00    000...00
    �      –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                                       Denormals
    �      �                    1       00...00    000...01
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    1       00...00    011...11
    �      �                                     ‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    1       00...01    Unnormals
    �      �                    ¨          ¨       000...00
    �      �                    ¨          ¨          ¨
    �    Reals                  ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨       011...11
    �      �                    ¨          ¨
    �      �                                     ‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Negatives  �                    ¨          ¨       Normals
    �      �                    ¨          ¨       100...00
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    1       11...10    11111...11
    �      ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �         ý                 1       11...11    100...00
    �      ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    1       11...11    100...00
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �         ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �     NaNs      Indefinite  1       11...11    110...00
    �      �         ‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    ¨          ¨          ¨
    �      �                    1       11...11    111...11
    ”‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                     �‘‘15 bits‘‘�‘‘64 bits‘‘�


Numeric Exceptions
Whenever the 80287 NPX attempts a numeric operation with invalid operands
or produces a result that cannot be represented, the 80287 recognizes a
numeric exception condition. Altogether, the 80287 checks for the following
six classes of exceptions while executing numeric instructions:

  1.  Invalid operation
  2.  Divide-by-zero
  3.  Denormalized operand
  4.  Numeric overflow
  5.  Numeric underflow
  6.  Inexact result (precision)


Invalid Operation
The 80287 reports an invalid operation if any of the following occurs:

  Ž  An attempt to load a register that is not empty (stack overflow).

  Ž  An attempt to pop an operand from an empty register (stack underflow).

  Ž  An operand is a NaN.

  Ž  The operands cause the operation to be indeterminate (square root of a
     negative number, 0/0).

An invalid operation generally indicates a program error.


Zero Divisor
If an instruction attempts to divide a finite nonzero operand by zero, the
80287 will report a zero divide exception.


Denormalized Operand
If an instruction attempts to operate on a denormal, the NPX reports the
denormalized operand exception. This exception allows users to implement in
software an option of the proposed IEEE standard specifying that operands
must be prenormalized before they are used.


Numeric Overflow and Underflow
If the exponent of a numeric result is too large for the destination real
format, the 80287 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 temporary real format (recommended as the
destination format for intermediates), overflow and underflow are
relatively rare events in most 80287 applications.


Inexact Result
If the result of an operation is not exactly representable in the
destination format, the 80287 rounds the number and reports the precision
exception. 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; it is provided for
applications that need to perform exact arithmetic only.


Handling Numeric Errors
When numeric errors occur, the NPX takes one of two possible courses of
action:

  Ž  The NPX can itself handle the error, 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
     error.

Each of the six exception conditions described above has a corresponding
flag bit in the 80287 status word and a mask bit in the 80287 control word.
If an exception is masked (the corresponding mask bit in the control
word = 1), the 80287 takes an appropriate default action and continues with
the computation. If the exception is unmasked (mask = 0), the 80287 asserts
the ERROR output to the 80286 to signal the exception and invoke a
software exception handler.

The NPX reports an exception by setting the corresponding flag in the NPX
status word to 1. The NPX then checks the corresponding exception mask in
the control word to determine if it should "field" the exception (mask = 1),
or if it should signal the exception to the CPU to invoke a software
exception handler (mask = 0).

If the mask is set, the exception is said to be masked (from user
software), and the NPX executes its on-chip masked response for that
exception. If the mask is not set (mask = 0), the exception is unmasked,
and the NPX performs its unmasked response. The masked response always
produces a standard result, then proceeds with the instruction. The unmasked
response always traps to a software exception handler, allowing the CPU to
recognize and take action on the exception. Table 1-17 gives a complete
description of all exception conditions and the NPX's masked response.

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 80287
could detect a denormalized operand, perform its masked response to this
exception, and then detect an underflow.


Table 1-17. Exception Conditions and Masked Responses


‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
      Condition                                         Masked Response
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                             Invalid Operation
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Source register is tagged empty        Return real indefinite.
(usually due to stack underflow).

Destination register is not tagged     Return real indefinite
empty (usually due to stack            (overwrite destination value).
overflow).

One or both operands is a NaN.         Return NaN with larger absolute
                                       value (ignore signs).

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

(Addition operations only): closure    Return real indefinite.
is affine and operands are
opposite-signed infinities; or
closure is projective and both
operands are ý (signs immaterial).

(Subtraction operations only):         Return real indefinite.
closure is affine and operands are
like-signed infinities; or closure
is projective and both operands are
ý (signs immaterial).

(Multiplication operations only):      Return real indefinite.
ý * 0; or 0 * ý.

(Division operations only):            Return real indefinite.
ý ÷ ý; or 0 ÷ 0; or 0 ÷ pseudo
zero; or divisor is denormal
or unormal.

(FPREM instruction only): modulus      Return real indefinite, set
(divisor) is unnormal or denormal;     condition code = "complete
or dividend is ý.                      remainder."

(FSQRT instruction only): operand      Return real indefinite.
is nonzero and negative; or operand
is denormal or unnormal; or closure
is affine and operand is -ý; or
closure is projective and operand
is ý.

(Compare operations only): closure     Set condition code = "not
is projective and ý is being           comparable."
compared with 0, a normal or ý.

(FTST instruction only): closure is    Set condition code = "not
projective and operand is ý.           comparable."

(FIST, FISTP instructions only):       Store integer indefinite.
source register is empty, a NaN,
denormal, unnormal, ý, or exceeds
representable range of destination.

(FBSTP instruction only): source       Stored packed decimal
register is empty, a NaN, denormal,    indefinite.
unnormal, ý, or exceeds 18 decimal
digits.

(FST, FSTP instructions only):         Store real indefinite.
destination is short or long real
and source register is an unnormal
with exponent in range.

(FXCH instruction only): one or        Change empty register(s) to
both registers is tagged empty.        real indefinite and then
                                       perform exchange.



‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
      Condition                                         Masked Response
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                             Denormalized Operand
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
(FLD instruction only): source         No special action; load as usual.
operand is denormal.

(Arithmetic operations only): one      Convert (in a work area) the
or both operands is denormal.          operand to the equivalent unnormal
                                       and proceed.

(Compare and test operations only):    Convert (in a work area) any
one or both operands is denormal       denormal to the equivalent
or unnormal other than pseudo          unnormal; normalize as much as
zero).                                 possible, and proceed with
                                       operation.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                 Zero Divide
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
(Division operations only):            Return ý signed with "exclusive or"
divisor = 0.                           of operand signs.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                  Overflow
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
(Arithmetic operations only):          Return properly signed ý and signal
rounding is nearest or chop, and       precision exception.
exponent of true result > 16,383.

(FST, FSTP instructions only):         Return properly signed ý and signal
rounding is nearest or chop, and       precision exception.
exponent of true result > +127
(short real destination) or > +1023
(long real destination).
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                                 Underflow
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
(Arithmetic operations only):          Denormalize until exponent rises to
exponent of true result < -16,382      -16,382 (true), round significand
(true).                                to 64 bits. If denormalized rounded
                                       significand = 0, then return true
                                       0; else, return denormal (tag =
                                       special, biased exponent = 0).

(FST, FSTP instructions only):         Denormalize until exponent rises to
destination is short real and          -126 (true), round significand to
exponent of true result < -126         24 bits, store true 0 if
(true).                                denormalized rounded significand = 0;
                                       else, store denormal (biased
                                       exponent = 0).

(FST, FSTP instructions only):         Denormalize until exponent rises to
destination is long real and           -1022 (true), round significand to
exponent of true result < -1022        53 bits, store true 0 if rounded
(true).                                denormalized significand = 0; else,
                                       store denormal (biased exponent = 0).
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
                               Precision
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
True rounding error occurs.            No special action.

Masked response to overflow            No special action.
exception earlier in instruction.


Automatic Exception Handling
As described in the previous section, when the 80287 NPX encounters an
exception condition whose corresponding mask bit in the NPX control word is
set, the NPX automatically performs an internal fix-up (masked-exception)
response. The 80287 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 1-11). If R{1} becomes zero, the circuit
resistance becomes zero. With the divide-by-zero and precision exceptions
masked, the 80287 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, programmers will find
that masking all exceptions other than Invalid Operation will yield
satisfactory results with the least programming effort. An Invalid
Operation exception normally indicates a fatal error in a program 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 1-11.  Arithmetic Example Using Infinity

      �                          �
      �                          �
      �           R{1}           �
      �‘‘‘‘‘‘‘‘‘/\/\/\/‘‘‘‘‘‘‘‘‘‘�                            1
      �                          �      EQUIVALENT   =   ‘‘‘‘‘‘‘‘‘‘‘‘
      �                          �      RESISTANCE        1    1    1
      �           R{2}           �                       ‘‘ + ‘‘ + ‘‘
      �‘‘‘‘‘‘‘‘‘/\/\/\/‘‘‘‘‘‘‘‘‘‘�                      R{1} R{2}  R{3}
      �                          �
      �                          �
      �           R{3}           �
      ”‘‘‘‘‘‘‘‘‘/\/\/\/‘‘‘‘‘‘‘‘‘‘•


Software Exception Handling
If the NPX encounters an unmasked exception condition, it signals the
exception to the 80286 CPU using the ERROR status line between the two
processors.

The next time the 80286 CPU encounters a WAIT or ESC instruction in its
instruction stream, the 80286 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 typically 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 80287
     environment and registers)

  Ž  Aborting further execution

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

Application programmers on 80286 systems having systems software support
for the 80287 NPX should consult their references for the appropriate system
response to NPX exceptions. For systems programmers, specific details on
writing software exception handlers are included in the section
"System-Level Numeric Programming" later in this manual.

The 80287 NPX differs from the 8087 NPX in the manner in which numeric
exceptions are signalled to the CPU; the 8087 requires an interrupt
controller (8259A) to interrupt the CPU, while the 80287 does not.
Programmers upgrading 8087 software to operate on an 80287 should be aware
of these differences and any implications they might have on numeric
exception-handling software. Appendix B explains the differences between
the 80287 and the 8087 NPX in greater detail.


Chapter 2  Programming Numeric Applications

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

Programmers developing applications for the 80287 have a wide range of
instructions and programming alternatives from which to choose.

The following sections describe the 80287 instruction set in detail, and
follow up with a discussion of several of the programming facilities that
are available to programmers of 80287.


The 80287 NPX Instruction Set
This section describes the operation of all 80287 instructions. Within this
section, the instructions are divided into six functional classes:

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

At the end of this section, each of the instructions is described in terms
of its execution speed, bus transfers, and exceptions, as well as a coding
example for each combination of operands accepted by the instruction. For
easy reference, this information is concentrated into a table, organized
alphabetically by instruction mnemonic.

Throughout this section, the instruction set is described as it appears to
the ASM286 programmer who is coding a program. Appendix A covers the actual
machine instruction encodings, which are principally of use to those reading
unformatted memory dumps, monitoring instruction fetches on the bus, or
writing exception handlers.


Compatibility with the 8087 NPX
The instruction set for the 80287 NPX is largely the same as that for the
8087 NPX used with 8086 and 8088 systems. Most object programs generated for
the 8087 will execute without change on the 80287. Several instructions are
new to the 80287, and several 8087 instructions perform no useful function
on the 80287. Appendix B at the back of this manual gives details of these
instruction set differences and of the differences in the ASM86 and ASM286
assemblers.


Numeric Operands
The typical NPX instruction accepts one or two operands as inputs, operates
on these, and produces a result as an output. Operands are most often (the
contents of) register or memory locations. The operands of some instructions
are predefined; for example, FSQRT always takes the square root of the
number in the top 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 stack element.

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:

  FADD
  FADD source
  FADD destination, source

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 modes. To
review these modes‘‘direct, register indirect, based, indexed, based
indexed‘‘refer to the 80286 Programmer's Reference Manual. Table 2-17 later
in this chapter also provides several addressing mode examples.


Data Transfer Instructions
These instructions (summarized in table 2-1) 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 temporary 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 80287 tag
word to reflect the register contents following the instruction.


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. The source may
be a register on the stack (ST(i)) or any of the real data types in memory.
Short and long real source operands are converted to temporary real
automatically. Coding FLD ST(0) duplicates the stack top.


FST destination
FST (store real) transfers the stack top to the destination, which may be
another register on the stack or a short or long real memory operand. If the
destination is short or long real, the significand is rounded to the width
of the destination according to the RC field of the control word, and the
exponent is converted to the width and bias of the destination format.

If, however, the stack top is tagged special (it contains ý, a NaN, or a
denormal) then the stack top's significand is not rounded but is chopped (on
the right) to fit the destination. Neither is the exponent converted, but 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) or a denormal
(exponent all zeros) so that it can be properly loaded and tagged later in
the program if desired.


Table 2-1. 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   �
”‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•


FSTP destination
FSTP (store real and pop) operates identically to FST except that the stack
is popped following the transfer. This is done by tagging the top stack
element empty and then incrementing ST. FSTP permits storing to a temporary
real memory variable, whereas FST does not. Coding FSTP ST(0) is equivalent
to popping the stack with no data transfer.


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 80287 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:

  FXCH ST(3)
  FSQRT
  FXCH ST(3)


FILD source
FILD (integer load) converts the source memory operand from its binary
integer format (word, short, or long) to temporary real and loads (pushes)
the result onto the stack. The (new) stack top is tagged zero if all bits in
the source were zero, and is tagged valid otherwise.


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


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


FBLD source
FBLD (packed decimal (BCD) load) converts the content of the source operand
from packed decimal to temporary real and loads (pushes) the result onto the
stack. 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
0-9H. The instruction does not check for invalid digits (A-FH) and the
result of attempting to load an invalid encoding is undefined.


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 produces a rounded integer from a
nonintegral value by adding 0.5 to the value and then chopping. Users who
are concerned about rounding may precede FBSTP with FRNDINT.


Arithmetic Instructions
The 80287's arithmetic instruction set (table 2-2) 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;
80287 programmers no longer need to spend valuable time eliminating square
roots from algorithms because they run too slowly. Other arithmetic
instructions perform exact modulo division, round real numbers to integers,
and scale values by powers of two.

The 80287's basic arithmetic instructions (addition, subtraction,
multiplication, and division) are designed to encourage the development of
very efficient algorithms. In particular, they allow the programmer to
minimize memory references and to make optimum use of the NPX register
stack.

Table 2-3 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: temporary real, long
     real, short real, short integer or word integer, with automatic
     conversion to temporary real performed by the 80287.

Five basic instruction forms may be used across all six operations, as
shown in table 2-3. The classicial stack form may be used to make the 80287
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 converse coding (ST is the source operand) allows, for
example, adding the 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 sourced 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 flexibity of the 80287's arithmetic
instructions. They permit a real number or a binary integer in memory to
be used directly as a source operand. This is a very useful facility in
situations where operands are not used frequently enough to justify
holding them in registers. Note that any memory addressing mode 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 paragraphs following
table 2-3, and descriptions of the remaining seven arithmetic operations
follow.

Table 2-2. Arithmetic 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
FRNDINT                 Round to integer
FXTRACT                 Extract exponent and significand
FABS                    Absolute value
FCHS                    Change sign


Table 2-3. Basic Arithmetic Instruction and Operands

’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
�  Instruction  � Mnemonic�      Operand Forms         � ASM286 Example�
�     Form      �   Form  �   destination, source      �               �
–‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�Classical stack�   Fop   � {ST(1),ST}                 � FADD          �
�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,} short-real/long-real � FDIV AZIMUTH  �
�Integer memory �   Flop  � {ST,} word-integer/        � FIDV N_PULSES �
�               �         �       short-integer        �               �
”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTES

  Braces({ }) surround inplicit operands; these are not coded, and
  are shown here for information only.

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


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)


NORMAL SUBTRACTION
FSUB   //source/destinaton,source
FSUBP  //destination/source
FISUB  source

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


REVERSED SUBTRACTION
FSUBR   //source/destinaton,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.


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.


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.


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.


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.)


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 integal powers of
2. It is particularly useful for scaling the elements of a vector.

Note that FSCALE assumes the scale factor in ST(1) is an integral value in
the range -2^(15) ¾ x < 2^(15). If the value is not integral, but is
in-range and is greater in magnitude than 1, FSCALE uses the nearest integer
smaller in magnitude; i.e., it chops the value toward 0. If the value is out
of range, or 0 < �x� < 1, the instruction will produce an undefined result
and will not signal an exception. The recommended practice is to load the
scale factor from a word integer to ensure correct operation.


FPREM
FPREM (partial remainder) performs modulo division of the top stack
element by the next stack element, i.e., ST(1) is the modulus. FPREM
produces an exact result; the precision exception does not occur. The sign
of the remainder is the same as the sign of the orginal dividend.

FPREM operates by performing successive scaled subtractions; obtaining the
exact remainder when the operands differ greatly in magnitude can consume
large amounts of execution time. Because the 80287 cas only be preempted
between instructions, the remainder function could seriously increase
interrupt latency in these cases. Accordingly, the instruction is designed
to be executed interactively in a software-controlled loop.

FPREM can reduce a magnitude difference of up to 264 in one execution. If
FPREM produces a remainder that is less than the modulus, 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 is ST is then called
the partial remainder. Software can inspect C2 by storing the status word
following execution of FPREM and re-execute the instruction (using the
partial remainder in ST as the dividend), until C2 is cleared.
Alternatively, a program can determine when the function is complete by
comparing ST to ST(1). If ST > ST(1), then FPREM must be executed again; if
ST = ST(1), then the remainder is 0; if ST < ST(1), then the remainder is
ST. A higher priority interrupting routine that needs the 80287 can force a
context switch between the instructions in the remainder loop.

An important use for FPREM is to reduce arguments (operands) of periodic
transcendental functions to the range permitted by these instructions. For
example, the FPTAN (tangent) instruction requires its argument to be less
than Ò/4. Using Ò/4 as a modulus, FPTAN will reduce an argument so that it
is in range of 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. (The
rounding of Ò does not create the effect of a rounded argument, but of a
rounded period.)

FPREM also provides the least-significant three bits of the quotient
generated by FPREM (in C{3}, C{1}, C{0}). This is also important for
trancendental argument reduction, because it locates the original angle in
the correct one of eight Ò/4 segments of the unit circle (see table 2-4).
If the quotient is less than 4, then C0 will be the value of C3 before
FPREM was executed. If the quotient is less than 2, then C3 will be the
value of C1 before FPREM was executed.


FRNDINT
FRNDINT (round to integer) rounds the top stack element to an integer. For
example, assume that ST contains the 80287 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.


FXTRACT
FXTRACT (extract exponent and significand) "decomposes" 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.
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 produces zeros in ST and
ST(1) and both are signed as the original operand.

To clarify the operation of FXTRACT, assume ST contains a number of 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 * 2^(2) = 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 -1.11 * 2^(2) = -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 in conjunction with FBSTP for converting numbers in 80287
temporary real format to decimal representations (e.g., for printing or
displaying). It can also be useful for debugging, because it allows the
exponent and significant parts of a real number to be examined separately.


FABS
FABS (absolute value) changes the top stack element to its absolute value
by making its sign positive.


FCHS
FCHS (change sign) complements (reverses) the sign of the top stack
element.


Table 2-4.  Condition Code Interpretation after FPREM

’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
� ’‘‘Condition Code‘‘“                                                �
� C3    C2    C1    C0      Interpretation after FPREM                �
–‘‘‘‘‘˜‘‘‘‘‘˜‘‘‘‘‘˜‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
�  X  �  1  �  X  �  X  �  Incomplete Reduction;                      �
�     �     �     �     �    further iteration is required for        �
�     �     �     �     �    complete reduction.                      �
�     �     �     �     �  Complete Reduction;                        �
�     �     �     �     �    C1, C3, and C0 contain the three least-  �
�     �     �     �     �    significant bits of quotient:            �
�  0  �  0  �  0  �  0  �  (Quotient) MOD 8 = 0                       �
�  0  �  0  �  0  �  1  �  (Quotient) MOD 8 = 4                       �
�  0  �  0  �  1  �  0  �  (Quotient) MOD 8 = 1                       �
�  0  �  0  �  1  �  1  �  (Quotient) MOD 8 = 5                       �
�  1  �  0  �  0  �  0  �  (Quotient) MOD 8 = 2                       �
�  1  �  0  �  0  �  1  �  (Quotient) MOD 8 = 6                       �
�  1  �  0  �  1  �  0  �  (Quotient) MOD 8 = 3                       �
�  1  �  0  �  1  �  1  �  (Quotient) MOD 8 = 7                       �
”‘‘‘‘‘™‘‘‘‘‘™‘‘‘‘‘™‘‘‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•


Comparison Instructions
Each of these instructions (table 2-5) analyzes the top stack element,
often in relationship to another operand, and reports the result in the
status word condition code. The basic operations are compare, test (compare
with zero), and examine (report tag, 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 for inspection.

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.


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 short or long real
memory operand. If an operand is not coded, ST is compared to ST(1).
Positive and negative forms of zero compare identically as if they were
unsigned. Following the instruction, the condition codes reflect the order
of the operands as shown in table 2-6.

NaNs and ý (projective) cannot be compared and return C3 = C0 = 1 as shown
in the table.


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


FCOMPP
FCOMPP (compare real and pop twice) operates like FCOM and additionally
pops the stack twice, discarding both operands. The comparison is of the
stack top to ST(1); no operands may be explicitly coded.


FICOM  source
FICOM (integer compare) converts the source operand, which may reference a
word or short binary integer variable, to temporary real and compares the
stack top to it.


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


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 2-7.


FXAM
FXAM (examine) reports the content of the top stack element as
positive/negative and NaN/unnormal/denormal/normal/zero, or empty.
Table 2-8 lists and interprets all the condition code values that FXAM
generates. Although four different encodings may be returned for an empty
register, bits C3 and C0 of the condition code are both 1 in all encodings.
Bits C2 and C1 should be ignored when examining for empty.


Table 2-5. 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
FXAM                 Examine


Table 2-6. Condition Code Interpretation after FCOM

’‘‘ Condition Code ‘‘“
C3    C2     C1     C0     Interpretation after FCOM

0     0      X      0      ST > source
0     0      X      1      ST < source
1     0      X      0      ST = source
1     1      X      1      ST is not comparable


Table 2-7. Condition Code Interpretation after FTST

’‘‘ Condition Code ‘‘“
C3    C2     C1     C0     Interpretation after FTST

0     0      X      0      ST > 0
0     0      X      1      ST < 0
1     0      X      0      ST = 0
1     1      X      1      ST is not comparable; (i.e., it
                           is a NaN or projective infinity)


Table 2-8. FXAM Condition Code Settings

’‘‘‘ Condition Code ‘‘‘“
C3     C2      C1     C0           Interpretation

0      0       0      0            + Unnormal
0      0       0      1            + NaN
0      0       1      0            - Unnormal
0      0       1      1            - NaN
0      1       0      0            + Normal
0      1       0      1            + ý
0      1       1      0            - Normal
0      1       1      1            - ý
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       0      1            Empty
1      1       1      0            - Denormal
1      1       1      1            Empty


Transcendental Instructions
The instructions in this group (table 2-9) perform the time-consuming core
calculations for all common trigonometric, inverse trigonometric,
hyperbolic, inverse hyperbolic, logarithmic, and exponential functions.
Prologue and epilogue software may be used to reduce arguments to the range
accepted by the instructions and to adjust the result to correspond to the
original arguments if necessary. The transcendentals operate on the top one
or two stack elements, and they return their results to the stack, also.

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTE
  The transcendental instructions assume that their operands are valid and
  in-range. The instruction descriptions in this section provide the
  allowed operand range of each instruction.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

All operands to a transcendental must be normalized; denormals, unnormals,
infinities, and NaNs are considered invalid. (Zero operands are accepted by
some functions and are considered out-of-range by others). If a
transcendental operand is invalid or out-of-range, the instruction will
produce an undefined result without signalling an exception. It is the
programmer's responsibility to ensure that operands are valid and in-range
before executing a transcendental. For periodic functions, FPREM may be
used to bring a valid operand into range.


FPTAN
0 ¾ ST(0) ¾ Ò/4

FPTAN (partial tangent) computes the function Y/X = TAN(Ú). Ú is taken
from the top stack element; it must lie in the range 0 ¾ Ú ¾ Ò/4. The result
of the operation is a ratio; Y replaces Ú in the stack and X is pushed,
becoming the new stack top.

The ratio result of FPTAN and the ratio argument of FPATAN are designed to
optimize the calculation of the other trigonometric functions, including
SIN, COS, ARCSIN, and ARCCOS. These can be derived from TAN and ARCTAN via
standard trigonometric identities.


FPATAN
0 ¾ ST(1) < ST(0) < ý

FPATAN (partial arctangent) computes the function Ú = ARCTAN(Y/X). X is
taken from the top stack element and Y from ST(1). Y and X must observe the
inequality 0 ¾ Y < X < ý. The instruction pops the stack and returns Ú to
the (new) stack top, overwriting the Y operand.


F2XM1
0 ¾ ST(0) ¾ 0.5

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 0 ¾ X ¾ 0.5. The result Y
replaces X at the stack top.

This instruction is designed to produce a very accurate result even when X
is close to 0. 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 * LOG{2}10)
   e^(x) = 2^(x * LOG{2}e)
   y^(x) = 2^(x * LOG{2}Y)

As shown in the next section, the 80287 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.


FYL2X
0 < ST(0) < ý - ý < ST(1) < ý

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
ranges 0 < X < ý and -ý < Y < +ý. The instruction pops the stack and returns
Z at the (new) stack top, replacing the Y operand.

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

  LOG{n}2 * LOG{2}X


FYL2XP1
0 ¾ �ST(0)� < (1 - (¹2/2))
-ý < ST(1) < ý

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
0 ¾ �X� < (1 - (¹2/2)). Y is taken from ST(1) and must be in the range
-ý < Y < ý. FYL2XP1 pops the stack and returns Z at the (new) stack top,
replacing Y.

The instruction provides improved accuracy over FYL2X when computing the
log 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 2-9. Transcendental Instructions

FPTAN        Partial tangent
FPATAN       Partial arctangent
F2XM1        2^(X) - 1
FYL2X        Y * log{2}X
FYL2XP1      Y * log{2}(X + 1)


Constant Instructions
Each of these instructions (table 2-10) loads (pushes) a commonly-used
constant onto the stack. The values have full temporary real precision (64
bits) and are accurate to approximately 19 decimal digits. Because a
temporary 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.

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

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

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

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

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

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

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


Table 2-10. 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


Processor Control Instructions
The processor control instructions shown in table 2-11 are not typically
used in calculations; they provide control over the 80287 NPX for
system-level activities. These activities include initialization, exception
handling, and task switching.

As shown in table 2-11, many of the NPX processor control instructions have
two forms of assembler mnemonic:

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

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

When the control instruction is coded using the no-wait form of the
mnemonic, the ASM286 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
80286, with the CPU testing the BUSY status line and only executing the
numeric instruction when this line is inactive. Because of this automatic
synchronization by the 80286, numeric instructions for the 80287 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 perform
no function in the 80287. If these opcodes are detected in an 80286/80287
instruction stream, the 80287 will perform no specific operation and no
internal states will be affected. For programmers interested in porting
numeric software from 8087 environments to the 80286, however, it should be
noted that program sections containing these exception-handling instructions
are not likely to be completely portable to the 80287. Appendix B contains
a more complete description of the differences between the 80287 and the
8087 NPX.


Table 2-11. Processor Control Instructions

FINIT/FNINIT                  Initialize processor
FSETPM                        Set Protected Mode
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


FINIT/FNINIT
FINIT/FNINIT (initialize processor) sets the 80287 NPX into a known state,
unaffected by any previous activity. The no-wait form of this instruction
will cause the 80287 to abort any previous numeric operations currently
executing in the NEU. This instruction performs the functional equivalent
of a hardware RESET, with one exception; FINIT/FNINIT does not affect the
current 80287 operating mode (either Real-Address mode or Protected mode).
FINIT checks for unmasked numeric exceptions, FNINIT does not.

Note that if FNINIT is executed while a previous 80287 memory-referencing
instruction is running, 80287 bus cycles in progress will be aborted. This
instruction may be necessary to clear the 80287 if a Processor Extension
Segment Overrun Exception (Interrupt 9) is detected by the CPU.


FSETPM
FSETPM (set Protected mode) sets the operating mode of the 80287 to
Protected Virtual-Address mode. When the 80287 is first initialized
following hardware RESET, it operates in Real-Address mode, just as does the
80286 CPU. Once the 80287 NPX has been set into Protected mode, only a
hardware RESET can return the NPX to operation in Real-Address mode.

When the 80287 operates in Protected mode, the NPX exception pointers are
represented differently than they are in Real-Address mode (see the FSAVE
and FSTENV instructions that follow). This distinction is evident primarily
to writers of numeric exception handlers, however. For general application
programmers, the operating mode of the 80287 need not be a concern.


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 80287'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 and clears the interrupt enable mask will generate an
immediate interrupt request before the next instruction is executed. When
changing modes, the recommended procedure is to first clear any exceptions
and then load the new control word.


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


FSTSW/FNSTSW destination
FSTSW/FNSTCW (store status word) writes the current value of the 80287
status word to the destination operand in memory. The instruction is used to

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

  Ž  Poll the 80287 to determine if it is busy (FNSTSW)

  Ž  Invoke exception handlers in environments that do not use interrupts
     (FSTSW).

FSTSW checks for unmasked numeric exceptions, FNSTSW does not.


FSTSW AX/FNSTSW AX
FSTSW AX/FNSTSW AX (store status word to AX) is a special 80287 instruction
that writes the current value of the 80287 status word directly into the
80286 AX register. This instruction optimizes conditional branching in
numeric programs, where the 80286 CPU must test the condition of various NPX
status bits. The waited form checks for unmasked numeric exceptions, the
non-waited for does not.

When this instruction is executed, the 80286 AX register is updated with
the NPX status word before the CPU executes any further instructions. In
this way, the 80286 can immediately test the NPX status word without any
WAIT or other synchronization instructions required.


FCLEX/FNCLEX
FCLEX/FNCLEX (clear exceptions) clears all exception flags, the error
status flag and the busy flag in the status word. As a consequence, the
80287's ERROR line goes inactive. FCLEX checks for unmasked numeric
exceptions, FNCLEX does not.


FSAVE/FNSAVE destination
FSAVE/FNSAVE (save state) writes the full 80287 state‘‘environment plus
register stack‘‘to the memory location defined by the destination operand.
Figure 2-1 shows the layout of the 94-byte save area; typically the
instruction will be coded to save this image on the CPU stack. 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 80287 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

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

  Ž  An exception handler needs to use the 80287.

  Ž  An application task wants to pass a "clean" 80287 to a subroutine.

FSAVE checks for unmasked numeric errors before executing, FNSAVE does not.
An FWAIT should be executed before CPU interrupts are enabled or any
subsequent 80287 instruction is executed. Other CPU instructions may be
executed between the FNSAVE/FSAVE and the FWAIT.


Figure 2-1.  FSAVE/FRSTOR Memory Layout

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NOTES:
  a = INSTRUCTION POINTER
  b = OPERAND POINTER
  S = Sign
  Bit 0 of each field is rightmost, least significant
        bit of corresponding register field.
  Bit 63 of significand is integer bit (assumed
        binary point is immediately to the right.)
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


FRSTOR source
FRSTOR (restore state) reloads the 80287 from the 94-byte 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. An FWAIT is not required after FRSTOR. FRSTOR will
automatically wait and check for interrupts until all data transfers are
completed before continuing to the next instruction.

Note that the 80287 "reacts" to its new state at the conclusion of the
FRSTOR; it will, for example, generate an exception request if the exception
and mask bits in the memory image so indicate when the next WAIT or
error-checking-ESC instruction is executed.


FSTENV/FNSTENV destination
FSTENV/FNSTENV (store environment) writes the 80287'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 processor.
FSTENV checks for pending errors before executing, FNSTENV does not.

Figure 2-2 shows the format of the environment data in memory. FNSTENV does
not store the environment until all NPX activity has completed. Thus, the
data saved by the instruction reflects the 80287 after any previously
decoded instruction has been executed. After writing the environment image
to memory, FNSTENV/FSTENV initializes the 80287 state as if FNINIT/FINIT
had been executed.

FSTENV/FNSTENV must be allowed to complete before any other 80287
instruction is decoded. When FSTENV is coded, an explicit FWAIT, or
assembler-generated WAIT, should precede any subsequent 80287 instruction.


Figure 2-2.  FSTENV/FLDENV Memory Layout

            REAL MODE                            PROTECTED MODE
  15                            0 MEMORY  15                        0 MEMORY
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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. An FWAIT is not
required after FLDENV. FLDENV will automatically wait for all data
transfers to complete before executing the next instruction.

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


FINCSTP
FINCSTP (increment stack pointer) adds 1 to the stack top pointer (ST) 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.


FDECSTP
FDECSTP (decrement 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.


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


FNOP
FNOP (no operation) stores the stack top to the stack top (FST ST,ST(0))
and thus effectively performs no operation.


FWAIT (CPU Instruction)
FWAIT is not actually an 80287 instruction, but an alternate mnemonic for
the CPU WAIT instruction. The FWAIT or WAIT mnemonic should be coded
whenever the programmer wants to synchronize the CPU to the NPX, that is, to
suspend further instruction decoding until the NPX has completed the current
instruction. FWAIT will check for unmasked numeric exceptions.

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTE
  A CPU instruction should not attempt to access a memory operand until the
  80287 instruction has completed. For example, the following coding shows
  how FWAIT can be used to force the CPU instruction to wait for the 80287:

     FIST     VALUE
     FWAIT    ; Wait for FIST to complete
     MOV      AX,VALUE
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

More information on when to code an FWAIT instruction is given in a
following section of this chapter, "Concurrent Processing with the 80287."


Instruction Set Reference Information
Table 2-14 later in this chapter lists the operating characteristics of all
the 80287 instructions. There is one table entry for each instruction
mnemonic; the entries are in alphabetical order for quick lookup. Each entry
provides the general operand forms accepted by the instruction as well as a
list of all exceptions that may be detected during the operation.

One entry exists for each combination of operand types that can be coded
with the mnemonic. Table 2-12 explains the operand identifiers allowed in
table 2-14. Following this entry are columns that provide execution time in
clocks, the number of bus transfers run during the operation, the length of
the instruction in bytes, and an ASM286 coding sample.


Instruction Execution Time
The execution of an 80287 instruction involves three principal activities,
each of which may contribute to the overall execution time of the
instruction:

  Ž  80286 CPU overhead involved in handling the ESC instruction opcode
     and setting up the 80287 NPX

  Ž  Instruction execution by the 80287 NPX

  Ž  Operand transfers between the 80287 NPX and memory or a CPU register

The timing of these various activities is affected by the individual clock
frequencies of the 80286 CPU and the 80287 NPX. In addition, slow memories
requiring the insertion of wait states in bus cycles, and bus contention due
to other processors in the system, may lengthen operand transfer times.

In calculating an overall execution time for an individual numeric
instruction, analysts must take each of these activities into account. In
most cases, it can be assumed that the numeric instructions have already
been prefetched by the 80286 and are awaiting execution.

  Ž  The CPU overhead in handling the ESC instruction opcode takes only a
     single CPU bus cycle before the 80287 begins its execution of the
     numeric instruction. The timing of this bus cycle is determined by the
     CPU clock. Additional CPU activity is required to set up the 80287's
     instruction and data pointer registers, but this activity occurs after
     the 80287 has begun executing its instruction, and so this parallel
     activity does not affect total execution time.

  Ž  The duration of individual numeric instructions executing on the 80287
     varies for each instruction. Table 2-14 quotes a typical execution
     clock count and a range for each 80287 instruction. Dividing the
     figures in the table by 10 (for a 10-MHz 80287 NPX clock) produces an
     execution time in microseconds. The typical case is an estimate for
     operand values that normally characterize most applications. The range
     encompasses best- and worst-case operand values that may be found in
     extreme circumstances.

  Ž  The operand transfer time required to transfer operands between the
     80287 and memory or a CPU register depends on the number of words to be
     transferred, the frequency of the CPU clock controlling bus timing, the
     number of wait states added to accommodate slower memories, and
     whether operands are based at even or odd memory addresses. Some
     (small) additional number of bus cycles may also be lost due to the
     asynchronous nature of the PEREQ/PEACK handshaking between the 80286
     and 80287, and this interaction varies with relative frequencies of
     the CPU and NPX clocks.

The execution clock counts for the NPX execution of instructions shown in
table 2-14 assume that no exceptions are detected during execution. Invalid
operation, denormalized operand (unmasked), and zero divide exceptions
usually decrease execution time from the typical figure, but execution
still falls within the indicated range. The precision exception has no
effect on execution time. Unmasked overflow and underflow, and masked
denormalized exceptions impose additional execution penalties as shown in
table 2-13. Absolute worst-case execution times are therefore the high
range figure plus the largest penalty that may be encountered.


Table 2-12. Key to Operand Types


Identifier      Explanation

ST              Stack top; the register currently at the top of the stack.

ST(i)           A register in the stack i (0¾i¾7) stack elements from the
                top. ST(1) is the next-on-stack register, ST(2) is below
                ST(1), etc.

Short-real      A short real (32 bits) number in memory.

Long-real       A long real (64 bits) number in memory.

Temp-real       A temporary real (80 bits) number in memory.

Packed-decimal  A packed decimal integer (18 digits, 10 bytes) in memory.

Word-integer    A word binary integer (16 bits) in memory.

Short-integer   A short binary integer (32 bits) in memory.

Long-integer    A long binary integer (64 bits) in memory.

nn-bytes        A memory area nn bytes long.


Bus Transfers
NPX instructions that reference memory require bus cycles to transfer
operands between the NPX and memory. The actual number of transfers depends
on the length of the operand and the alignment of the operand in memory.
In table 2-14, the first figure gives execution clocks for even-addressed
operands, while the second gives the clock count for odd-addressed operands.

For operands aligned at word boundaries, that is, based at even memory
addresses, each word to be transferred requires one bus cycle between the
80286 data channel and memory, and one bus cycle to the NPX. For operands
based at odd memory addresses, each word transfer requires two bus cycles
to transfer individual bytes between the 80286 data channel and memory, and
one bus cycle to the NPX.

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTE
  For best performance, operands for the 80287 should be aligned along word
  boundaries; that is, based at even memory addresses. Operands based at odd
  memory addresses are transferred to memory essentially byte-at-a-time and
  may take half again as long to transfer as word-aligned operands.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘

Additional transfer time is required if slow memories are being used,
requiring the insertion of wait states into the CPU bus cycle. In
multiprocessor environments, the bus may not be available immediately; this
overhead can also increase effective transfer time.


Table 2-13. Execution Penalties

Exception                 Additional Clocks
Overflow (unmasked)              14
Underflow (unmasked)             16
Denormalized (masked)            33


Instruction Length
80287 instructions that do not reference memory are two bytes long. Memory
reference instructions vary between two and four bytes. The third and fourth
bytes are for the 8- or 16-bit displacement values used in conjunction with
the standard 80286 memory-addressing modes.

Note that the lengths quoted in table 2-14 for the processor control
instructions (FNINIT, FNSTCW, FNSTSW, FNSTSW AX, FNCLEX, FNSTENV, and
FNSAVE) do not include the one-byte CPU wait instruction inserted by the
ASM286 assembler if the control instruction is coded using the wait form of
the mnemonic (e.g. FINIT, FSTCW, FSTSW, FSTSW AX, FCLEX, FSTENV, and
FSAVE). Wait and no-wait forms of the processor control instructions have
been described in the preceding section titled "Processor Control
Instructions."


Table 2-14. Instruction Set Reference Data


                     ®Execution Clocks¬     Operand Word    Code
Operands              Typical    Range       Transfers      Bytes   Coding Example
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FABS                  FABS (no operands)
                      Absolute value                        Exceptions: I

(no operands)         14         10-17          0           2       FABS
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FADD                  FADD\\source\destination,source
                      Add real                              Execptions: I,D,O,U,P

\\ST,ST(i)\ST(i),ST   85         70-100         0           2       FADD,ST,ST(4)
short-real            105        90-120         2           2-4     FADD AIR_TEMP [SI]
long-real             110        95-125         4           2-4     FADD [BX].MEAN
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FADDP                 FADDP destination, source
                      Add real and pop                      Exceptions: I,D,O,U,P

ST(i),ST              90         75-105         0           2       FADDP ST(2),ST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FBLD                  FBLD source
                      Packed decimal (BCD) load             Exceptions: I

packed-decimal        300        290-310        5           2-4     FBLD YTD_SALES
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FBSTP                 FBSTP destination
                      Packed decimal (BCD) store and pop    Exceptions: I

packed-decimal        530        520-540        5           2-4     FBSTP [BX].FORECAST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FCHS                  FCHS (no operands)
                      Change sign                           Exceptions: I
(no operands)         15         10-17          0           2       FCHS
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FCLEX/FNCLEX          FCLEX/FNCLEX(no operands)
                      Clear exceptions                      Exceptions: None

(no operands)         5          2-8            0           2       FNCLEX
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FCOM                  FCOM //source
                      Compare real                          Exceptions: I, D

//ST(i)               45         40-50          0           2       FCOM ST(1)
short-real            65         60-70          2           2-4     FCOM [BP].UPPER_LIMIT
long-real             70         65-75          4           2-4     FCOM WAVELENGTH
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FCOMP                 FCOMP //source
                      Compare real and pop                  Exceptions: I, D

//ST(i)               47         42-52          0           2       FCOMP ST(2)
short-real            68         63-73          2           2-4     FCOMP [BP + 2].N_READINGS
long-real             72         67-77          4           2-4     FCOMP DENSITY
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FCOMPP                FCOMPP (no operands)
                      Compare real and pop twice                    Exceptions: I, D

(no operands)         50         45-55          0           2       FCOMPP
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FDECSTP               FDECSTP (no operands)
                      Decrement stack pointer               Exceptions: None

(no operands)         9          6-12           0           2       FDECSTP
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FDIV                  FDIV //source/destination,source
                      Divide real                           Exceptions: I, D, Z, O, U, P

//ST(i),ST            198        193-203        0           2       FDIV
short-real            220        215-225        2           2-4     FDIV DISTANCE
long-real             225        220-230        4           2-4     FDIV ARC [DI]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FDIVP                 FDIVP destination, source
                      Divide real and pop                   Exceptions: I, D, Z, O, U, P

ST(i),ST              202        197-207        0           2       FDIVP ST(4),ST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FDIVR                 FDIVR //source/destination, source
                      Divide real reversed                  Exceptions: I, D, Z, O, U, P

//ST,ST(i)/ST(i),ST   199        194-204        0           2       FDIVR ST(2),ST
short-real            221        216-226        2           2-4     FDIVR [BX].PULSE_RATE
long-real             226        221-231        4           2-4     FDIVR RECORDER.FREQUENCY
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FDIVRP                FDIVRP destination, source
                      Divide real reversed and pop          Exceptions: I, D, Z, O, U, P

ST(i),ST              203        198-208        0           2       FDIVRP ST(1),ST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FFREE                 FFREE destination
                      Free register                         Exceptions: None

ST(i)                 11         9-16           0           2       FFREE ST(1)
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FIADD                 FIADD source
                      Integer add                           Exceptions: I, D, O, P

word-integer          120        102-137        1           2-4     FIADD DISTANCE_TRAVELLED
short-integer         125        108-143        2           2-4     FIADD PULSE_COUNT [SI]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FICOM                 FICOM source
                      Integer compare                       Exceptions: I, D

word-integer          80         72-86          1           2-4     FICOM TOOL.N_PASSES
short-integer         85         78-91          2           2-4     FICOM [BP+4].PARM_COUNT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FICOMP                FICOMP source
                      Integer compare and pop               Exceptions: I, D

word-integer          82         74-88          1           2-4     FICOMP [BP].LIMIT [SI]
short-integer         87         80-93          2           2-4     FICOMP N_SAMPLES
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FIDIV                 FIDIV source
                      Integer divide                        Exceptions: I, D, Z, O, U, P

word-integer          230        224-238        1           2-4     FIDIV SURVEY.OBSERVATIONS
short-integer         236        230-243        2           2-4     FIDIV RELATIVE_ANGLE [DI]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FIDIVR                FIDIVR source
                      Integer divide reversed               Exceptions: I, D, Z, O, U, P

word-integer          230        225-239        1           2-4     FIDIVR [BP].X_COORD
short-integer         237        231-245        2           2-4     FIDIVR FREQUENCY
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FILD                  FILD source
                      Integer load                          Exceptions: I

word-integer          50         46-54          1           2-4     FILD [BX].SEQUENCE
short-integer         56         52-60          2           2-4     FILD STANDOFF [DI]
long-integer          64         60-68          4           2-4     FILD RESPONSE.COUNT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FIMUL                 FIMUL source
                      Integer multiply                      Exceptions: I, D, O, P

word-integer          130        124-138        1           2-4     FIMUL BEARING
short-integer         136        130-144        2           2-4     FIMUL POSITION.Z_AXIS
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FINCSTP               FINCSTP (no operands)
                      Increment stack pointer               Exceptions: None

(no operands)         9          6-12           0           2       FINCSTP
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FINIT/FNINIT          FINIT/FNINIT (no operands)
                      Initialize processor                  Exceptions: None

(no operands)         5          2-8            0           2       FINIT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FIST                  FIST destination
                      Integer store                         Exceptions: I, P

word-integer          86         80-90          1           2-4     FIST OBS.COUNT[SI]
short-integer         88         82-92          2           2-4     FIST [BP;].FACTORED_PULSES
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FISTP                 FISTP destination
                      Integer store and pop                 Exceptions: I, P

word-integer          88         82-92          1           2-4     FISTP [BX].ALPHA_COUNT [SI]
short-integer         90         84-94          2           2-4     FISTP CORRECTED_TIME
long-integer          100        94-105         4           2-4     FISTP PANEL.N_READINGS
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FISUB                 FISUB source
                      Integer subtract                      Exceptions: I, D, O, P

word-integer          120        102-137        1           2-4     FISUB BASE_FREQUENCY
short-integer         125        108-143        2           2-4     FISUB TRAIN_SIZE [DI]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FISUBR                FISUBR source
                      Integer subtract reversed             Exceptions: I, D, O, P

word-integer          120        103-139        1           2-4     FISUBR FLOOR [BX] [SI]
short-integer         125        109-144        2           2-4     FISUBR BALANCE
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLD                   FLD source
                      Load real                             Exceptions: I, D

ST(i)                 20         17-22          0           2       FLD ST(0)
short-real            43         38-56          2           2-4     FLD READING [SI].PRESSURE
long-real             46         40-60          4           2-4     FLD BP].TEMPERATURE
temp-real             57         53-65          5           2-4     FLD SAVEREADING
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDCW                 FLDCW source
                      Load control word                     Exceptions: None

2-bytes               10         7-14           1           2-4     FLDCW CONTROL_WORD
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDENV                FLDENV source
                      Load environment                      Exceptions: None

14-bytes              40         35-45          7           2-4     FLDENV [BP + 6]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDLG2                FLDLG2 (no operands)
                      Load log{10}2                         Exceptions: I

(no operands)         21         18-24          0           2       FLDLG2
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDLN2                FLDLN2 (no operands)
                      Load log{e}2                          Exceptions: I

(no operands)         20         17-23          0           2       FLDLN2
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDL2E                FLDL2E (no operands)
                      Load log{2}e                          Exceptions: I

(no operands)         18         15-21          0           2       FLDL2E
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDL2T                FLDL2T (no operands)
                      Load log{2}10                         Exceptions: I

(no operands)         19         16-22          0           2       FLDL2T
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDPI                 FLDPI (no operands)
                      Load Ò                                Exceptions: I

(no operands)         19         16-22          0           2       FLDPI
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLDZ                  FLDZ (no operands)
                      Load +0.0                             Exceptions: I

(no operands)         14         11-17          0           2       FLDZ
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FLD1                  FLD1 (no operands)
                      Load +1.0                             Exceptions: I

(no operands)         18         15-21          0           2       FLD1
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FMUL                  FMUL //source/destination,source
                      Multiply real                         Exceptions: I, D, O, U, P

//ST(i),ST/T,ST(i)
Occurs when one or both operands is "short"‘‘it has 40 trailing zeros
in its fraction (e.g., it was loaded from a short-real memory operand).    97         90-105        0           2       FMUL ST,ST(3)
//ST(i),ST/ST,ST(i)   138        130-145        0           2       FMUL ST,ST(3)
short-real            118        110-125        2           2-4     FMUL SPEED_FACTOR
long-real
Occurs when one or both operands is "short"‘‘it has 40 trailing zeros
in its fraction (e.g., it was loaded from a short-real memory operand).            120        112-126        4           2-4     FMUL [BP].HEIGHT
long-real             161        154-168        4           2-4     FMUL [BP].HEIGHT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FMULP                 FMULP destination, source
                      Multiply real and pop                 Exceptions: I, D, O, U, P

ST(i),ST
Occurs when one or both operands is "short"‘‘it has 40 trailing zeros
in its fraction (e.g., it was loaded from a short-real memory operand).             100        94-108         0           2       FMULP ST(1),ST
ST(i),ST              142        134-148        0           2       FMULP ST(1),ST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FNOP                  FNOP (no operands)
                      No operation                          Exceptions: None

(no operands)         13         10-16          0           2       FNOP
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FPATAN                FPATAN (no operands)
                      Partial arctangent                    Exceptions: U, P (operands not checked)

(no operands)         650        250-800        0           2       FPATAN
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FPREM                 FPREM (no operands)
                      Partial remainder                     Exceptions: I, D, U

(no operands)         125        15-190         0           2       FPREM
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FPTAN                 FPTAN (no operands)
                      Partial tangent                       Exceptions: I, P (operands not checked)

(no operands)         450        30-540         0           2       FPTAN
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FRNDINT               FRNDINT (no operands)
                      Round to integer                      Exceptions: I, P

(no operands)         45         16-50          0           2       FRNDINT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FRSTOR                FRSTOR source
                      Restore saved state                   Exceptions: None

94-bytes                            (
The 80287 execution clock count for this instruction is not meaningful
in determining overall instruction execution time. For typical frequency
ratios of the 80286 and 80287 clocks, 80287 execution occurs in parallel
with the operand transfers, with the operand transfers determining the
overall execution time of the instruction. For 80286:80287 clock frequency
ratios of 4:8, 1:1, and 8:5, the overall execution clock count for this
instruction is estimated at 490, 302, and 227 80287 clocks, respectively.)         47          2-4     FRSTOR [BP]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSAVE/FNSAVE          FSAVE/FNSAVE destination
                      Save state                            Exceptions: None

94-bytes                            (
The 80287 execution clock count for this instruction is not meaningful in
determining overall instruction execution time. For typical frequency
rations of the 80286 and 80287 clocks, 80287 execution occurs in parallel
with the operand transfers, with the operand transfers determining the
overall execution time of the instruction. For 80286:80287 clock frequency
ratios of 4:8, 1:1, and 8:5, the overall execution clock count for this
instruction is estimated at 376, 233, and 174 80287 clocks, respectively.)         47          2-4     FSAVE [BP]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSCALE                FSCALE (no operands)
                      Scale                                 Exceptions: I, O, U

(no operands)         35         32-38           0           2      FSCALE
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSETPM                FSETPM (no operands)
                      Set protected mode                    Exceptions: None

(no operands)                     2-8            0           2      FSETPM
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSQRT                 FSQRT (no operands)
                      Square root                           Exceptions: I, D, P

(no operands)         183          80-186         0         2       FSQRT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FST                   FST destination
                      Store real                            Exceptions: I, O, U, P

ST(i)                  18          15-22          0         2       FST ST(3)
short-real             87          84-90          2         2-4     FST CORRELATION [DI]
long-real             100          96-104         4         2-4     FST MEAN_READING
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSTCW/                FSTCW destination
FNSTCW                Store control word                    Exceptions: None

2-bytes               15           12-18          1         2-4     FSTCW SAVE_CONTROL
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSTENV/               FSTENV destination
FNSTENV               Store environment                     Exceptions: None

14-bytes              45           40-50          7         2-4     FSTENV [BP]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSTP                  FSTP destination
                      Store real and pop                    Exceptions: I, O, U, P

ST(i)                  20          17-24          0         2       FSTP ST(2)
short-real             89          86-92          2         2-4     FSTP [BX].ADJUSTED_RPM
long-real             102          98-106         4         2-4     FSTP TOTAL_DOSAGE
temp-real              55          52-58          5         2-4     FSTP REG_SAVE [SI]
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSTSW/                FSTSW destination
FNSTSW                Store status word                     Exceptions: None

2-bytes               15           12-18          1         2-4     FSTSW SAVE_STATUS
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSTSW AX/             FSTSW AX
FNSTSWAX              Store status word to AX               Exceptions: None

AX                                 10-16          1         2       FSTSW AX
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSUB                  FSUB //source/destination,source
                      Subtract real                         Exceptions: I, D, O, U, P

//ST,ST(i)/ST(i),ST    85          70-100         0         2       FSUB ST,ST(2)
short-real            105          90-120         2         2-4     FSUB BASE_VALUE
long-real             110          95-125         4         2-4     FSUB COORDINATE.X
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSUBP                 FSUBP destination, source
                      Subtract real and pop                 Exceptions: I, D, O, U, P

ST(i),ST              90           75-105         0         2       FSUBP ST(2),ST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSUBR                 FSUBR //source/destination, source
                      Subtract real reversed                Exceptions: I, D, O, U, P

//ST,ST(i)/ST(i),ST    87          70-100         0         2       FSUBR ST,ST(1)
short-real            105          90-120         2         2-4     FSUBR VECTOR[SI]
long-real             110          95-125         4         2-4     FSUBR [BX].INDEX
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FSUBRP                FSUBRP destination, source
                      Subtract real reversed and pop        Exceptions: I, D, O, U, P

ST(i),ST              90           75-105         0         2       FSUBRP ST(1),ST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FTST                  FTST (no operands)
                      Test stack top against +0.0           Exceptions: I, D
(no operands)         42           38-48          0         2       FTST
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FWAIT                 FWAIT (no operands)
                      (CPU) Wait while 80287 is busy        Exceptions: None (CPU instruction)

(no operands)         3+5n
n = number of times CPU examines BUSY line before 80287 completes
execution of previous instruction.        3+5n
n = number of times CPU examines BUSY line before 80287 completes
execution of previous instruction.          0         1       FWAIT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FXAM                  FXAM (no operands)
                      Examine stack top                     Exceptions: None

(no operands)         17           12-23          0         2       FXAM
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FXCH                  FXCH //destination
                      Exchange registers                            Exceptions: I

//ST(i)               12           10-15          0         2       FXCH ST(2)
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FXTRACT               FXTRACT (no operands)
                      Extract exponent and significant              Exceptions: I

(no operands)         50           27-55          0         2       FXTRACT
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FYL2X                 FYL2X (no operands)
                      Y * Log{2}X                           Exceptions: P (operands not checked)

(no operands)         950        900-1100         0         2       FYL2X
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
FYL2XP1               FYL2XP1 (no operands)
                      Y * log{2}(X + 1)                     Exceptions: P (operands not checked)

(no operands)         850        700-1000         0         2       FYL2XP1
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
F2XM1                 F2XM1 (no operands)
                      2^(2-1)                             Exceptions: U, P (operands not checked)

(no operands)         500        310-630          0         2       F2XM1
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘



Programming Facilities
As described previously, the 80287 NPX is programmed simply as an extension
of the 80286 CPU. This section describes how programmers in ASM286 and in a
variety of higher-level languages can work with the 80287.

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 80287, but
this information does not document the full capabilities of these
facilities. For a complete list of documentation on all the languages
available for 80286 systems, readers should consult Intel's Literature
Guide.


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 80287
NPX when appropriate. These languages include

  PL/M-286
  FORTRAN-286
  PASCAL-286
  C-286

Each of these high-level languages has special numeric libraries allowing
programs to take advantage of the capabilities of the 80287 NPX. No special
programming conventions are necessary to make use of the 80287 NPX when
programming numeric applications in any of these languages.

Programmers in PL/M-286 and ASM286 can also make use of many of these
library routines by using routines contained in the 80287 Support Library,
described in the 80287 Support Library Reference Manual, Order Number
122129. These library routines provide 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 80287 instruction set.


PL/M-286
Programmers in PL/M-286 can access a very useful subset of the 80287's
numeric capabilities. The PL/M-286 REAL data type corresponds to the NPX's
short real (32-bit) format. This data type provides a range of about
8.43*10^(-37) ¾ ABS(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-286 compiler's
practice of holding intermediate results in the 80287's temporary real
format. This means that the full range and precision of the processor are
utilized for intermediate results. Underflow, overflow, and rounding errors
are most likely to occur during intermediate computations rather than during
calculation of an expression's final result. Holding intermediate results in
temporary 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 80287 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 2-15) give the PL/M-286 programmer access
to 80287 functions manipulated by the processor control instructions. Prior
to any arithmetic operations, a typical PL/M-286 program will set up the NPX
after power up using the INIT$REAL$MATH$UNIT procedure and then issue
SET$REAL$MODE to configure the NPX. SET$REAL$MODE loads the 80287 control
word, and its 16-bit parameter has the format shown in figure 1-5. The
recommended value of this parameter is 033EH (projective closure, 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 by CPU
interrupt pointer (vector) number 16. The exception handler can use the
GET$REAL$ERROR procedure to obtain the low-order byte of the 80287 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
multi-tasking environments where a running task that uses the 80287 may be
preempted by another task that also uses the 80287. It is the responsibility
of the preempting task to issue SAVE$REAL$STATUS before it executes any
statements that affect the 80287; these include the INIT$REAL$MATH$UNIT and
SET$REAL$MODE procedures as well as arithmetic expressions.
SAVE$REAL$STATUS saves the 80287 state (registers, status, and control
words, etc.) on the CPU's stack. RESTORE$REAL$STATUS reloads the state
information; the preempting task must invoke this procedure before
terminating in order to restore the 80287 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 2-15. PL/M-286 Built-In Procedures

                         80287
Procedure                Instruction        Description

INIT$REAL$MATH$UNIT
Also initializes interrupt pointers for emulation.     FINIT              Initialize processor.

SET$REAL$MODE            FLDCW              Set exception masks, rounding
                                            precision, and infinity
                                            controls.

GET$REAL$ERROR
Returns low-order byte of status word.          FNSTSW & FNCLEX    Store, then clear, exception
                                            flags.

SAVE$REAL$STATUS         FNSAVE             Save processor state.

RESTORE$REAL$STATUS      FRSTOR             Restore processor state.


ASM286
The ASM286 assembly language provides programmmers with complete access to
all of the facilities of the 80286 and 80287 processors.

The programmer's view of the 80286/80287 hardware is a single machine with
these resources:

  Ž  160 instructions
  Ž  12 data types
  Ž  8 general registers
  Ž  4 segment registers
  Ž  8 floating-point registers, organized as a stack


Defining Data
The ASM286 directives shown in table 2-16 allocate storage for 80287
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 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 stack location where the
processor's 94-byte state record has been previously saved.

The initial values for 80287 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 nonnormalized numbers. Most programmers will find that ordinary decimal
and scientific decimal provide the simplest way to initialize 80287
constants. Figure 2-3 compares several ways of setting the various 80287
data types to the same initial value.

Note that preceding 80287 variables and constants with the ASM286 EVEN
directive ensures that the operands will be word-aligned in memory. This
will produce the best system performance. All 80287 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 2-16. 80287 Storage Allocation Directives

Directive    Interpretation        Data Types

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


Records and Structures
The ASM286 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 2-4 shows one definition of the
status word and how it might be used in a routine that polls the 80287 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 2-5 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 2-3.  Sample 80287 Constants

  ;  THE FOLLOWING ALL ALLOCATE THE CONSTANT: -126
  ;  NOTE TWO'S COMPLETE STORAGE OF NEGATIVE BINARY INTEGERS.
  ;
     EVEN                                  ; FORCE WORK ALIGNMENT
  WORD_INTEGER    DW  111111111000010B     ; BIT STRING
  SHORT_INTEGER   DD  OFFFFFF82H           ; HEX STRING MUST START
                                           ; WITH DIGIT
  LONG_INTEGER    DQ  -126                 ; ORDINARY DECIMAL
  SHORT_REAL      DD  -126.0               ; NOTE PRESENCE OF '.'
  LONG_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.
  ;
  TEMP_REAL       DT  0C0057E00000000000000R   ; HEX STRING


Figure 2-4.  Status Word RECORD Definition

;  RESERVE SPACE FOR STATUS WORD
STATUS_WORD
;  LAY OUT STATUS WORD FIELDS
STATUS RECORD
&    BUSY:               1,
&    COND_CODE 3:        1,
&    STACK_TOP:          3,
&    COND_CODE 2:        1,
&    COND_CODE 1:        1,
&    COND_CODE 0:        1,
&    INT_REQ:            1,
&    RESERVED:           1,
&    P_FLAG:             1,
&    U_FLAG:             1,
&    O_FLAG:             1,
&    Z_FLAG:             1,
&    D_FLAG:             1,
&    I_FLAG:             1,
;  POLL STATUS WORD UNTIL 80287 IS NOT BUSY
POLL:   FNSTSW   STATUS_WORD
        TEST     STATUS_WORD, MASK_BUSY
        JNZ      POLL


Figure 2-5.  Structure Definition

  SAMPLE        STRUC

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


Addressing Modes
80287 memory data can be accessed with any of the CPU's five memory
addressing modes. This means that 80287 data types can be incorporated in
data aggregates ranging from simple to complex according to the needs of the
application. The addressing modes, and the ASM286 notation used to specify
them in instructions, make the accessing of structures, arrays, arrays of
structures, and other organizations direct and straightforward. Table 2-17
gives several examples of 80287 instructions coded with operands that
illustrate different addressing modes.


Table 2-17. Addressing Mode 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).


Comparative Programming Example
Figures 2-6 and 2-7 show the PL/M-286 and ASM286 code for a simple 80287
program, called ARRSUM. The program references an array (X$ARRAY), which
contains 0-100 short real values; the integer variable N$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: projective closure, round to nearest, 64-bit precision,
interrupts enabled, and all exceptions masked 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 80287
without altering the code shown.

The PL/M-286 version of ARRSUM (figure 2-6) is very straightforward and
illustrates how easily the 80287 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$ARRAY with a DO-loop. The
loop control takes into account PL/M-286's practice of considering the index
of the first element of an array to be 0. In the computation of SUM$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 temporary real
format).

The ASM286 version (figure 2-7) defines the external procedure INIT287,
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
INIT287 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 2-8, 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 CX, which LOOP automatically decrements, is loaded with
N_OF_X, the number of array elements to be summed. Register SI is used to
select (index) the array elements. The program steps through X_ARRAY from
back to front, so SI is initialized to point at the element just beyond the
first element to be processed. The ASM286 TYPE operator is used to determine
the number of bytes in each array element. This permits  changing X_ARRAY to
a long real array by simply changing its definition (DD to DQ) and
reassembling.

Figure 2-8 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 2-6.  Sample PL/M-286 Program

PL/M-286 COMPILER   ARRAYSUM

SERIES-III PL/M-286 V 1.0 COMPILATION OF MODULE ARRAYSUM
OBJECT MODULE PLACED IN :F6:D.OBJ
COMPILER INVOLKED BY   PLM286.86 :F6:D.SRC XREF

            /******************************************
            *                                         *
            *     ARRAYSUM MOD                        *
            *                                         *
            ******************************************/

 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$287 literally '033eh';

            /* Assume x$array and n$of$x are initalized */

            /* Prepare the 80287 of its emulator */
 6    1     call init$real$math$unit;
 7    1     call set$real$mode(control$287);

            /* 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;

PL/M-286 COMPILER     ARRAYSUM
                      CROSS-REFERENCE LISTING

DEFN   ADDR   SIZE  NAME,ATTRIBUTES, AND REFERENCES

    1  0006H   117  ARRAYSUM             PROCEDURE STACK=002H
    5               CONTROL287           LITERALLY '033eh'    7
                    FLOAT                BUILTIN             11
    4  019EH     2  I                    INTEGER              9*    9    10    11    12    13
                    INITREALMATHUNIT     BUILTIN              6
    4  019CH     2  NDFX                 INTEGER              9
                    SETREALMODE          BUILTIN              7
    2  0004H     4  SUMINDEXES           REAL          8*    11    11*
    2  0008H     4  SUMSQAURES           REAL          8*    12    12*
    2  0000H     4  SUMX                 REAL          8*    10    10*
    3  000CH   400  XARRAY               REAL ARRAY(100)            10    11    12

MODULE INFORMATION
      CODE AREA SIZE     = 0077H    119D
      CONSTANT AREA SIZE = 0004H      4D
      VARIABLE AREA SIZE = 01A0H    416D
      MAXIMUM STACK SIZE = 0002H      2D
      33 LINES READ
      0 PROGRAM WARNINGS
      0 PROGRAM ERRORS

DICTIONARY SUMMARY
      96KB MEMORY AVILABLE
      3KB MEMORY USED  (3%)
      0KB DISK SPACE USED

END OF PL/M-286 COMPILATION


Figure 2-7.  Sample ASM286 Program

iAPX286 MACRO ASSEMBLER     EXAMPLE ASM286_PROGRAM

SERIES-III iAPX286 MACRO ASSEMBLER X108 ASSEMBLY OF MODULE EXAMPLE_ASM286_PROGRAM
OBJECT MODULE PLACED IN :F6:287EXP.OBJ
ASSEMBLER INVOKED BY   ASM286 B6 :F6:287EXP.SRC XREF

LOC  OBJ              LINE     SOURCE

                         1                 name    example_ASM286_program
                         2     ; Define intitialization routine
                         3                 extrn       init287:far
                         4
                         5     ;  Allocate space for date
----                     6     data        segment rw public
0000 3E03                7     control_287     dw      033eh
0002 ????                8     n_of_x          dw      ?
0004 (000                9     x_array         dd      100 dwp (?)
     ????????
     )
0194 ????????           10     sum_squares     dd      ?
019B ????????           11     sum_indexes     dd      ?
019C ????????           12     sum_x           dd      ?
----                    13     data     ends
                        14
                        15     ; Allocate CPU stack space
----                    16     stack       stackseg 400
                        17
                        18     ; Begin code
----                    19     code        segment or public
                        20                 assumes ds: data, ss: stack, es: nothing
0000                    21     start:
0000 BB----       R     22                 mov     ax,data
0003 BED8               23                 mov     ds,ax
0005 B8----       R     24                 mov     ax,stack
0008 BED0               25                 mov     ss,ax
000A BCFEFF       R     26                 mov     sp,stackstart stack
                        27
                        28     ; Assume x_array and n_of_x are initialized
                        29     ;           this pprogram zeroes n_of_x
                        30
                        31     ; Prepare the 80287 or its emulator
000D 9A0000----   E     32                 call    init287
0012 D92E0000     R     33                 fldcw   control_287
                        34
                        35     ; Clear three registers to hold running sums
0016 D9EE               36                 fldz
0018 D9EE               37                 fldz
001A D9EE               38                 fldz
                        39
                        40     ; Setup CX as loop counter and
                        41     ; SI as index to x_array
001C 8B0E0200     R     42                 mov     cx,n_of_x
0020 F7E9               43                 imul    cx
0022 8BF0               44                 mov     si,ax
                        45
                        46     ; SI now contains index of last element + 1
                        47     ; Loop thru x_array, accumulating sums
0024                    48     sum_next:
0024 8E3304             49                 sub     si,type x_array ;backup one element
0027 D9840400     R     50                 fld     x_array[si]     ;push it on the stack
002B DCC3               51                 fadd    st(3),st        ;add into sum of x
002D D9C0               52                 fld     st              ;duplicate x on tap
002F DCC8               53                 fmul    st,st           ;square it
0031 DEC2               54                 faddp   st(2),st        ;add into sum of (index+x)
                        55                                         ;  and discard
0033 FF0E0200     R     56                 dec     n_of_x          ;reduce index for next iteration
0037 E2EB               57                 loop    sum_next                ;continue
                        58
                        59     ; Pop running sums into memory
0039                    60     pop_results:
0039 D91E9401     R     61                 fstp    sum_squares
003D D91E9801     R     62                 fstp    sum_indexes
0041 D91E9C01     R     63                 fstp    sum_x
0045 9B                 64                 fwait
                        65
                        66     ;
                        67     ; Etc.
                        68     ;
----                    69     code        ends
                        70                 end     start

iAPX286 MACRO ASSEMBLER     EXAMPLE_ASM286_PROGRAM
XREF SYMBOL TABLE LISTING

NAME          TYPE      VALUE   ATTRIBUTES, XREFS

CODE          SEGMENT           SIZE=0046H ER PUBLIC  19# 69
CONTROL_287   V WORD    0000H   DATA  7# 33
DATA          SEGMENT           SIZE=01A0H RW PUBLIC   6# 13 20 22
INIT287       L FAR     0000H   EXTR   3# 32
N_OF_X        V WORD    0002H   DATA   8# 42 56
POP_RESULTS   L NEAR    0039H   CODE   60#
STACK         STACK             SIZE=0190H RW PUBLIC   16# 20 24 26
START         L NEAR    0000H   CODE   21# 70
SUM_INDEXES   V DWORD   0198H   DATA   11# 62
SUM_NEXT      L NEAR    0024H   CODE   48# 57
SUM_SQUARES   V DWORD   0194H   DATA   10# 61
SUM_X         V DWORD   019CH   DATA   12# 63
X_ARAY        V DWORD   0004H   (100) DATA   9# 49 50

END OF SYMBOL TABLE LISTING

ASSEMBLY COMPLETE,                  NO ERRORS


Figure 2-8.  Instructions and Register Stack

          FLDZ,FLDZ,FLDZ                    FDL X_ARRAY[SI]
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             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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            FMUL ST,ST�                     FADDP ST(2),ST
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        ST(0)€ 6.25€ X_ARRAY(19)^(2)      ST(0)€ 2.5 € X_ARRAY(19)
             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
        ST(2)€ 0.0 € SUM_SQUARES          ST(2)€ 0.0 € SUM_INDEXES
             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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             Ñ‘‘‘‘               ’‘‘‘‘‘‘‘‘‘‘‘„�����…
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             „�����…               �
                       ’‘‘‘‘‘‘‘‘‘‘‘•
           FIMUL N_OF_X�                     FADDP ST(2),ST
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        ST(0)€ 50.0€ X_ARRAY(19)*20       ST(0)€ 6.25€ SUM_SQUARES
             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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             Ñ‘‘‘‘                           Ñ‘‘‘‘Â
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80287 Emulation
The programming of applications to execute on both 80286 and 80287 is made
much easier by the existence of an 80287 emulator for 80286 systems. The
Intel E80287 emulator offers a complete software counterpart to the 80287
hardware; NPX instructions can be simply emulated in software rather than
being executed in hardware. With software emulation, the distinction
between 80286 and 80287 systems is reduced to a simple performance
differential (see Table 1-2 for a performance comparison between an actual
80287 and an emulator 80287).  Identical numeric programs will simply
execute more slowly on 80286 systems (using software emulation of NPX
instructions) than on executing NPX instructions directly.

When incorporated into the systems software, the emulation of NPX
instructions on the 80286 systems is completely transparent to the
programmer. Applications software needs no special libraries, linking, or
other activity to allow it to run on an 80286 with 80287 emulation.

To the applications programmer, the development of programs for 80286
systems is the same whether the 80287 NPX hardware is available or not. The
full 80287 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 80287 emulators are described
in a later section of this supplement. An E80287 software emulator for 80286
systems is contained in the iMDX 364 8086 Software Toolbox, available from
Intel and described in the 8086 Software Toolbox Manual.


Concurrent Processing with the 80287
Because the 80286 CPU and the 80287 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 80286 CPU and
80287 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.


Managing Concurrency
Concurrent execution of the host and 80287 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.

Managing concurrency is necessary because both the arithmetic and control
areas must converge to a well-defined state before starting another numeric
operation. A well-defined state means all previous arithmetic and control
operations are complete and valid.

Normally, the host waits for the 80287 to finish the current numeric
operation before starting another. This waiting is called synchronization.

Managing concurrent execution of the 80287 involves three types of
synchronization:

  1.  Instruction synchronization
  2.  Data synchronization
  3.  Error synchronization

For programmers in higher-level languages, all three types of
synchronization are automatically provided by the appropriate compiler. For
assembly-language programmers, instruction synchronization is guaranteed by
the NPX interface, but data and error synchronization are the
responsibility of the assembly-language programmer.


Instruction Synchronization
Instruction synchronization is required because the 80287 can perform only
one numeric operation at a time. Before any numeric operation is started,
the 80287 must have completed all activity from its previous instruction.

Instruction synchronization is guaranteed for most ESC instructions because
the 80286 automatically checks the BUSY status line from the 80287
before commencing execution of most ESC instructions. No explicit WAIT
instructions are necessary to ensure proper instruction synchronization.


Data Synchronization
Data synchronization addresses the issue of both the CPU and the NPX
referencing the same memory values within a given block of code.
Synchronization ensures that these two processors access the memory operands
in the proper sequence, just as they would be accessed by a single
processor with no concurrency. Data synchronization is not a concern when
the CPU and NPX are using different memory operands during the course of one
numeric instruction.

The two cases where data synchronization might be a concern are

  1.  The 80286 CPU reads or alters a memory operand first, then invokes
      the 80287 to load or alter the same operand.

  2.  The 80287 is invoked to load or alter a memory operand, after which
      the 80286 CPU reads or alters the same location.

Due to the instruction synchronization of the NPX interface, data
synchronization is automatically provided for the first case‘‘the 80286 will
always complete its operation before invoking the 80287.

For the second case, data synchronization is not always automatic. In
general, there is no guarantee that the 80287 will have finished its
processing and accessed the memory operand before the 80286 accesses the
same location.

Figure 2-9 shows examples of the two possible cases of the CPU and NPX
sharing a memory value. In the examples of the first case, the CPU will
finish with the operand before the 80287 can reference it. The NPX interface
guarantees this. In the examples of the second case, the CPU must wait for
the 80287 to finish with the memory operand before proceeding to reuse it.
The FWAIT instructions shown in these examples are required in order to
ensure this data synchronization.

There are several NPX control instructions where automatic data
synchronization is provided; however, the FSTSW/FNSTSW, FSTCW/FNSTCW, FLDCW,
FRSTOR, and FLDENV instructions are all guaranteed to finish their execution
before the CPU can read or alter the referenced memory locations.

The 80287 provides data synchronization for these instructions by making a
request on the Processor Extension Data Channel before the CPU executes its
next instruction. Since the NPX data transfers occur before the CPU regains
control of the local bus, the CPU cannot change a memory value before the
NPX has had a chance to reference it. In the case of the FSTSW AX
instruction, the 80286 AX register is explicitly updated before the CPU
continues execution of the next instruction.

For the numeric instructions not listed above, the assembly-language
programmer must remain aware of synchronization and recognize cases
requiring explicit data synchronization. Data synchronization can be
provided either by programming an explicit FWAIT instruction, or by
initiating a subsequent numeric instruction before accessing the operands or
results of a previous instruction. After the subsequent numeric instruction
has started execution, all memory references in earlier numeric
instructions are complete. Reaching the next host instruction after the
synchronizing numeric instruction indicates that previous numeric operands
in memory are available.

The data-synchronization function of any FWAIT or numeric instruction must
be well-documented, as shown in figure 2-10. Otherwise, a change to the
program at a later time may remove the synchronizing numeric instruction and
cause program failure.

High-level languages automatically establish data synchronization and
manage it, but there may be applications where a high-level language may not
be appropriate.

For assembly-language programmers, automatic data synchronization can be
obtained using the assembler, although concurrency of execution is lost as a
result. To perform automatic data synchronization, the assembler can be
changed to always place a WAIT instruction after the ESCAPE instruction.
Figure 2-11 shows an example of how to change the ASM286 Code Macro for the
FIST instruction to automatically place a WAIT instruction after the ESCAPE
instruction. This Code Macro is included in the ASM286 source module. The
price paid for this automatic data synchronization is the lack of any
possible concurrency between the CPU and NPX.


Figure 2-9.  Synchronizing References to Shared Data

  Case 1:                Case 2:
       MOV     I,  1          FILD      I
       FILD    I              FWAIT
                              MOV       I, 5

       MOV     AX, I          FISTP     I
       FISTP   I              FWAIT
                              MOV       AX, I


Figure 2-10.  Documenting Data Synchronization

  FISTP   I
  FMUL                ; I is updated before FMUL is executed
  MOV     AX, I       ; I is now safe to use


Figure 2-11.  Nonconcurrent FIST Instruction Code Macro

  ;
  ; This is an ASM286 code macro to redefine the FIST
  ; instruction to prevent any concurrency
  ; while the instruction runs. A wait
  ; instruction is placed immediately after the
  ; escape to ensure the store is done
  ; before the program may continue.
  CodeMacro FIST memop: Mw
  RfixM 111B, memop
  ModRM 010B, memop
  RWfix
  EndM


Error Synchronization
Almost any numeric instruction can, under the wrong circumstances, produce
a numeric error. Concurrent execution of the CPU and NPX requires
synchronization for these errors just as it does for data references and
numeric instructions. In fact, the synchronization required for data and
instructions automatically provides error synchronization.

However, incorrect data or instruction synchronization may not be
discovered until a numeric error occurs. A further complication is that a
programmer may not expect his numeric program to cause numeric errors, but
in some systems, they may regularly happen. To better understand these
points, let's look at what can happen when the NPX detects an error.

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 errors.
     Programs can mask individual error types to indicate that the NPX
     should generate a safe, reasonable result whenever that error occurs.
     The default error fix-up activity is treated by the NPX as part of the
     instruction causing the error; no external indication of the error is
     given. When errors 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 errors, then error
     synchronization is never exercised. This is no reason to ignore error
     synchronization, however.

  Ž  As an alternative to the NPX default fix-up of numeric errors, the
     80286 CPU can be notified whenever an exception occurs. The CPU can
     then implement any sort of recovery procedures desired, for any numeric
     error detectable by the NPX. When a numeric error is unmasked and the
     error 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. Some ESC instructions do not check for errors. These are the
     nonwaited forms FNINIT, FNSTENV, FNSAVE, FNSTSW, FNSTCW, and FNCLEX.

When the NPX signals an unmasked exception condition, it is requesting
help. The fact that the error 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.

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


Incorrect Error Synchronization
An example of how some instructions written without error synchronization
will work initially, but fail when moved into a new environment is shown in
figure 2-12.

In figure 2-12, three instructions are shown to load an integer, calculate
its square root, then increment the integer. The NPX interface and
synchronous execution of the NPX emulator will allow this program to execute
correctly when no errors occur on the FILD instruction.

This situation changes if the 80287 numeric register stack is extended to
memory. To extend the NPX stack to memory, the invalid error is unmasked. A
push to a full register or pop from an empty register will cause an invalid
error. The recovery routine for the error 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.


Figure 2-12. Error 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  COUNT   ; subsequent NPX instruction - error from
               ;    previous NPX instruction detected here
INC    COUNT   ; CPU instruction alters operand


Proper Error Synchronization
Error Synchronization relies on the WAIT instructions required by
instruction and data synchronization and the BUSY and ERROR signals of
the 80287. When an unmasked error occurs in the 80287, it asserts the
ERROR signal, signalling to the CPU that a numeric error has occurred.
The next time the CPU encounters an error-checking ESC or WAIT instruction,
the CPU acknowledges the ERROR signal by trapping automatically to
Interrupt #16, the Processor Extension Error 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 error.


Chapter 3  System-Level Numeric Programming

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

System programming for 80287 systems requires a more detailed understanding
of the 80287 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.


80287 Architecture
On a software level, the 80287 NPX appears as an extension of the 80286
CPU. On the hardware level, however, the mechanisms by which the 80286 and
80287 interact are a bit more complex. This section describes how the 80287
NPX and 80286 CPU interact and points out features of this interaction that
are of interest to systems programmers.


Processor Extension Data Channel
All transfers of operands between the 80287 and system memory are performed
by the 80286's internal Processor Extension Data Channel. This independent,
DMA-like data channel permits all operand transfers of the 80287 to come
under the supervision of the 80286 memory-management and protection
mechanisms. The operation of this data channel is completely transparent to
software.

Because the 80286 actually performs all transfers between the 80287 and
memory, no additional bus drivers, controllers, or other components are
necessary to interface the 80287 NPX to the local bus. Any memory accessible
to the 80286 CPU is accessible by the 80287. The Processor Extension Data
Channel is described in more detail in Chapter Six of the 80286 Hardware
Reference Manual.


Real-Address Mode and Protected Virtual-Address Mode
Like the 80286 CPU, the 80287 NPX can operate in both Real-Address mode and
in Protected mode. Following a hardware RESET, the 80287 is initially
activated in Real-Address mode. A single, privileged instruction (FSETPM) is
necessary to set the 80287 into Protected mode.

As an extension to the 80286 CPU, the 80287 can access any memory location
accessible by the task currently executing on the 80286. When operating in
Protected mode, all memory references by the 80287 are automatically
verified by the 80286'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 80286 to
trap to an appropriate exception handler.

To the programmer, these two 80287 operating modes differ only in the
manner in which the NPX instruction and data pointers are represented in
memory following an FSAVE or FSTENV instruction. When the 80287 operates in
Protected mode, its NPX instruction and data pointers are each represented
in memory as a 16-bit segment selector and a 16-bit offset. When the 80287
operates in Real-Address mode, these same instruction and data pointers are
represented simply as the 20-bit physical addresses of the operands in
question (see figure 1-7 in Chapter One).


Dedicated and Reserved I/O Locations
The 80287 NPX does not require that any memory addresses be set aside for
special purposes. The 80287 does make use of I/O port addresses in the range
00F8H through 00FFH, although these I/O operations are completely
transparent to the 80286 software. 80286 programs must not reference these
reserved I/O addresses directly.

To prevent any accidental misuse or other tampering with numeric
instructions in the 80287, the 80286's I/O Privilege Level (IOPL) should be
used in multiuser reprogrammable environments to restrict application
program access to the I/O address space and so guarantee the integrity of
80287 computations. Chapter Eight of the 80286 Operating System Writer's
Guide contains more details regarding the use of the I/O Privilege Level.


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 80287 NPX. In this section, issues
related to system initialization and control are described, including
recognition of the NPX, emulation of the 80287 NPX in software if the
hardware is not available, and the handling of exceptions that may occur
during the execution of the 80287.


System Initialization
During initialization of an 80286 system, systems software must

  Ž  Recognize the presence or absence of the NPX

  Ž  Set flags in the 80286 MSW to reflect the state of the numeric
     environment

If an 80287 NPX is present in the system, the NPX must be

  Ž  Initialized

  Ž  Switched into Protected mode (if desired)

All of these activities can be quickly and easily performed as part of the
overall system initialization.


Recognizing the 80287 NPX
Figure 3-1 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 3-1.  Software Routine to Recognize the 80287

;  The following algorithm detects the presence of the 8087 as well as the
;  80287 in a system.  This will make it easier for ISVs to port their 8086-87
;  software to 286-287 systems.
;
            cc_cr               equ     0DH     ;  carriage return
            cc_lf               equ     0AH     ;  line feed

assume      cz:code, ds:data
;
code        segment             public
start:
            mov     ax,data                 ;  set data segment
            mov     ds,ax
;
;  Test if 8087 is present in PC or PC/XT, or 80287 is in PC/AT
;
            fninit                          ;  initialize coprocessor
            xor     ah,ah                   ;  zero ah register and memory byte
            mov     byte ptr control + 1,ah
            fnstcw  control                 ;  store coprocessor's control word in
                                            ;  memory
            mov     ah,byte ptr control+1
            cmp     ah,03h                  ;  upper byte of control work will be
                                            ;  03 if 8087 or 80287 coprocessor
                                            ;  is present
            jne     no_coproc
;
coproc:
            mov     ah,09h                  ;  print string-coprocessor present
            mov     dx,offset msg_yes
            int     21h
            jmp     done

;
no_coproc:
            mov     ah,09h                  ; print string-coprocessor not
                                            ; present
            mov     dx,offset msg_no
            int     21h
;
done:
            mov     ah,4CH                  ; terminate program
            int     21h
code        ends

data        segment         public
control     dw              00
msg_yes     db      cc_cr,cc_lf,
            db      'System has an 8087 or 80287',cc_cr, cc_lf, '$'
msg_no      db      cc_cr,cc_lf,
            db      'System does not have an 8087 or 80287',cc_cr, cc_lf,
                     '$'
data        ends
            end     start                   ; start is the entry point


Configuring the Numerics Environment
Once the 80286 CPU has determined the presence or absence of the 80287 NPX,
the 80286 must set either the MP or the EM bit in its own machine status
word accordingly. The initialization routine can either

  Ž  Set the MP bit in the 80286 MSW to allow numeric instructions to be
     executed directly by the 80287 NPX component

  Ž  Set the EM bit in the 80286 MSW to permit software emulation of the
     80287 numeric instructions

The Math Present (MP) flag of the 80286 machine status word indicates to
the CPU whether an 80287 NPX is physically available in the system. The MP
flag controls the function of the WAIT instruction. When executing a WAIT
instruction, the 80286 tests only the Task Switched (TS) bit if MP is set;
if it finds TS set under these conditions, the CPU traps to exception #7.

The Emulation Mode (EM) bit of the 80286 machine status word indicates to
the CPU 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 80287. The 80286 EM flag can be changed only by
using the LMSW (load machine status word) instruction (legal only at
privilege level 0) and examined with the aid of the SMSW (store machine
status word) instruction (legal at any privilege level).

The EM bit also controls the function of the WAIT instruction. If the CPU
finds EM set while executing a WAIT, the CPU does not check the ERROR
pin for an error indication.

For correct 80286 operation, the EM bit must never be set concurrently with
MP. The EM and MP bits of the 80286 are described in more detail in the
80286 Operating System Writer's Guide. More information on software
emulation for the 80287 NPX is described in the "80287 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.


Initializing the 80287
Initializing the 80287 NPX simply means placing the NPX in a known state
unaffected by any activity performed earlier. The example software routine
to recognize the 80287 (figure 3-1) performed this initialization using a
single FNINIT instruction. This instruction causes the NPX to be
initialized in the same way as that caused by the hardware RESET signal to
the 80287. All the error masks are set, all registers are tagged empty, the
ST is set to zero, and default rounding, precision, and infinity controls
are set. Table 3-1 shows the state of the 80287 NPX following
initialization.

Following a hardware RESET signal, such as after initial power-up, the
80287 is initialized in Real-Address mode. Once the 80287 has been switched
to Protected mode (using the FSETPM instruction), only another hardware
RESET can switch the 80287 back to Real-Address mode. The FNINIT instruction
does not switch the operating state of the 80287.


80287 Emulation
If it is determined that no 80287 NPX is available in the system, systems
software may decide to emulate ESC instructions in software. This emulation
is easily supported by the 80286 hardware, because the 80286 can be
configured to trap to a software emulation routine whenever it encounters
an ESC instruction in its instruction stream.

As described previously, whenever the 80286 CPU encounters an ESC
instruction, and its MP and EM status bits are set appropriately (MP = 0,
EM = 1), the 80286 will automatically trap 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 80286 system with 80287
emulation is almost indistinguishable from execution on an 80287  system,
except for the difference in execution speeds.

There are several important considerations when using emulation on an 80286
system:

  Ž  When operating in Protected-Address 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 80286 in emulation mode. The
     instructions necessary to place the 80286 in Emulation mode are
     privileged instructions, and are not typically accessible to an
     application.

An emulator package (E80287) that runs on 80286 systems is available from
Intel in the 8086 Software Toolbox, Order Number 122203. This emulation
package operates in both Real and Protected mode, providing a complete
functional equivalent for the 80287 emulated in software.

When using the E80287 emulator, writers of numeric exception handlers
should be aware of one slight difference between the emulated 80287 and the
80287 hardware:

  Ž  On the 80287 hardware, exception handlers are invoked by the 80286 at
     the first WAIT or ESC instruction following the instruction causing the
     exception. The return link, stored on the 80286 stack, points to this
     second WAIT or ESC instruction where execution will resume following a
     return from the exception handler.

  Ž  Using the E80287 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
     E80287 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 80287's instruction and data pointers.


Table 3-1. NPX Processor State Following Initialization


Field                    Value         Interpretation

Control Word
 Infinity Control         0            Projective
 Rounding Control         00           Round to nearest
 Precision Control        11           64 bits
 Interrupt-Enable Mask    1            Interrupts disabled
 Exception Masks          111111       All exceptions masked

Status Word
 Busy                     0            Not busy
 Condition Code           ????         (Indeterminate)
 Stack Top                000          Empty stack
 Interrupt Request        0            No interrupt
 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


Handling Numeric Processing Exceptions
Once the 80287 has been initialized and normal execution of applications
has been commenced, the 80287 NPX may occasionally require attention in
order to recover from numeric processing errors. This section provides
details for writing software exception handlers for numeric exceptions.
Numeric processing exceptions have already been introduced in previous
sections of this manual.

As discussed previously, the 80287 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
     80286 CPU using the ERROR status line between the two processors.
     Each time the 80286 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 80286 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 save environment.

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

  Ž  Return to the interrupted program and resume normal execution.

It should be noted that the NPX exception pointers contained in the stored
NPX environment will take different forms, depending on whether the NPX is
operating in Real-Address mode or in Protected mode. The earlier discussion
of Real versus Protected mode details how this information is presented in
each of the two operating modes.


Simultaneous Exception Response
In cases where multiple exceptions arise simultaneously, the 80287 signals
one exception according to the precedence sequence shown in table 3-2. This
means, for example, that zero divided by zero will result in an invalid
operation, and not a zero divide exception.


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 error 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 errors and associated recovery
routines in a manner transparent to the application programmer.

Some other possible system-dependent actions, mentioned previously, may
include:

  Ž  Incrementing an exception counter for later display or printing

  Ž  Printing or displaying diagnostic information (e.g., the 80287
     environment and registers)

  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 implementation. Once the exception handler corrects the error 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 from the NPX 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 Four include an
outline of several exception handlers to process numeric exceptions for the
80287.


Table 3-2. Precedence of NPX Exceptions

Signaled First:       Denormalized operand (if unmasked)
                      Invalid operation
                      Zero divide
                      Denormalized (if masked)
                      Over/Underflow
Signaled Last:        Precision


Chapter 4  Numeric Programming Examples

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

The following sections contain examples of numeric programs for the 80287
NPX written in ASM286. These examples are intended to illustrate some of the
techniques for programming the 80287 computing system for numeric
applications.


Conditional Branching Examples
As discussed in Chapter Two, several numeric instructions post their
results to the condition code bits of the 80287 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. (80287 allows storing status directly into AX
     register.)

  Ž  Inspect the condition code bits.

  Ž  Jump on the result.

Figure 4-1 is a code fragment that illustrates how two memory-resident long
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 80287 register
stack and then comparing it to B, while popping the stack with the same
instruction. The status word is then written into the 80286 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 4-2 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 16-bit
displacement of 16 labels, one for each possible condition code setting.
Note that four of the table entries contain the same value, because four
condition code settings correspond to "empty."

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 labelled
routine that contains code (not shown in the example) to process each
possible result of the FXAM instruction.


Figure 4-1.  Conditional Branching for Compares

            .
            .
            .
  A         DQ        ?
  B         DQ        ?
            .
            .
            .
            FLD       A         ; LOAD A ONTO TOP OF 287 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 4-2.  Conditional Branching for FXAM

            ; JUMP TABLE FOR EXAMINE ROUTINE
            ;
  FXAM_TBL  DW 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
            FSTSW AX
            ;
            ;  CALCULATE OFFSET INTO JUMP TABLE
            MOV       BH,0      ; CLEAR UPPER HALF OF BX,
            MOV       BL,AH     ; LOAD CONDITION CODE INTO BL
            AND       BL,00000111B    ; CLEAR ALL BITS EXCEPT C2-C0
            AND       AH,01000000B    ; CLEAR ALL BITS EXCEPT C3
            SHR       AH,2      ;  SHIFT C3 TWO PLACES RIGHT
            SAL       BX,1      ;  SHIFT C2-C0 1 PLACE LEFT (MULTIPLY
                                ;  BY 2)
            OR        BL,AH     ;  DROP C3 BACK IN ADJACENT TO C2
                                ;  (000XXXX0)
            ;
            ;  JUMP TO THE ROUTINE `ADDRESSED' BY CONDITION CODE
            JMP       FXAM_TBL[BX]
            ;
            ;  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:


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. (For compatibility with
the 80287 emulators, this procedure should be invoked by interrupt pointer
(vector) 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 will involve saving
CPU registers and transferring diagnostic information from the 80287 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 exception handler body 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 80287 or another exception
will be requested immediately.

Figures 4-3, 4-4 and 4-5 show the ASM286 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.

Figure 4-3 and 4-4 are very similar; their only substantial difference is
their choice of instructions to save and restore the 80287. 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 will
not recognize another interrupt request (unless it is a nonmaskable
interrupt).

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 80287 are
cleared to zero prior to reloading (in fact, in these examples, the entire
status word image is cleared).

The examples in figures 4-3 and 4-4 assume that the exception handler
itself will not cause an unmasked exception. Where this is a possibility,
the general approach shown in figure 4-5 can be employed. The basic
technique is to save the full 80287 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 4-3.  Full-State Exception Handler

  SAVE_ALL            PROC
  ;
  ; SAVE CPU REGISTERS, ALLOCATE STACK SPACE FOR 80287 STATE IMAGE
            PUSH      BP
            MOV       BP,SP
            SUB       SP,94
  ; SAVE FULL 80287 STATE, WAIT FOR COMPLETION, ENABLE CPU INTERRUPTS
            FNSAVE    [BP-94]
            FWAIT
            STI
  ;
  ; APPLICATION-DEPENDENT EXCEPTION HANDLING CODE GOES HERE
  ;
  ; CLEAR EXCEPTION FLAGS IN STATUS WORD RESTORE MODIFIED STATE IMAGE
            MOV       BYTE PTR [BP-92], 0H
            FRSTOR    [BP-94]
  ; DE-ALLOCATE STACK SPACE, RESTORE CPU REGISTERS
            MOV       SP,BP
            .
            .
            POP       BP
  ;
  ; RETURN TO INTERRUPTED CALCULATION
            IRET
  SAVE_ALL            ENDP


Figure 4-4.  Reduced-Latency Exception Handler

  SAVE_ENVIRONMENT PROC
  ;
  ; SAVE CPU REGISTERS, ALLOCATE STACK SPACE FOR 80287 ENVIRONMENT
            PUSH      BP
            .
            MOV       BP,SP
            SUB       SP,14
  ; SAVE ENVIRONMENT, WAIT FOR COMPLETION, ENABLE CPU INTERRUPTS
            FNSTENV   [BP-14]
            FWAIT
            STI
  ;
  ; APPLICATION EXCEPTION-HANDLING CODE GOES HERE
  ;
  ; CLEAR EXCEPTION FLAGS IN STATUS WORD RESTORE MODIFIED
  ; ENVIRONMENT IMAGE
            MOV       BYTE PTR [BP-12], 0H
            FLDENV    [BP-14]
  ; DE-ALLOCATE STACK SPACE, RESTORE CPU REGISTERS
            MOV       SP,BP
            POP       BP
  ;
  ; RETURN TO INTERRUPTED CALCULATION
            IRET
  SAVE_ENVIRONMENT ENDP


Figure 4-5.  Reentrant Exception Handler

            .
            .
            .
         LOCAL_CONTROL  DW  ?  ; ASSUME INITIALIZED
            .
            .
            .
  REENTRANT           PROC
  ;
  ; SAVE CPU REGISTERS, ALLOCATE STACK SPACE FOR
  ; 80287 STATE IMAGE
         PUSH     BP
            .
            .
            .
        MOV       BP,SP
        SUB       SP,94
  ; SAVE STATE, LOAD NEW CONTROL WORD, FOR COMPLETION, ENABLE CPU
  ; INTERRUPTS
        FNSAVE    [BP-94]
        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 RESTORE MODIFIED STATE IMAGE
        MOV       BYTE PTR [BP-92], 0H
        FRSTOR    [BP-94]
  ; DE-ALLOCATE STACK SPACE, RESTORE CPU REGISTERS
        MOV       SP,BP
            .
            .
            .
        POP       BP
  ; RETURN TO POINT OF INTERRUPTION
        IRET
  REENTRANT           ENDP


Floating-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 4-6 can be invoked from PL/M-286,
Pascal-286, FORTRAN-286, or ASM286 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 4-6.  Floating-Point to ASCII Conversion Routine

iAPX286 MACRO ASSEMBLER   80287 Floating-Point to 18-Digit ASCII Conversion          10:12:38  09/25/83  PAGE    1

SERIES-III iAPX286 MACRO ASSEMBLER X108 ASSEMBLY OF MODULE FLOATING_TO_ASCII
OBJECT MODULE PLACED IN :F3:FPASC.OBJ
ASSEMBLER INVOKED BY:  ASM286.86 :F3:FPASC.AP2

LOC  OBJ           LINE       SOURCE

                      1  +1   $title("80287 Floating-Point to 18-Digit ASCII Conversion")
                      2
                      3                       name    floating_to_ascii
                      4
                      5                       public  floating_to_ascii
                      6                       extrn   get_power_IO near.tos_status near
                      7       ;
                      8       ;        This subroutine will convert the floating point number in the
                      9       ;        top of the 80287 stack to an ASCII string and separate power of 10
                     10       ;        scaling value (in binary).  The maximum width of the ASCII string
                     11       ;        formed is controlled by a parameter which must be > 1.  Unnormal values,
                     12       ;        denormal values, and psuedo zeroes will be correclty converted.
                     13       ;        A returned value will indicate how many binary bits of
                     14       ;        precision were lost in an unnormal or denormal value.  The magnitude
                     15       ;        (in terms of binary power) of a psuedo zero will also be indicated.
                     16       ;        Integers less than 10**18 in magnitude are accurately converted if the
                     17       ;        destination ASCII string field is wide enough to hold all the
                     18       ;        digits.  Otherwise the value is converted to scientific notation.
                     19       ;
                     20       ;          The status of the conversion is identified by the return value,
                     21       ;        it can be:
                     22       ;
                     23       ;                0       conversion complete, string_size is defined
                     24       ;                1       invalid arguments
                     25       ;                2       exact integer conversion, string_size is defined
                     26       ;                3       indefinite
                     27       ;                4       + NAN (Not A Number)
                     28       ;                5       - NAN
                     29       ;                6       + Infinity
                     30       ;                7       - Infinity
                     31       ;                8       psuedo zero found, string_size is defined
                     32       ;
                     33       ;            The PLM/286 calling convention is
                     34       ;
                     35       ;floating_to_ascii:
                     36       ;        procedure (number,denormal ptr, string ptr, size_ptr, field_size,
                     37       ;                power_ptr) word external.
                     38       ;        declare (denormal_ptr, string ptr, power ptr, size_ptr) pointer,
                     39       ;        declare field_size word, string_size basd size_ptr word,
                     40       ;        delcare number real;
                     41       ;        declare denormal integer based denormal_ptr,
                     42       ;        declare power integer based power_ptr,
                     43       ;        and floating_to_ascii,
                     44       ;
                     45       ;          The floating point value is expected to be on the top of the NPX
                     46       ;        stack.  This subroutine expects 3 free entries on the NPX stack and
                     47       ;        will pop the passed value off when done.  The generated ASCII string
                     48       ;        will have a leading character either '-' or '+' indicating the sign
                     49       ;        of the value.  The ASCII decimal digits will immediately follow.
                     50       ;        The numeric value of the ASCII string is (ASCII STRING )*10**POWER.
                     51       ;        If the given number was zero, the ASCII string will contain a sign
                     52       ;        and a single zero character.  The value string_size indicates the total
                     53       ;        length of the ASCII string including the sign character.  String(0) will
                     54       ;        always hold the sign.  It is possible for string_size to be less than
                     55       ;        field_size.  This occurs for zeroes or integer values.  A psuedo zero
                     56       ;        will return a special return code.  The denormal count will indicate
                     57       ;        the power of the two originally associated with the value.  The power of
                     58       ;        ten and ASCII string will be as if the value was an ordinary zero.
                     59       ;
                     60       ;          The subroutine is accurate up to a maximum of 18 decimal digits for
                     61       ;        integers.  Integer values will have a decimal power of zero associated
                     62       ;        with them.  For non-integers, the result will be accurate to within 2
                     63       ;        decimal digits of the 16th decimal place (double pracision).  The
                     64       ;        exponentiate instruction is also used for scaling the value into the
                     65       ;        range acceptable for the BCD data type.  The rounding mode in effect
                     66       ;        on entry to the subroutine is used for the conversion.
                     67       ;
                     68       ;          The following registers are not tranparent
                     69       ;
                     70       ;                ax bx cx dx si di flags
                     71       ;
                     72  +1  $eject
                     73       ;
                     74       ;           Define the stack layout
                     75       ;
 0000[]              76       bp_save     equ     word ptr [bp]
 0002[]              77       es_save     equ     bp_save + size bp_save
 0004[]              78       return_ptr  equ     es_save + size es_save
 0006[]              79       power_ptr   equ     return_ptr _ size return_ptr
 0008[]              80       field_size  equ     power_ptr + size power_ptr
 000A[]              81       size_ptr    equ     field_size + size field_size
 000C[]              82       string_ptr  equ     size_ptr + size size_ptr
 000E[]              83       denormal_ptr    equ     string_ptr + size string_ptr
                     84
                     85       parms_size  equ     size power_ptr + size field_size + size_ptr +
 000A                86       &                   size string_ptr + size denormal_ptr
                     87
                     88                   Define constants used
                     89
 0012                90       BCD_DIGIIS  equ     18          ; Number of digits in bcd_value
 0002                91       WORD-SIZE   equ     2
 000A                92       BCD_SIZE    equ     10
 0001                93       MINUS       equ     1           ; Define return values
 0004                94       NAN         equ     4           ; The exact values chosen here are
 0006                95       INFINITY    equ     6           ; important.  They must correspond to
 0003                96       INDEFINITE  equ     3           ; the possible return values and be in
 0008                97       PSUEDO_ZERO equ     8           ; the same numeric order as tested by
-0002                98       INVALID     equ     -2          ; the program.
-0004                99       ZERO        equ     -4
-0006               100       DENORMAL    equ     -6
 0008               101       UNNORMAL    equ     -8
 0000               102       NORMAL      equ     0
 0002               103       EXACT       equ     2
                    104       ;
                    105       ;           Define layout of temporary storage area
                    106       ;
-0002[]             107       status      equ     word ptr [bp-WORD_SIZE]
-0004[]             108       power_two   equ     status - WORD_SIZE
-0006[]             109       power_ten   equ     power_two - WORD_SIZE
-0010[]             110       bcd_value   equ     tbyte ptr power_ten - BCD_SIZE
-0010[]             111       bcd_byte    equ     byte ptr bcd_value
-0010[]             112       fraction    equ     bcd_value
                    113
                    114       local_size  equ     size status + size pwer_two + size power_ten
 0010               115       &                   + size bcd_value
                    116
----                117       stack       stackseg (local_size+6) ; Allocate stack space for locals
                    118   +1  $eject
----                119       code        segment or public
                    120                   extrn   power_table:qword
                    121       ;
                    122       ;           Constants used by this function
                    123       ;
                    124                       even                ; Optimize for 16 bits
0000 0A00           125       const10         dw      10          ; Adjustment value for too big BCD
                    126       ;
                    127       ;         Convert the C3,C2,C1,C0 encoding from tos_status into meaningful bit
                    128       ;       flags and values.
                    129       ;
0002 F8             130       status_table    db  UNNORMAL, NAN, UNNORMAL + MINUS, NAN + MINUS
0003 04
0004 F9
0005 05
0006 00             131       &                   NORMAL, INFINITY, NORMAL + MINUS, INFINITY + MINUS
0007 06
0008 01
0009 07
000A FC             132       &                   ZERO, INVALID, ZERO + MINUS, INVALID
000B FE
000C FD
000D FE
000E FA             133       &                   DENORMAL, INVALID, DENORMAL + MINUS, INVALID
000F FE
0010 FB
0011 FE
                    134
0012                135       floating_to_ascii proc
                    136
0012 E80000         137               call    tos_status              ; look at status of ST(0)
0015 8BD8           138               mov     bx,ax                   ; Get descriptor from table
0017 2E8A870200     139               mov     al,status_table[bx]
001C 3CFE           140               cmp     al,INVALID              ; Look for empty ST(0)
001E 752B           141               jne     not_empty
                    142       ;
                    143       ;        ST(0) is empty!  Return the status value
                    144       ;
0020 C20A00         145               ret     parms_size
                    146       ;
                    147       ;         Remove infinity from stack and exit
                    148       ;
0023                149       found_infinity
                    150
0023 DDD8           151               fstp    st(0)                   ; OR to leave fstp running
0025 EB02           152               jmp     short exit_proc
                    153       ;
                    154       ;         String space is too small!  Return invalid code
                    155       ;
0027                156       small_string
                    157
0027 B0FE           158               mov     al,INVALID
                    159
0029                160       exit_proc:
                    161
0029 C9             162               leave                           ; Restore stack
002A 07             163               pop     es
002B C20A00         164               ret     parms_size
                    165       ;
                    166       ;         ST(0) is NAN or indefinite.  Store the value in memory and look
                    167       ;       at the fraction field to separate indefinite from an ordinary NAN.
                    168       ;
002E                169       NAN_or_indefinite:
002E DB7EF0         170
0031 A801           171               fstp    fraction                ; Remove value from stack for examination
0033 9B             172               test    al,MINUS                ; Look at sign bit
0034 74F3           173               fwait                           ; Insure store is done
                    174               jz      exit_proc               ; Can't be indefinite if positive
0036 BB00C0         175
0039 2B5EF6         176               mov     bx,0C000H               ; Match against upper 16 bits of fraction
003C 0B5EF4         177               sub     bx,word ptr fraction+6  ; Compare bits 63-4B
003F 0B5EF2         178               or      bx,word ptr fraction+4  ; Bits 32-47 must be zero
0042 0B5EF0         179               or      bx,word ptr fraction+2  ; Bits 31-16 must be zero
0045 75E2           180               or      bx,word ptr fraction    ; Bits 15-0 must be zero
                    181               jnz     exit_proc
0047 B003           182
0049 EBDE           183               mov     al,INDEFINITE           ; Set return value for indefinite value
                    184               jmp     exit_proc
                    185       ;
                    186       ;         Allocate stack space for local variables and establish parameter
                    187       ;       addressibility.
                    188       ;
004B                189       not_empty:
                    190
004B 06             191               push    es                      ; Save working register
004C C8100000       192               enter   local_size,0            ; Format stack
                    193
0050 8B4E08         194               mov     cx,field_ize            ; Check for enough string space
0053 83F902         195               cmp     cx,2
0056 7CCF           196               jl      sjall_string
                    197
005B 49             198               dec     cx                      ; Adjust for sign character
0059 83F912         199               cmp     cx,BCD_DIGITS           ; See if string is too large for BCD
005C 7603           200               jbe     size_ok
                    201
005E B91200         202               mov     cx,BCD_DIGITS           ; Else set maximum string size
                    203
0061                204       size_ok:
                    205
0061 3C06           206               cmp     al,INFINITY             ; Look for infinity
0063 7DBE           207               jge     found_infinity          ; Return status value for + or - inf
                    208
0065 3C04           209               cmp     al,NAN                  ; Look for NAN or INDEFINITE
0067 7DC5           210               jge     NAN_or_indefinite
                    211       ;
                    212       ;         Set default return values and check that the number is normalized
                    213       ;
0069 D9E1           214               fabs                            ; Use positive value only
                    215                                               ; sign bit in al has true sign of value
006B 8BD0           216               mov     dx,ax                   ; Save return value for later
006D 33C0           217               xor     ax,ax                   ; Form 0 constant
006F 8B7E0E         218               mov     di,denormal_ptr         ; Zero denormal count
0072 8905           219               mov     word ptr [di],ax
0074 8B5E06         220               mov     bx,power_ptr            ; Zero power of ten value
0077 B907           221               mov     word ptr [bx],ax
0079 80FAFC         222               cmp     dl,ZERO                 ; Test for zero
007C 732B           223               jae     real_zero               ; Skip power code if value is zero
                    224
007E 80FAFA         225               cmp     dl,DENORMAL             ; Look for a denormal value
008A 732C           226               jae     found_denormal          ; Handle it specially
                    227
0083 D9F4           228               fxtract                         ; Separate exponent from signifand
0085 80FAF8         229               cmp     dl,UNNORMAL             ; Test for unnormal value
0088 7240           230               jb      normal_value
                    231
008A 80EAF8         232               sub     dl,UNNORMAL-NORMAL      ; Return normal status with correct sign
                    233       ;
                    234       ;         Normalize the fraction, adjust the power of two in ST(1) and set
                    235       ;       the denormal count value
                    236       ;
                    237       ;       Assert 0 <= ST(0) < 1.0
                    238       ;
008D D9E8           239               fld1                            ; Load constant to normalize fraction
                    240
008F                241       normalize_fraction
                    242
008F DCC1           243               fadd    st(1),st                ; Set integer bit in fraction
0091 DEE9           244               fsub                            ; Form normalized fraction in ST(0)
0093 D9F4           245               fxtract                         ; Power of two field will be negative
                    246                                               ; of denormal count
0095 D9C9           247               fxch                            ; Put denormal count in ST(0)
0097 DF15           248               fist    word ptr [di]           ; Put negative of denormal count in memory
0099 DEC2           249               faddp   st(2),st                ; Form correct power of two in st(1)
                    250                                               ; OK to use word ptr [di] now
009B F71D           251               neg     word ptr [di]           ; Form positive denormal count
009D 752B           252               jnz     not_psuedo_zero
                    253       ;
                    254       ;         A psuedo zero will appear as an unnormal number.  When attempting
                    255       ;       to normalize it, the resultant fraction field will be zero.  Performing
                    256       ;       an fxtract on zero will yield a zero exponent value.
                    257       ;
009F D9C9           258               fxch                        ; Put power of two value in st(0)
00A1 DF1D           259               fistp   wrd ptr [di]        ; Set denormal count ot power of two value.
                    260                                           ; Word ptr [di] is not used by convert
                    261                                           ; integer, OK to leave running
00A3 B0EAF8         262               sub     dl,NORMAL-PSUEDO_ZERO ; Set return value saving the sign bit
00A6 E9A400         263               jmp     convert_integer     ; Put zero value into memory
                    264       ;
                    265       ;         The number is a real zero, set the return value and setup for
                    266       ;       conversion to BCD.
                    267       ;
00A9                268       real_zero
                    269
00A9 80EAF0         270               sub     dl,ZERO-NORMAL          ; Convert status to normal value
00AC E99E00         271               jmp     convert_integer         ; Treat the zero as an integer
                    272       ;
                    273       ;         The number is a denormal.  FXTRACT will not work correctly in this
                    274       ;       case.  To correctly separate the exponent and fraction, add a fixed
                    275       ;       constant to the exponent to guarantee the rsult is not a denormal.
                    276       ;
00AF                277       found_denormal:
                    278
00AF D9E8           279               fld1                            ; Prepare to bump exponent
00B1 D9C9           280               fxch
00B3 D9F8           281               fprem                           ; Force denormal to smallest representable
                    282                                               ; extended real format exponent
00B5 D9F4           283               fxtract                         ; This will work correctly now
                    284       ;
                    285       ;         The power of the original enormal value has been safely isolated.
                    286       ;       Check if the fraction value is an unnormal.
                    287       ;
00B7 D9E5           288               fxam                            ; See if the fraction is an unnormal
00B9 9BDFE0         289               fstsw   ax                      ; Save 80287 status in CPU AX reg for later
00BC D9C9           290               fxch                            ; Put exponent in ST(0)
00BE D9CA           291               fxch    st(2)                   ; Put 1.0 into ST(0), exponent in ST(2)
00C0 80EAFA         292               sub     dl,DENORMAL-NORMAL      ; Return normal status with correct sign
00C3 A90044         293               test    ax,4400H                ; See if C0=C2=0 impling unnormal or NAN
00C6 74C7           294               jz      normalize_fraction      ; Jump if fraction is an unnormal
                    295
00C8 DDD8           296               fstp    st(0)                   ; Remove unnecessary 1.0 from st(0)
                    297       ;
                    298       ;         Calculate the decimal magnitude associated with this number to
                    299       ;       within one order.  This error will always be inevitable due to
                    300       ;       rounding and lost precision.  As a result, we will deliberately fail
                    301       ;       to consider the LOQ10 of the fraction value in calcuating the order.
                    302       ;       Since the fraction will always be 1 <= F < 2, its LOQ10 will not change
                    303       ;       the basic accuracy of the function.  To get the decimal order of magnitude,
                    304       ;       simply multiply the power of two by LOQ10(2) and truncate the result to
                    305       ;       an integer.
                    306       ;
00CA                307       normal_value:
00CA                308       not_pseudo_zero:
                    309
00CA DB7EF0         310               fstp    fraction                ; Save the fraction field for later use
00CD DF56FC         311               fist    power_two               ; Save power of two
00D0 D9EC           312               fldlg2                          ; Get LOQ10(2)
                    313                                               ; Power_two is now safe to use
00D2 DEC9           314               fmul                            ; Form LOQ10(of exponent of number)
00D4 DF5EFA         315               fistp   power_ten               ; Any rounding mode will work here
                    316       ;
                    317       ;         Check if the magnitude of the number rules out treating it as
                    318       ;       an integer.
                    319       ;
                    320       ;       CX has the maximum number of decimal digits allowed.
                    321       ;
00D7 7B             322               fwait                           ; Wait for power_ten to be valid
00D8 3B46FA         323               mov     ax,power_ten            ; Get power of ten of value
00DB 2BC1           324               sub     ax,cx                   ; Form scaling factor necessary in ax
00DD 7722           325               ja      adjust_result           ; Jump if number will not fit
                    326       ;
                    327       ;         The number is between 1 and 10**(field_size).
                    328       ;       Test if it is an integer.
                    329       ;
00DF 0F46FC         330               fild    power_two               ; Restore original number
00E2 8BF2           331               mov     si,dx                   ; Save return value
00E4 80EAFE         332               sub     dl,NORMAL-EXACT         ; Convert to exact return value
00E7 0B6EF0         333               fld     fraction
00EA 09FD           334               fscale                          ; Form full value, this is safe here
00EC DDD1           335               fst     st(1)                   ; Copy value for compare
00EE 09FC           336               frndint                         ; Test if its an integer
00F0 08D9           337               fcomp                           ; Compare values
00F2 7BDD7EFE       338               fstsw   status                  ; Save status
00F6 F746FE0040     339               test    status,4000H            ; C3=1 implies it was an integer
00FB 7550           340               jnz     convert_integer
                    341
00FD DDD8           342               fstp    st(0)                   ; Remove non integer value
00FF 8BD6           343               mov     dx,si                   ; Restore original return value
                    344       ;
                    345       ;         Scale the number to within the range allowed by the BCD format
                    346       ;       The scaling operation should produce a number within one decimal order
                    347       ;       of magnitude of the largest decimal number representable within the
                    348       ;       given string width.
                    349       ;
                    350       ;         The scaling power of ten value is in ax.
                    351       ;
0101                352       adjust_result:
                    353
0101 8907           354               mov     word ptr [bx],ax        ; Set initial power of ten return value
0103 F7D8           355               neg     ax                      ; Substract one for each order
                    356                                               ; of magnitude the value is scaled by
0105 E80000     E   357               call    get_power_10            ; Scaling factor is returned as exponent
                    358                                               ; and fraction
0108 DB6EF0         359               fld     fraction                ; Get fraction
010B DEC9           360               fmul                            ; Combine fractions.
010D 8BF1           361               mov     si,cx                   ; Form power of ten of the maximum
010F D1E6           362               shl     si,1                    ; BCD value to fit in the string
0111 D1E6           363               shl     si,1                    ; Index in si
0113 D1E6           364               shl     si,1
0115 DF46FC         365               fild    power_two               ; Combine powers of two
0118 DEC2           366               faddp   st(2),st
011A D9FD           367               fscale                          ; Form full value, exponent was safe
011C DDD9           368               fstp    st(1)                   ; Remove exponent
                    369       ;
                    370       ;         Test the adjusted value against a table of exact powers of ten.
                    371       ;       The combined errors of the magnitude estimate and power function
                    372       ;       result in a value one order of magnitude too small or too large to fit
                    373       ;       correctly in the BCD field.  To handle this problem, pretest the
                    374       ;       adjusted value, if it is too small or large, then adjust it by ten and
                    375       ;       adjust the power of ten value.
                    376       ;
011E                377       test_power:
011E 2EDC940800  E  378
                    379               fcom    power_table[si]_type power_table; Compare against exact power
                    380                                               ; entry.  Use the next entry since cx
                    381                                               ; has been decremented by one.
0123 9BDFE0         382               fstsw   ax                      ; No wait is necessary
0126 690041         383               test    ax,4100H                ; If C3 = C0 = 0 then too big
0129 750C           384               jnz     text_for_small
                    385
012B 2EDE360000  R  386               fidiv   const10                 ; Else adjust value
0130 80E2FD         387               and     dl,not EXACT            ; Remove exact flag
0133 FF07           388               inc     word ptr [bx]           ; Adjust power of ten value
0135 EB14           389               jmp     short in_range          ; Convert the value to a BCD integer
                    390
0137                391       test_for_small
                    392
0137 2EDC940000  E  393               fcom    power_table[si]         ; Test relative size
013C 9BDFE0         394               fstsw   ax                      ; No wait is necessary
013F A90001         395               test    ax,100H                 ; If C0 = 0 then st(0) >= lower bound
0142 7407           396               jz      in_range                ; Convert the value to a BCD integer
                    397
0144 2EDE0E0000  R  398               fimul   const10                 ; Adjust value into range
0149 FF0F           399               dec     word ptr [bx]           ; Adjust power of ten value
                    400
014B                401       in_range:
                    402
014B D9FC           403               frndint                         ; Form integer value
                    404       ;
                    405       ;       Assert: 0 <= TOS <= 999,999,999,999,999,999
                    406       ;       The TOS number will be exactly representable in 18 digit BCD format
                    407       ;
014D                408       convert_integer:
                    409
014D DF76F0         410               fbstp   bcd_value               ; Store ax BCD format number
                    411       ;
                    412       ;         While the store BCD runs, setup registers for the conversion to
                    413       ;       ASCII.
                    414       ;
0150 BE0800         415               mov     si,BCD_SIZE-2           ; Initial BCD index value
0153 B9040F         416               mov     cx,0f04h                ; Set shift count and mask
0156 BB0100         417               mov     bx,1                    ; Set initial size of ASCII field for sign
0159 8B730C         418               mov     di,string_ptr           ; Get address of start of ASCII string
015C BCD8           419               mov     ax,ds                   ; Copy ds to es
015E BEC0           420               mov     es,ax
0160 FC             421               cld                             ; Set autoincrement mode
0161 B02B           422               mov     al,'+'                  ; Clear sign field
0163 F6C201         423               text    dl,MINUS                ; Look for negative value
0166 7402           424               jr      positive_result
                    425
0168 B02D           426               mov     al,'-'
                    427
016A                428       positive_result:
                    429
016A AA             430               stash                           ; Bump string pointer past sign
016B 809E2FE        431               and     dl,not MINUS            ; Turn off sign bit
016E 9B             432               fwait                           ; Wait for fbstp to finish
                    433       ;
                    434       ;         Register usage:
                    435       ;                               ah:     BCD byte value in use
                    436       ;                               al:     ASCII character value
                    437       ;                               dx:     Return value
                    438       ;                               ch:     BCD mask = ofh
                    439       ;                               cl      BCD shift count = 4
                    440       ;                               bx:     ASCII string field width
                    441       ;                               si:     BCD field index
                    442       ;                               di:     ASCII string field pointer
                    443       ;                               ds,es:  ASCII string segment base
                    444       ;
                    445       ;         Remove leading zeroes from the number.
                    446       ;
016F                447       skip_leading_zeroes
                    448
016F 8A62F0         449               move    ah,bcd_byte[si]         ; Get BCD byte
0172 BAC4           450               move    al,ah                   ; Copy value
0174 D2E8           451               shr     al,cl                   ; Get high order digit
0176 22C5           452               and     al,ch                   ; Set zero flag
0178 7516           453               jnz     enter_odd               ; Enter loop if leading non zero found
                    454
017A 8AC4           455               mov     al,ah                   ; Get BCD byte again
017C 22C5           456               and     al,ch                   ; Get low order digit
017E 7518           457               jnz     enter_even
                    458
0180 4E             459               dec     si                      ; Decrement BCD index
0181 79EC           460               jns     skip_leading_zeroes
                    461       ;
                    462       ;         The significand was all zeroes
                    463       ;
0183 B030           464               mov     al,'0'                  ; Set initial zero
0185 AA             465               stosb
0186 43             466               inc     bx                      ; Bump string length
0187 EB16           467               jmp     short exit_with_value
                    468       ;
                    469       ;         Now expand the BCD string into digit per byte values 0-9
                    470       ;
0189                471       digit_loop
                    472
0189 8A62F0         473               mov     ah,bcd_byte[si]         ; Get BCD byte
018C 8AC4           474               mov     al,ah
018E D2E8           475               shr     al,cl                   ; Get high order digit
                    476
0190                477       enter_odd
                    478
0190 0430           479               add     al,'0'                  ; Convert to ASCII
0192 AA             480               stosb                           ; Put digit into ASCII string area
0193 8AC4           481               mov     al,ah                   ; Get low order digit
0195 22C5           482               and     al,ch
0197 43             483               inc     bx                      ; Bump field size counter
                    484
0198 0430           485       enter_even
019A AA             486
019B 43             487               add     al,'0'                  ; Convert to ASCII
019C 4E             488               stosb                           ; Put digit into ASCII area
019D 79EA           489               inc     bx                      ; Bump field size counter
                    490               dec     si                      ; Go to next BCD byte
                    491               jns     digit_loop
                    492       ;
                    493       ;         Conversion complete.  Set the string size and remainder
                    494       ;
019F                495       exit_with_value:
                    496
019F 8B7E0A         497               move    di,size_ptr
01A2 891D           498               mov     word ptr [di],bx
01A4 8BC2           499               mov     ax,x                    ; Set return value
01A6 E980FE         500               jmp     exit_proc
                    501
                    502       floating_to_ascii       endp
----                503       code                    ends
                    504                               end
ASSEMBLY COMPLETE,          NO WARNINGS,    NO ERRORS


iAPX286 MACRO ASSEMBLER     Calculate the value of 10**ax            12:11:08 09/25/83 PAGE  1

SERIES-III iAPX286 MACRO ASSEMBLER X108 ASSEMBLY OF MODULE GET_POWER_10
OBJECT MODULE PLACED IN :F3:POW10.OBJ
ASSEMBLER INVOKED BY:  ASM286.86 :F3:POW10.AP2

LOC  OBJ           LINE       SOURCE

                      1  +1   $title("Calculate the value of 10**ax")
                      2       ;
                      3       ;         This subroutine will calculate the value of 10**ax.
                      4       ;       For values of 0 <= ax <19, the result will exact.
                      5       ;       All 80286 registers are transparent and the value is returned on
                      6       ;       the TOS as two numbers, exponent in ST(1) and fraction in ST(0).
                      7       ;       The exponent value can be larger than the largest exponent of an
                      8       ;       extended real format number.  Three stack entries are used.
                      9       ;
                     10                       name    get_power_10
                     11
                     12                       public  get_power_10,power_table
                     13
----                 14       stack           stackseg 8
                     15
----                 16       code            segment or public
                     17       ;
                     18       ;         Use exact values from 1:0 to 1e18
                     19       ;
                     20                       even                        ; Optimize 16 bit access
0000 000000000000F0  21       power_table     dq      1.0,1e1,1e2,1e3
     3F
0008 00000000000024
     40
0010 00000000000059
     40
0018 0000000000408F
     40
0020 000000000088C3  22                       dq      1e4,1e5,1e6,1e7
     40
0028 00000000006AF8
     40
0030 0000000080842E
     41
0038 00000000D01263
     41
0040 0000000084D797  23                       dq      1e8,1e9,1e10,1e11
     41
0048 0000000065CDCD
     41
0050 000000205FA002
     42
0058 000000E8764837
     42
0060 000000A2941A6D  24                       dq      1e12,1e13,1e14,1e15
     42
0068 000040E59C30A2
     42
0070 0000901EC4BCD6
     42
0078 00003426F56B0C
     43
0080 0080E03779C341  25                       dq      1e16,1e17,1e18
     43
0088 00A0D885573476
     43
0090 00C84E676DC1AB
     43
0098                 26
                     27       get_power_10    proc
0098 3D1200          28
009B 770F            29               cmp     ax,18                       ; Test for 0 <= ax < 19
                     30               ja      out_of_range
009D 53              31
009E 8BD8            32               push    bx                          ; Get working index register
00A0 C1E303          33               mov     bx,ax                       ; Form table index
00A3 2EDD870000    R 34               shl     bx,3
00A8 5B              35               fld     power_table[bx]             ; Get exact value
00A9 D9F4            36               pop     bx                          ; Restore register value
00AB C3              37               fxtract                             ; Separate power and fraction
                     38               ret                                 ; OK to leave fxtract running
                     39       ;
                     40       ;         Calculate the value using the exponentiate instruction.
                     41       ;       The following relations are used:
                     42       ;               10**x = 2**(log2(10)*x)
                     43       ;               2**(I+F) = 2**I * 2**F
                     44       ;               if st(1) = I and st(0) = 2**F then fscale produces 2**(I+F)
                     45       ;
00AC                 46       out_of_range:
                     47
00AC D9E9            48               fld12t                              ; TOS = LOG2(10)
00AE C8040000        49               enter   4.0                         ; Format stack
00B2 8946FE          50               mov     [bp-2],ax                   ; Save power of 10 value
00B5 DE4EFE          51               fimul   word ptr [bp-1]             ; TOS, x= LOG2(10)*P = LOG2(10**P)
00B8 9BD97EFC        52               fstcw   word ptr [bp-4]             ; Get current control word
00BC 8B46FC          53               mov     ax,word ptr [bp-4]          ; Get control word, no wait necessary
00BF 25FFF3          54               and     ax,not OCOOH                ; Mask off current rounding field
00C2 0D0004          55               or      ax,0400H                    ; Set round to negative infinity
00C5 6746FC          56               xchg    ax,word ptr [bp-4]          ; Put new control word in memory
                     57                                                   ; old control word is in ax
00C8 D9E8            58               fld1                                ; Set TOS = -1.0
00CA D9E0            59               fchs
00CC D9C1            60               fld     st(1)                           ; Copy power value in base two
00CE D96EFC          61               fldcw   word ptr [bp-4]                 ; Set new control word value
00D1 D9FC            62               frndint                                 ; TOS = I: -inf < I <= X, I is an integer
00D3 8946FC          63               mov     word ptr [bp-4],ax              ; Restore original rounding control
00D6 D96EFC          64               fldcw   word ptr [bp-4]
00D9 D9CA            65               fxch    st(2)                           ; TOS = X, ST(1) = -1.0, St(2) = I
00DB DBE2            66               fsub    st,st(2)                        ; TOS,F=X-I; 0 <= TOS < 1.0
00DD 8B46FE          67               mov     ax,[bp-2]                       ; Restore power of ten
00E0 D9FD            68               fscale                                  ; TOS = F/2: 0 <= TOS < 0.5
00E2 D9F0            69               f2xm1                                   ; TOS = 2**(F/2) - 1.0
00E4 C9              70               leave                                   ; Restore stack
00E5 DEE1            71               fsubr                                   ; Form 2**(F/2)
00E7 DCC8            72               fmul    st,st(0)                        ; Form 2**F
00E9 C3              73               ret                                     ; OK to leave fmul running
                     74
                     75       get_power_10    endp
                     76
----                 77       code            ends
                     78                       end
ASSEMBLY COMPLETE,        NO WARNINGS,   NO ERRORS


iAPX286 MACRO ASSEMBLER   Determine TOS register contents          12:12:13 09/25/83 PAGE   1

SERIES-III iAPX286 MACRO ASSEMBLER X108 ASSEMBLY OF MODULE TOS_STATUS
OBJECT MODULE PLACED IN :F3:T0SST.OBJ
ASSEMBLER INVOKED BY:  ASM286.86 :F3:TOSST.AP2

LOC  OBJ           LINE      SOURCE

                      1   +1  $title("Determine TOS register contents")
                      2       ;
                      3       ;         This subroutine will return a value from 0-15 in AX corresponding
                      4       ;       to the contents of 80287 TOS.  All registers are transparent and no
                      5       ;       errors are possible.  The return value corresponds to c3,c2,c1,c0
                      6       ;       of FXAM instruction.
                      7       ;
                      8               name    tos_status
                      9
                     10               public  tos_status
                     11
----                 12       stack           stackseg 6              ; Allocate space on the stack
                     13
----                 14       code            segment er public
                     15
0000                 16       tos_status      proc
                     17
0000 D9E5            18               fxam                            ; Get register contents status
0002 9BDFE0          19               fstsw   ax                      ; Get status
0005 8AC4            20               mov     al,ah                   ; Put bit 10-8 into bits 2-0
0007 250740          21               and     ax,4007h                ; Mask out bits c3,c2,c1,c0
000A C0EC03          22               shr     ah,3                    ; Put bit c3 into bit 11
000D 0AC4            23               or      al,ah                   ; Put c3 into bit 3
000F B400            24               mov     ah,0                    ; Clear return value
0011 C3              25               ret
                     26
                     27       tos_status      endp
                     28
----                 29       code            ends
                     30                       end
ASSEMBLY COMPLETE,       NO WARNINGS,   NO ERRORS


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, will identify what, if
anything, is in the top of the numeric register stack.


Exception Considerations
Care is taken inside the function to avoid generating exceptions. Any
possible numeric value will be accepted. The only exceptions possible would
occur if insufficient space exists on the numeric register stack.

The value passed in the numeric stack is checked for existence, type (NaN
or infinity), and status (unnormal, 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 will return with an
error code.

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


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


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 <= 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 Not a Number (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, unnormals, and
pseudo zero all have a numeric value but should be recognized, because all
of them 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 unnormal flags, the value must be scaled to
the BCD range.


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 given value. After scaling the number, a check will be 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 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 will produce 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) will be used.

Due to restrictions on the range of values allowed by the F2XM1
instruction, the power of 2 value will be 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.


Inaccuracy in Scaling
The inaccuracy of these operations arises because of the trailing zeros
placed into the fraction value 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.


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 will involve small integers, all easily represented by the NPX.


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) would produce 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.


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.


Trigonometric Calculation Examples
The 80287 instruction set does not provide a complete set of trigonometric
functions that can be used directly in calculations. Rather, the basic
building blocks for implementing trigonometric functions are provided by the
FPTAN and FPREM instructions. The example in figure 4-7 shows how three
trigonometric functions (sine, cosine, and tangent) can be implementing
using the 80287. All three functions accept a valid angle argument between
-2^(62) and +2^(62). These functions may be called from PL/M-286,
Pascal-286, FORTRAN-286, or ASM286 routines.

These trigonometric functions use the partial tangent instruction together
with trigonometric identities to calculate the result. They are accurate to
within 16 units of the low 4 bits of an extended precision value. The
functions are coded for speed and small size, with tradeoffs available for
greater accuracy.

Figure 4-7. Calculating Trigonometric Functions


iAPX286 MACRO ASSEMBLER   80287 Trigonometric functions           10:13:51 09/25/83 PAGE   1

SERIES-888 iAPX286 MACRO ASSEMBLER X108 ASSEMBLY OF MODULE TRIG_FUNCTIONS
OBJECT MODULE PLACED IN :F3:TRIG.OBJ
ASSEMBLER INVOKED BY:  ASM286.86 :F3:TRIG.AP2

LOC  OBJ           LINE     SOURCE

                      1  +1   $title("80287 Trigonometric Functions")
                      2
                      3                       name    trig_functions
                      4                       public  sine,cosine,tangent
                      5
----                  6       stack           stackseg        6           ; Reserve local space
                      7
   #                  8       sw_287          record  res1:1,cond3:1,top:3,cond2:1,cond1:1,cond0:1,
                      9                               res2:8
                     10
----                 11       code            segment er public
                     12       ;
                     13       ;         Define local constants
                     14       ;
                     15                       even
0000 35C26821A2DA0F  16       pi_quarter      dt      3FFEC90FDAA22168C235R ; PI/4
     C9FE3F
000A 0000C0FF        17       indefinite      dd      0FFC000000R         ; Indefinite special value
                     18  +1   $eject
                     19       ;
                     20       ;         This subroutine calculates the sine or cosine of the angle, given in
                     21       ;       radians.  The angle is in ST(0), the returned value will be in ST(0).
                     22       ;       The result is accurate to within 7 units of the least significant three
                     23       ;       bits of the NPX extended real format.  The PLM/86 definition is:
                     24       ;
                     25       ;       sine:   procedure (angle) real external,
                     26       ;               declare angle real;
                     27       ;               and sine;
                     28       ;
                     29       ;       cosine: procedure (angle) real external;
                     30       ;               declare angle real;
                     31       ;               and cosine;
                     32       ;
                     33       ;         Three stack registers are required.  The result of the function
                     34       ;       defined as follows for the following arguments:
                     35       ;
                     36       ;                       angle                                   result
                     37       ;
                     38       ;               valid or unnormal less than 2**62 in magnitude  correct value
                     39       ;               zero                                            0 or 1
                     40       ;               denormal                                        correct denormal
                     41       ;               valid or unnormal greater than 2**62            indefinite
                     42       ;               infinity                                        indefinite
                     43       ;               NAN                                             NAN
                     44       ;               empty                                           empty
                     45 +1    $eject
                     46       ;
                     47       ;         This function is based on the NPX fptan instruction.  The fptan
                     48       ;       instruction will only work with an angle of from 0 to PI/4.  With this
                     49       ;       instruction, the sine or cisone of angles from 0 to PI/4 can be accurately
                     50       ;       calculated.  The technique used by this routine can calculate a general
                     51       ;       sine or cosine by using one of four possible operations:
                     52       ;
                     53       ;                   Let R = |angle mod PI/4|
                     54       ;                       S = -1 or 1, according to the sign of the angle
                     55       ;
                     56       ;       1) sin(R)   2) cos(R)   3) sin(PI/4-R)  4) cos(PI/4-R)
                     57       ;
                     58       ;         The choice of the relation and the sign of the result follows
                     59       ;       decision table shown below based on the octant the angle falls in:
                     60       ;
                     61       ;               octant          sine            cosine
                     62       ;
                     63       ;                 0              s*1               2
                     64       ;                 1              s*4               3
                     65       ;                 2              s*2            -1*1
                     66       ;                 3              s*3            -1*4
                     67       ;                 4             -s*1            -1*2
                     68       ;                 5             -s*4            -1*3
                     69       ;                 6             -s*2               1
                     70       ;                 7             -s*3               4
                     71       ;
                     72 +1    $eject
                     73       ;
                     74       ;         Angle to sine function is a zero or unnormal
                     75       ;
000E                 76       sine_zero_unnormal:
                     77
000E DDD9            78               fstp    st(1)                           ; Remove PI/4
0010 7501            79               jnz     enter_sine_normalize            ; Jump if angle is unnormal
                     80       ;
                     81       ;         Angle is a zero.
                     82       ;
0012 C3              83               ret
                     84       ;
                     85       ;         Angle is an unnormal
                     86       ;
0013                 87       enter_sine_normalize:
                     88
0013 E80901          89               call    normalize_value
0016 EB2F            90               jmp     short enter_sine
                     91
0018                 92       cosine  proc                                    ; Entry point to cosine
                     93
0018 D9E5            94               fxam                                    ; Look at the value
00A1 9BDF30          95               fstsw   ax                              ; Store status value
001D 2EDB2E0000   R  96               fld     pi_quarter                      ; Setup for angle reduce
0022 B101            97               mov     c1,1                            ; Signal cosine function
0024 9E              98               sahf                                    ; ZF = C3, PF = C2, CF = C0
0025 7263            99               jc      funny_parameter                 ; Jump if parameter is
                    100                                                       ; empty, NAN, or infinity
                    101       ;
                    102       ;         Angle is unnormal, normal, zero, denormal.
                    103       ;
0027 D9C9           104               fxch                                    ; st(0) = angle, st(1) = PI/4
0029 7A1C           105               jpe     enter_sine                      ; Jump if normal or denormal
                    106       ;
                    107       ;         Angle is an unnormal or zero
                    108       ;
002B DDD9           109               fstp    st(1)                           ; Remove PI/4
002D 75E4           110               jnz     enter_sine_normalize
                    111       ;
                    112       ;         Angle is a zero, cos(0) = 1.0
                    113       ;
002F DDD8           114               fstp    st(0)                       ; Remove 0
0031 D9E8           115               fldl                                ; Return 1
0033 C3             116               ret
                    117       ;
                    118       ;         All work is done as a sine function.  By adding PI/2 to the angle
                    119       ;       a cosine is converted to a sine.  Of course the angle addition is not
                    120       ;       done to the argument but rather to the program logic control values.
                    121       ;
0034                122       sine                                        ; Entry point for sine function
                    123
0034 D9E5           124               fxam                                ; Look at the parameter
0036 9BDFE0         125               fstsw   ax                          ; Look at fxam status
0039 2EDB2E0000   R 126               fld     pi_quarter                  ; Get PI/4 value
003E 9E             127               sahf                                ; CF = C0, PF = C2, ZF - C3
003F 7249           128               jc      funny_parameter             ; Jump if empty, NAN, or infinity
                    129       ;
                    130       ;         Angle is unnormal, normal, zero, or denormal
                    131       ;
0041 D9C9           132               fxch                                ; ST(1) = PI/4, st(0) angle
0043 B100           133               mov     c1,0                        ; Signal sine
0045 7BC7           134               jpo     sine_zero_unnormal          ; Jump if zero or unnormal
                    135       ;
                    136       ;         ST(0) is either a normal or denormal value.  Both will work
                    137       ;       Use the fprem instruction to accurately reduce the range of the given
                    138       ;       angle to within 0 and PI/4 in magnitude.  If fprem cannot reduce the
                    139       ;       angle in one shot, the angle is too big to be meaningful, >2**62
                    140       ;       radians.  Any roundoff error in the calculation of the angle given
                    141       ;       could completely change the result of this function.  It is safest to
                    142       ;       call this very rare case an error.
                    143       ;
0047                144       enter_sine
0047 D9F8           145               fprem                               ; Reduce angle
                    146                                                   ; Note that fprem will force a
                    147                                                   ; denormal to a very small unnormal
                    148                                                   ; Fptan of a very small unnormal
                    149                                                   ; will be the same very small
                    150                                                   ; unnormal, which is correct.
0049 93             151               xchg    ax,bx                       ; Save old status in BX
004A 9BDFE0         152               fstsw   ax                          ; Check if reduction was complete
                    153                                                   ; Quotient in C0,C3,C1
004D 93             154               xchg    ax,bx                       ; Put new status in bx
004E F6C704         155               test    bh,high(mask cond2)         ; sin(2*N*PI+x) = sin(x)
0051 7544           156               jnz     angle_too_big
                    157       ;
                    158       ;         Set sign flags and test for which eighth of the revolution the
                    159       ;       angle fell intl.
                    160       ;
                    161       ;       Assert -PI/4 < st(0) < PI/4
                    162       ;
0053 D9E1           163               fabs                                ; Force the argument positive
                    164                                                   ; cond1 bit in bx holds the sign.
0055 0AC9           165               or      c1,c1                       ; Test for sine or cosine function
0057 740F           166               jz      sine_select                 ; Jump if sine function
                    167       ;
                    168       ;         This is a cosine function.  Ignore the original sign of the angle
                    169       ;       and add a quarter revolution to the octant id from the fprem instruction
                    170       ;       cos(A) = sin(A+PI/2) and cos(|A|) = cos(A)
                    171       ;
0059 B0E4FD         172               and     ah,not high(mask cond1)     ; Turn off sign of argument
005C B0CF80         173               or      by,80H                      ; Prepare to add 010 to c0,c3,c1
                    174                                                   ; status value in ax
                    175                                                   ; Set busy bit so carry out from
005F 80C740         176               add     bh,high(mask cond3)         ; C3 will go into the carry flag
0062 B000           177               mov     al,0                        ; Extract carry flag
0064 D0D0           178               rcl     al,1                        ; Put carry flag in low bit
0066 32FB           179               xor     bh,al                       ; Add carry to C0 not changing
                    180                                                   ; C1 flag
                    181       ;
                    182       ;         See if the argument should be reversed, depending on the octant in
                    183       ;       which the argument fell during fprem.
                    184       ;
0068                185       sine_select:
                    186
0068 F6C702         187               test    bh,high(mask cond1)         ; Reverse angle if C1 = 1
006B 7404           188               jz      no_sine_reverse
                    189       ;
                    190       ;         Angle was in octants 1,3,5,7.
                    191       ;
006D DEE9           192               fsub                                ; Invert sense of rotation
006F EB0E           193               jmp     short do_sine_fptan         ; 0 < arg <= PI/4
                    194       ;
                    195       ;         Angle was in octants 0,2,4,6
                    196       ;       Test for a zero argument since fptan will not work if st(0) = 0
                    197       ;
0071                198       no_sine_reverse:
                    199
0071 D9E4           200               ftst                                ; Test for zero angle
0073 91             201               xchg    ax,cx
0074 9BDFE0         202               fstsw   ax                          ; cond3 = 1 if st(0) = 0
0077 91             203               xchg    ax,cx
0078 DDD9           204               fstp    st(1)                       ; Remove PI/4
007A F6C540         205               test    ch,high(mask cond3)         ; If c3=1, argument is zero
007D 7514           206               jnz     sine_argument_zero
                    207       ;
                    208       ;       Assert: 0 < st(0) <= PI/4
                    209       ;
007F                210       do_sine_fptan:
                    211
007F D9F2           212               fptan                                   ; TAN ST(0) = ST(1)/ST(0) = Y/X
                    213
0081                214       after_sine_fptan:
                    215
0081 F6C742         216               test    bh,high(mask cond3 + mask cond1); Look at octant angle fell into
0084 7B1A           217               jpo     x_numerator                     ; Calculate cosine for octants
                    218                                                       ; 1,2,5,6
                    219       ;
                    220       ;         Calculate the sine of the argument
                    221       ;       sine(A) = tan(A)/sqrt(1+tan(A)**2)      if tan(A) = Y/X then
                    222       ;       sin(A) = Y/sqrt(X*X + Y*Y)
                    223       ;
0086 D9C1           224               fld     st(1)                           ; Copy Y value
0088 EB1A           225               jmp     short finish_sine               ; Put Y value in numerator
                    226       ;
                    227       ;         The top of the stack is either NAN, infinity, or empty
                    228       ;
008A                229       funny_parameter:
                    230
008A DDD8           231               fstp    st(0)                           ; Remove PI/4
008C 7404           232               jz      return_empty                    ; Return empty if no parm
                    233
008E 7B02           234               jpo     return_NAN                      ; Jump if st(0) is NAN
                    235       ;
                    236       ;         st(0) is infinity.  Return an indefinite value.
                    237       ;
0090 D9F8           238               fprem                                   ; ST(1) can be anything
                    239
0092                240       return_NAN:
0092                241       return_empty:
                    242
0092 C3             243               ret                                     ; OK to leave fprem running
                    244       ;
                    245       ;       Simulate fptan with st(0) = 0
                    246       ;
0093                247       sine_argument_zero:
                    248
0093 D9EB           249               fld1                                    ; Simulate tan(0)
0095 EBEA           250               jmp     after_sine_fptan                ; Return the zero value
                    251       ;
                    252       ;         The angle was too large.  Remove the modulus and dividend from the
                    253       ;       stack and return an indefinite result.
                    254       ;
0097                255       angle_too_big:
                    256
0097 DED9           257               fcompp                                  ; Pop two values from the stack
0099 2ED9060A00   R 258               fld     indefinite                      ; Return indefinite
009E 9B             259               fwait                                   ; Wait for load to finish
009F C3             260               ret
                    261       ;
                    262       ;         Calculate the cosine of the argument
                    263       ;       cos(A) = 1/sqrt(1+tan(A)**2)    if tan(A) - Y/X then
                    264       ;       cos(A) = X/sqrt(X*X + Y*Y)
                    265       ;
00A0                266       X_numerator:
                    267
00A0 D9C0           268               fld     st(0)                           ; Copy X value
00A2 D9CA           269               fxch    st(2)                           ; Put X in numerator
                    270
00A4                271       finish_sine:
                    272
00A4 DCCB           273               fmul    st,st(0)                        ; Form X*X + Y*Y

00A6 D9C9           274               fxch
00AB DCC8           275               fmul    st,st(0)
00AA DEC1           276               fadd                                    ; st(0) = X*X + Y*Y
00AC D9FA           277               fsqrt                                   ; st(0) = sqrt(X*X + Y*Y)
                    278
                    279
                    280       ;         Form the sign of th result.  The two conditions are the C1 flag from
                    281       ;       FXAM in bh and the CO flag from fprem in ah
                    282       ;
00AE 80E701         283               and     bh,high(mask cond0)             ; Look at the fprem C0 flag
00B1 80E402         284               and     ah,high(mask cond1)             ; Look at the fxam C1 flag
00B4 0AFC           285               or      bh,ah                           ; Even number of flags cancel
00B6 7A02           286               jpe     positive_sine                   ; Two negatives make a positive
                    287
00B8 D9E0           288               fchs                                    ; Forc result negative
                    289
00BA                290       positive_sine:
                    291
00BA DEF9           292               fdiv                                    ; Form final result
00BC C3             293               ret                                     ; Ok to leave fdiv running
                    294
                    295       cosine endp
                    296   +1  $eject
                    297       ;
                    298       ;         This function will calculate the tangent of an angle.
                    299       ;       The angle, in radians is passed in ST(0), the tangent is returned
                    300       ;       in ST(0).  The tangent is calculated to an accuracy of 4 units in the
                    301       ;       least three significant bits of an extended real format number.  The
                    302       ;       PLM/86 calling format is:
                    303       ;
                    304       ;     tangent   procedure (angle) real external;
                    305       ;               declare angle real;
                    306       ;               end tangent;
                    307       ;
                    308       ;         Two stack registers are used.  The result of the tangent function is
                    309       ;       defined for the following cases:
                    310       ;
                    311       ;                               angle                           result
                    312       ;
                    313       ;               valid or unnormal < 2**62 in magnitude          correct value
                    314       ;               0                                               0
                    315       ;               denormal                                        correct denormal
                    316       ;               valid or unnormal > 2**62 in magnitude          indefinite
                    317       ;               NAN                                             NAN
                    318       ;               infinity                                        indefinite
                    319       ;               empty                                           empty
                    320       ;
                    321       ;         The tangent instruction uses the fptan instruction.  Four possible
                    322       ;       relations are used:
                    323       ;
                    324       ;         Let R = |angle MOD PI/4|
                    325       ;             S = -1 or 1 depending on the sign of the angle
                    326       ;
                    327       ;       1) tan(R)   2) tan(PI/4-R)  3) 1/tan(R)     4) 1/tan(PI/4-R)
                    328       ;
                    329       ;         The following table is used to decide which relation to use depending
                    330       ;       on in which octant the angle fell.
                    331       ;
                    332       ;       octant          relation
                    333       ;
                    334       ;         0              s*1
                    335       ;         1              s*4
                    336       ;         2             -s*3
                    337       ;         3             -s*2
                    338       ;         4              s*1
                    339       ;         5              s*4
                    340       ;         6             -s*3
                    341       ;         7             -s*2
                    342       ;
00BD                343       tangent proc
                    344
00BD D9E5           345               fram                                    ; Look at the parameter
00BF 9BDFE0         346               fstw    ax                              ; Get fxam status
00C2 2EDB2E0000  R  347               fld     pi_quarter                      ; Get PI/4
00C7 9E             348               sahf                                    ; CF = C1, PF = C2, ZF = C3
00C8 72C0           349               jc      funny_parameter
                    350       ;
                    351       ;         Angle is unnormal, normal, zero, or denormal.
                    352       ;
00CA D9C9           353               fxch                                    ; st(0) = angle, st(1) = PI/4
00CC 7A17           354               jpe     tan_zero_unnormal
                    355       ;
                    356       ;         Angle is either an normal or denormal.
                    357       ;       Reduc the angle to the range -PI/4 < result < PI/4
                    358       ;       If fprem cannot perform this operation in one try, the magnitude of the
                    359       ;       angle must be > 2**62.  Such an angle is so large that any rounding
                    360       ;       errors could make a very large difference in the reduced angle.
                    361       ;       It is safest to call this very rare case an error.
                    362       ;
00CE                363       tan_normal
                    364
00CE D9F8           365               fprem                                   ; Quotient in C0,C3,C1
                    366                                                       ; Convert denormals into unnormals
00D0 93             367               xchg    ax,bx
00D1 9BDFE0         368               fstsw   ax                              ; Quotient identifies octant
                    369                                                       ; original angle fell into
00D4 93             370               xchg    ax,bx
00D5 F6C704         371               test    bh,high(mask cond2)             ; Test for complete reduction
00D8 7BD            372               jnz     angle_too_big                   ; Exit if angle was too big
                    373       ;
                    374       ;         See if the angle must be reversed.
                    375       ;
                    376       ;       Assert -PI/4 < st(0) < PI/4
                    377       ;
00DA D9E1           378               fabs                                    ; 0 <= st(0) < PI/4
                    379                                                       ; C3 in bx has the sign flag
00DC F6C702         380               test    bh,high(mask cond1)             ; must be reversed
00DF 740E           381               jz      no_tan_reverse
                    382       ;
                    383       ;         Angle fell in octants 1,3,5,7.  Reverse it, subtract it from PI/4.
                    384       ;
00E1 DEE9           385               fsub                                    ; Reverse angle
00E3 EB18           386               jmp     short do_tangent
                    387       ;
                    388       ;         Angle is either zero or an unnormal
                    389       ;
00E5                390       tan_zero_unnormal:
                    391
00E5 DDD9           392               fstp    st(1)                           ; Remove PI/4
00E7 7405           393               jz      tan_angle_zero
                    394       ;
                    395       ;       Angle is an unnormal.
                    396       ;
00E9 E83300         397               call    normalize_value
00EC EBE0           398               jmp     tan_normal
                    399
00EE                400       tan_angle_zero:
                    401
00EE C3             402               ret
                    403       ;
                    404                 Angle fell in octants 0,2,4,6. Test for st(0) = 0, fptan won't work
                    405       ;
00EF                406       no_tan_reverse:
                    407
00EF D9E4           408               ftst                                    ; Test for zero angle
00F1 91             409               xchg    ax,cx
00F2 9BDFE0         410               fstsw   ax                              ; C3 = 1 if st(0) = 0
00F5 91             411               fstp    st(1)
00F6 DDD9           412               test    ch,high(mask cond3)
00F8 F6C540         413               jnz     tan_zero
00FB 7515           414
                    415       do_tangent:
00FD                416
                    417               fptan                                   ; tan ST(0) = ST(0)/ST(0)
00FD D9F2           418
                    419       after_tangent:
00FF                420       ;
                    421       ;         Decide on the order of the operands and their sign for the divide
                    422       ;       operation while the fptan instruction is working.
                    423       ;
                    424               mov     al,bh                           ; Get a copy of fprem C3 flag
00FF 8AC7           425               and     ax,mask cond1 + high(mask cond3); Examine fprem C3 flag and
0101 254002         426                                                       ; FXAM C1 flag
                    427               test    bh,high(mask cond1 + mask cond3); Use reverse divide if in
0104 F6C742         428                                                       ; octants 1,2,5,6
                    429               jpo     reverse_divide                  ; Note: parity works on low
0107 7B0D           430                                                       ;  8 bits only!
                    431       ;
                    432       ;         Angle was in octants 0,3,4,7
                    433       ;       Test for the sign of the result.  Two negatives cancel.
                    434       ;
                    435               or      al,ah
0109 0AC4           436               jpe     positive_divide
010B 7A02           437
                    438               fchs                                    ; Force result negative
010D D9E0           439
                    440       positive_divide:
010F                441
                    442               fdiv                                    ; Form result
010F DEF9           443               ret                                     ; Ok to leave fdiv running
0111 C3             444
                    445       tan_zero:
0112                446
                    447               fld1                                    ; Force 1/0 = tan(PI/2)
0112 D9E8           448       ;
0114 EBE9           449       ;         Angle was in octants 1,2,5,6
                    450       ;       Set the correct sign of the result
                    451       ;
                    452       reverse_divide:
                    453
0116                454               or      al,ah
                    455               jpe     positive_r_divide
0116 0AC4           456
0118 7A02           457               fchs                                    ; Force result negative
                    458
011A D9E0           459       positive_r_divide:
                    460
011C                461               fdivr                                   ; Form reciprocal of result
                    462               ret                                     ; Ok to leave fdiv running
011C DEF1           463
011E C3F1           464       tangent endp
                    465       ;
                    466       ;         This function will normalize the value in st(0)
                    467       ;       Then PI/4 is placed into st(1).
                    468       ;
                    469       normalize_value:
                    470
011F                471               fabs                                    ; Force value positive
                    472               fxtract                                 ; 0 <= st(0) < 1
011F D9E1           473               fld1                                    ; Get normalize bit
0121 D9F4           474               fadd    st(1),st                        ; Normalize fraction
0123 D9E8           475               fsub                                    ; Resotre original value
0125 DCC1           476               fscale                                  ; Form original normalized value
0127 DEE9           477               fstp    st(1)                           ; Remove scale factor
0129 D9FD           478               fld     pi_quarter                      ; Get PI/4
012B DDD9           479               fxch
012D 2EDB2E0000  R  480               ret
0132 D9C9           481
0134 C3             482       code    ends
                    483
----                484
                    485

ASSEMBLY COMPLETE,            NO WARNINGS,   NO ERRORS


FPTAN and FPREM
These trigonometric functions use the FPTAN instruction of the NPX. FPTAN
requires that the angle argument be between 0 and Ò/4 radians, 0 to 45
degrees. The FPREM instruction is used to reduce the argument down to this
range. The low three quotient bits set by FPREM identify which octant the
original angle was in.

One FPREM instruction iteration can reduce angles of 10^(18) radians or less
in magnitude to Ò/4! Larger values can be reduced, but the meaning of the
result is questionable, because any errors in the least significant bits of
that value represent changes of 45 degrees or more in the reduced angle.


Cosine Uses Sine Code
To save code space, the cosine function uses most of the sine function
code. The relation sin (�A� + Ò/2) = cos(A) is used to convert the
cosine argument into a sine argument. Adding Ò/2 to the angle is performed
by adding 010{2} to the FPREM quotient bits identifying the argument's
octant.

It would be very inaccurate to add Ò/2 to the cosine argument if it was
very much differentfrom Ò/2.

Depending on which octant the argument falls in, a different relation will
be used in the sine and tangent functions. The program listings show which
relations are used.

For the tangent function, the ratio produced by FPTAN will be directly
evaluated. The sine function will use either a sine or cosine relation
depending on which octant the angle fell into. On exit, these functions will
normally leave a divide instruction in progress to maintain concurrency.

If the input angles are of a restricted range, such as from 0 to 45
degrees, then considerable optimization is possible since full angle
reduction and octant identification is not necessary.

All three functions begin by looking at the value given to them. Not a
Number (NaN), infinity, or empty registers must be specially treated.
Unnormals need to be converted to normal values before the FPTAN instruction
will work correctly. Denormals will be converted to very small unnormals
that do work correctly for the FPTAN instruction. The sign of the angle is
saved to control the sign of the result.

Within the functions, close attention was paid to maintain concurrent
execution of the 80287 and host. The concurrent execution will effectively
hide the execution time of the decision logic used in the program.


Appendix A  Machine Instruction Encoding and Decoding

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

Machine instructions for the 80287 come in one of five different forms as
shown in table A-1. In all cases, the instructions are at least two bytes
long and begin with the bit pattern 11011B, which identifies the ESCAPE
class of instructions. Instructions that reference memory operands are
encoded much like similar CPU instructions, because all of the CPU
memory-addressing modes may be used with ESCAPE instructions.

Note that several of the processor control instructions (see table 2-11 in
Chapter Two) may be preceded by an assembler-generated CPU WAIT instruction
(encoding: 10011011B) if they are programmed using the WAIT form of their
mnemonics. The ASM286 assembler inserts a WAIT instruction only before these
specific processor control instructions‘‘all of the numeric instructions are
automatically synchronized by the 80286 CPU and an explicit WAIT
instruction, though allowed, is not necessary.

Table A-2 lists all 80287 machine instructions in binary sequence. This
table may be used to "disassemble" instructions in unformatted memory dumps
or instructions monitored from the data bus. Users writing exception
handlers may also find this information useful to identify the offending
instruction.


Table A-1. 80287 Instruction Encoding

  ’‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘“
  � Lower-Addressed Byte  �     Higher-Addressed Byte   �  0, 1, or 2 bytes �
  –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘˜‘‘‘‘‘‘‘š‘‘‘˜‘‘‘‘‘‘‘˜‘‘‘˜‘‘‘‘‘˜‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
1
Memory transfers, including applicable processor control instructions;
0, 1, or 2 displacement bytes may follow.� 1  1  0  1  1 � OP-A  � 1 �  MOD  � 1 �OP-B �  R/M  �   DISPLACEMENT    �
  –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘š‘‘‘š‘‘‘‘‘‘‘š‘‘‘™‘‘‘‘‘š‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘—
2
Memory arithmetic and comparison instructions; 0, 1, or 2 displacement
bytes may follow.� 1  1  0  1  1 �FORMAT �OP-A  MOD  �  OP-B   �  R/M  �   DISPLACEMENT    �
  –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘˜‘‘‘š‘‘‘š‘‘‘˜‘‘‘š‘‘‘‘‘‘‘‘‘š‘‘‘‘‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘•
3
Stack arithmetic and comparison instructions.� 1  1  0  1  1 � R � P �OP-A 1 � 1 �  OP-B   �  REG  �
  –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘š‘‘‘š‘‘‘š‘‘‘š‘‘‘š‘‘‘˜‘‘‘‘‘™‘‘‘‘‘‘‘—
4
Constant, transcendental, some arithmetic instructions.� 1  1  0  1  1 � 0 � 0 � 1 � 1 � 1 � 1 �     OP      �
  –‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘š‘‘‘š‘‘‘š‘‘‘š‘‘‘š‘‘‘š‘‘‘š‘‘‘‘‘‘‘‘‘‘‘‘‘—
5
Processor control instructions that do not reference memory.� 1  1  0  1  1 � 0 � 1 � 1 � 1 � 1 � 1 �     OP      �
  ”‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘™‘‘‘‘‘‘‘‘‘‘‘‘‘•
    7  6  5  4  3   2   1   0   7   6   5   4  3  2  1  0

‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NOTES

  OP, OP-A, OP-B: Instruction opcode, possibly split into two fields.

  MOD: Same as 80286 CPU mode field.

  R/M: Same as 80286 CPU register/memory field.

  FORMAT: Defines memory operand
    00 = short real
    01 = short integer
    10 = long real
    11 = word integer

  R: 0 = return result to stack top
     1 = return result to other register

  P: 0 = do not pop stack
     1 = pop stack after operation

  REG: register stack element
   000 = stack top
   001 = next on stack
   010 = third stack element, etc.
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


Table A-2. Machine Instruction Decoding Guide


’‘1st  Byte‘‘“                                        ASM286 Instruction
Hex     Binary     2nd Byte        Bytes 3, 4               Format

D8   1101  1000   MOD00  0R/M  (disp-lo),(disp-hi)    FADD     short-real
D8   1101  1000   MOD00  1R/M  (disp-lo),(disp-hi)    FMUL     short-real
D8   1101  1000   MOD01  0R/M  (disp-lo),(disp-hi)    FCOM     short-real
D8   1101  1000   MOD01  1R/M  (disp-lo),(disp-hi)    FCOMP    short-real
D8   1101  1000   MOD10  0R/M  (disp-lo),(disp-hi)    FSUB     short-real
D8   1101  1000   MOD10  1R/M  (disp-lo),(disp-hi)    FSUBR    short-real
D8   1101  1000   MOD11  0R/M  (disp-lo),(disp-hi)    FDIV     short-real
D8   1101  1000   MOD11  1R/M  (disp-lo),(disp-hi)    FDIVR    short-real
D8   1101  1000   1100   0REG                         FADD     ST,ST(i)
D8   1101  1000   1100   1REG                         FMUL     ST,ST(i)
D8   1101  1000   1101   0REG                         FCOM     ST(i)
D8   1101  1000   1101   1REG                         FCOMP    ST(i)
D8   1101  1000   1110   0REG                         FSUB     ST,ST(i)
D8   1101  1000   1110   1REG                         FSUBR    ST,ST(i)
D8   1101  1000   1111   0REG                         FDIV     ST,ST(i)
D8   1101  1000   1111   1REG                         FDIVR    ST,ST(i)
D9   1101  1001   MOD00  0R/M  (disp-lo),(disp-hi)    FLD      short-real
D9   1101  1001   MOD00  1R/M                         reserved
D9   1101  1001   MOD01  0R/M  (disp-lo),(disp-hi)    FST      short-real
D9   1101  1001   MOD01  1R/M  (disp-lo),(disp-hi)    FSTP     short-real
D9   1101  1001   MOD10  0R/M  (disp-lo),(disp-hi)    FLDENV   14-bytes
D9   1101  1001   MOD10  1R/M  (disp-lo),(disp-hi)    FLDCW    2-bytes
D9   1101  1001   MOD11  0R/M  (disp-lo),(disp-hi)    FSTENV   14-bytes
D9   1101  1001   MOD11  1R/M  (disp-lo),(disp-hi)    FSTCW    2-bytes
D9   1101  1001   1100   0REG                         FLD      ST(i)
D9   1101  1001   1100   1REG                         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   1REG                         (1)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows: FSTP ST(i)



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                         reserved
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                         reserved
D9   1101  1001   1111   1100                         FRNDINT
D9   1101  1001   1111   1101                         FSCALE
D9   1101  1001   1111   111-                         reserved
DA   1101  1010   MOD00  0R/M  (disp-lo),(disp-hi)    FIADD    short-integer
DA   1101  1010   MOD00  1R/M  (disp-lo),(disp-hi)    FIMUL    short-integer
DA   1101  1010   MOD01  0R/M  (disp-lo),(disp-hi)    FICOM    short-integer
DA   1101  1010   MOD01  1R/M  (disp-lo),(disp-hi)    FICOMP   short-integer
DA   1101  1010   MOD10  0R/M  (disp-lo),(disp-hi)    FISUB    short-integer
DA   1101  1010   MOD10  1R/M  (disp-lo),(disp-hi)    FISUBR   short-integer
DA   1101  1010   MOD11  0R/M  (disp-lo),(disp-hi)    FIDIV    short-integer
DA   1101  1010   MOD11  1R/M  (disp-lo),(disp-hi)    FIDIVR   short-integer
DA   1101  1010   11--  ----                          reserved
DB   1101  1011   MOD00  0R/M  (disp-lo),(disp-hi)    FILD     short-integer
DB   1101  1011   MOD00  1R/M  (disp-lo),(disp-hi)    reserved
DB   1101  1011   MOD01  0R/M  (disp-lo),(disp-hi)    FIST     short-integer
DB   1101  1011   MOD01  1R/M  (disp-lo),(disp-hi)    FISTP    short-integer
DB   1101  1011   MOD10  0R/M  (disp-lo),(disp-hi)    reserved
DB   1101  1011   MOD10  1R/M  (disp-lo),(disp-hi)    FLD      temp-real
DB   1101  1011   MOD11  0R/M  (disp-lo),(disp-hi)    reserved
DB   1101  1011   MOD11  1R/M  (disp-lo),(disp-hi)    FSTP     temp-real
DB   1101  1011   110-   ----                         reserved
DB   1101  1011   1110   0000                         reserved (8087 FENI)
DB   1101  1011   1110   0001                         reserved (8087 FDISI)
DB   1101  1011   1110   0010                         FCLEX
DB   1101  1011   1110   0011                         FINIT
DB   1101  1011   1110   0100                         FSETPM
DB   1101  1011   1110   1---                         reserved
DB   1101  1011   1111   ----                         reserved
DC   1101  1100   MOD00  0R/M  (disp-lo),(disp-hi)    FADD     long-real
DC   1101  1100   MOD00  1R/M  (disp-lo),(disp-hi)    FMUL     long-real
DC   1101  1100   MOD01  0R/M  (disp-lo),(disp-hi)    FCOM     long-real
DC   1101  1100   MOD01  1R/M  (disp-lo),(disp-hi)    FCOMP    long-real
DC   1101  1100   MOD10  0R/M  (disp-lo),(disp-hi)    FSUB     long-real
DC   1101  1100   MOD10  1R/M  (disp-lo),(disp-hi)    FSUBR    long-real
DC   1101  1100   MOD11  0R/M  (disp-lo),(disp-hi)    FDIV     long-real
DC   1101  1100   MOD11  1R/M  (disp-lo),(disp-hi)    FDIVR    long-real
DC   1101  1100   1100   0REG                         FADD     ST(i),ST
DC   1101  1100   1100   1REG                         FMUL     ST(i),ST
DC   1101  1100   1101   0REG                         (2)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FCOM ST(i)



DC   1101  1100   1101   1REG                         (3)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FCOMP ST(i)



DC   1101  1100   1110   0REG                         FSUB     ST(i),ST
DC   1101  1100   1110   1REG                         FSUBR    ST(i),ST
DC   1101  1100   1111   0REG                         FDIV     ST(i),ST
DC   1101  1100   1111   1REG                         FDIVR    ST(i),ST
DD   1101  1101   MOD00  0R/M  (disp-lo),(disp-hi)    FLD      long-real
DD   1101  1101   MOD00  1R/M                         reserved
DD   1101  1101   MOD01  0R/M  (disp-lo),(disp-hi)    FST      long-real
DD   1101  1101   MOD01  1R/M  (disp-lo),(disp-hi)    FSTP     long-real
DD   1101  1101   MOD10  0R/M  (disp-lo),(disp-hi)    FRSTOR   94-bytes
DD   1101  1101   MOD10  1R/M  (disp-lo),(disp-hi)    reserved
DD   1101  1101   MOD11  0R/M  (disp-lo),(disp-hi)    FSAVE    94-bytes
DD   1101  1101   MOD11  1R/M  (disp-lo),(disp-hi)    FSTSW    2-bytes
DD   1101  1101   1100   0REG                         FFREE    ST(i)
DD   1101  1101   1100   1REG                         (4)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FXCH ST(i)



DD   1101  1101   1101   0REG                         FST      ST(i)
DD   1101  1101   1101   1REG                         FSTP     ST(i)
DD   1101  1101   111-   ----                         reserved
DE   1101  1110   MOD00  0R/M  (disp-lo),(disp-hi)    FIADD    word-integer
DE   1101  1110   MOD00  1R/M  (disp-lo),(disp-hi)    FIMUL    word-integer
DE   1101  1110   MOD01  0R/M  (disp-lo),(disp-hi)    FICOM    word-integer
DE   1101  1110   MOD01  1R/M  (disp-lo),(disp-hi)    FICOMP   word-integer
DE   1101  1110   MOD10  0R/M  (disp-lo),(disp-hi)    FISUB    word-integer
DE   1101  1110   MOD10  1R/M  (disp-lo),(disp-hi)    FISUBR   word-integer
DE   1101  1110   MOD11  0R/M  (disp-lo),(disp-hi)    FIDIV    word-integer
DE   1101  1110   MOD11  1R/M  (disp-lo),(disp-hi)    FIDIVR   word-integer
DE   1101  1110   1100   0REG                         FADDP    ST(i),ST
DE   1101  1110   1100   1REG                         FMULP    ST(i),ST
DE   1101  1110   1101   0---                         (5)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FCOMP ST(i)



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   0REG                         FSUBP    ST(i),ST
DE   1101  1110   1110   1REG                         FSUBRP   ST(i),ST
DE   1101  1110   1111   0REG                         FDIVP    ST(i),ST
DE   1101  1110   1111   1REG                         FDIVRP   ST(i),ST
DF   1101  1111   MOD00  0R/M  (disp-lo),(disp-hi)    FILD     word-integer
DF   1101  1111   MOD00  1R/M  (disp-lo),(disp-hi)    reserved
DF   1101  1111   MOD01  0R/M  (disp-lo),(disp-hi)    FIST     word-integer
DE   1101  1110   MOD01  1R/M  (disp-lo),(disp-hi)    FISTP    word-integer
DF   1101  1111   MOD10  0R/M  (disp-lo),(disp-hi)    FBLD     packed-decimal
DF   1101  1111   MOD10  1R/M  (disp-lo),(disp-hi)    FILD     long-integer
DF   1101  1111   MOD11  0R/M  (disp-lo),(disp-hi)    FBSTP    packed-decimal
DF   1101  1111   MOD11  1R/M  (disp-lo),(disp-hi)    FISTP    long-integer
DF   1101  1111   1100   0REG                         (6)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FFREE ST(i) and pop stack



DF   1101  1111   1100   1REG                         (7)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FXCH ST(i)



DF   1101  1111   1101   0REG                         (8)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FSTP ST(i)



DF   1101  1111   1101   1REG                         (9)
The marked encodings are not generated by the language translators. If,
however, the 80287 encounters one of these encodings in the instruction
stream, it will execute it as follows:  FSTP ST(i)



DF   1101  1111   1110   000                          FSTSW AX
DF   1101  1111   1111   XXX                          reserved


Appendix B  Compatibility Between the 80287 NPX and the 8087

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The 80286/80287 operating in Real-Address mode will execute 8087 programs
without major modification. However, because of differences in the handling
of numeric exceptions by the 80287 NPX and the 8087 NPX, exception-handling
routines may need to be changed.

This appendix summarizes the differences between the 80287 NPX and the 8087
NPX, and provides details showing how 8087 programs can be ported to the
80287.

  1.  The 80287 signals exceptions through a dedicated ERROR line to the
      80286. The 80287 error signal does not pass through an interrupt
      controller (the 8087 INT signal does). 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 80287. If the 80287 encounters one of these opcodes in
      its instruction stream, the instruction will effectively be ignored‘‘
      none of the 80287 internal states will be updated. While 8087 code
      containing these instructions may be executed on the 80287, it is
      unlikely that the exception-handling routines containing these 
      instructions will be completely portable to the 80287.

  3.  Interrupt vector 16 must point to the numeric exception handling
      routine.

  4.  The ESC instruction address saved in the 80287 includes any leading
      prefixes before the ESC opcode. The corresponding address saved in the
      8087 does not include leading prefixes.

  5.  In Protected-Address mode, the format of the 80287'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 80286 when executing ESC instructions
      with either TS (task switched) or EM (emulation) of the 80286 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 80287
      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 in 80287
      code to report these programming errors.

  8.  Except for the processor control instructions, all of the 80287
      numeric instructions are automatically synchronized by the 80286 CPU‘‘
      the 80286 automatically tests the BUSY line from the 80287 to ensure
      that the 80287 has completed its previous instruction 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 80287 without
      reassembly, these WAIT instructions are unnecessary.

  9.  Since the 80287 does not require WAIT instructions before each
      numeric instruction, the ASM286 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 80286/20, reassembly using ASM286
      may result in a more compact code image.

      The processor control instructions for the 80287 may be coded using
      either a WAIT or No-WAIT form of mnemonic. The WAIT forms of these
      instructions cause ASM286 to precede the ESC instruction with a
      CPU WAIT instruction, in the identical manner as does ASM86.

  10. A recommended way to detect the presence of an 80287 in an 80286
      system (or an 8087 in an 8086 system) is shown below. It assumes that
      the sytem hardware causes the data bus to be high if no 80287 is
      present to drive the data lines during the FSTSW (Store 80287 Status
      Word) instruction.

      FND_287: FNINIT            ; initialize numeric processor.
               FSTSTW  STAT      ; store status word into location
                                 ; STAT.
               MOV     AX,STAT
               OR      AL,AL     ; Zero Flag reflects result of OR.
               JZ      GOT_287   ; Zero in AL means 80287 is present.
      ;
      ;    No 80287 Present
      ;
               SMSW    AX
               OR      AX,0004H  ; set EM bit in Machine Status Word.
               LMSW    AX        ; to enable software emulation of 287.
               JMP     CONTINUE
      ;
      ;    80287 is present in system
      ;
      GOT_287: SMSW    AX
               OR      AX,0002H  ; set MP bit in Machine Status Word
               LMSW    AX        ; to permit normal 80287 operation
      ;
      ;    Continue . . .
      ;
      CONTINUE:                  ; and off we go

      An 80286/80287 design must place a pullup resistor on one of the low
      eight data bus bits of the 80286 to be sure it is read as a high when
      no 80287 is present.


Appencix C  Implementing the IEEE P754 Standard

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

The 80287 NPX and standard support library software, provides an
implementation of the IEEE "A Proposed Standard for Binary Floating-Point
Arithmetic," Draft 10.0, Task P754, of December 2, 1982. The 80287 Support
Library, described in 80287 Support Library Reference Manual, Order Number
122129, is an example of such a support library.

This appendix describes the relationship between the 80287 NPX and the IEEE
Standard. Where the Standard has options, Intel's choices in implementing
the 80287 are described. Where portions of the Standard are implemented
through software, this appendix indicates which modules of the 80287 Support
Library implement the Standard. Where special software in addition to the
Support Library may be required by your application, this appendix indicates
how to write this software.

This appendix contains many terms with precise technical meanings,
specified in the 754 Standard. Where these terms are used, they have been
capitalized to emphasize the precision of their meanings. The Glossary
provides the definitions for all capitalized phrases in this appendix.


Options Implemented in the 80287
The 80287 SHORT_REAL and LONG_REAL formats conform precisely to the
Standard's Single and Double Floating-Point Numbers, respectively. The 80287
TEMP_REAL format is the same as the Standard's Double Extended format. The
Standard allows a choice of Bias in representing the exponent; the 80287
uses the Bias 16383 decimal.

For the Double Extended format, the Standard contains an option for the
meaning of the minimum exponent combined with a nonzero significand. The
Bias for this special case can be either 16383, as in all the other cases,
or 16382, making the smallest exponent equivalent to the second-smallest
exponent. The 80287 uses the Bias 16382 for this case. This allows the 80287
to distinguish between Denormal numbers (integer part is zero, fraction is
nonzero, Biased exponent is 0) and Unnormal numbers of the same value (same
as the denormal except the Biased Exponent is 1).

The Standard allows flexibility in specifying which NaNs are trapping and
which are nontrapping. The EH287.LIB module of the 80287 Support Library
provides a software implementation of nontrapping NaNs, and defines one
distinction between trapping and nontrapping NaNs: If the most significant
bit of the fractional part of a NaN is 1, the NaN is nontrapping. If it is
0, the NaN is trapping.

When a masked Invalid Operation error involves two NaN inputs, the Standard
allows flexibility in choosing which NaN is output. The 80287 selects the
NaN whose absolute value is greatest.


Areas of the Standard Implemented in Software
There are five areas of the Standard that are not implemented directly in
the 80287 hardware; these areas are instead implemented in software as part
of the 80287 Support Library.

  1.  The Standard requires that a Normalizing Mode be provided, in which
      any nonnormal operands to functions are automatically normalized
      before the function is performed. The NPX provides a "Denormal
      operand" exception for this case, allowing the exception handler
      the opportunity to perform the normalization specified by the
      Standard. The Denormal operand exception handler provided by
      EH287.LIB implements the Standard's Normalizing Mode completely for
      Single- and Double-precision arguments. Normalizing mode for Double
      Extended operands is implemented in EH287.LIB with one non-Standard
      feature, discussed in the next section.

  2.  The Standard specifies that in comparing two operands whose
      relationship is "unordered," the equality test yield an answer of
      FALSE, with no errors or exceptions. The 80287 FCOM and FTST
      instructions themselves issue an Invalid Operation exception in this
      case. The error handler EH287.LIB filters out this Invalid Operation
      error using the following convention: Whenever an FCOM or FTST
      instruction is followed by a MOV AX,AX instruction (8BC0 Hex), and
      neither argument is a trapping NaN, the error handler will assume that
      a Standard equality comparison was intended, and return the correct
      answer with the Invalid Operation exception flag erased. Note that the
      Invalid Operation exception must be unmasked for this action to
      occur.

  3.  The Standard requires that two kinds of NaN's be provided: trapping
      and nontrapping. Nontrapping NaNs will not cause further Invalid
      Operation errors when they occur as operands to calculations. The NPX
      hardware directly supports only trapping NaN's; the EH287.LIB
      software implements nontrapping NaNs by returning the correct answer
      with the Invalid Operation exception flag erased. Note that the
      Invalid Operation exception must be unmasked for this action to occur.

  4.  The Standard requires that all functions that convert real numbers to
      integer formats automatically normalize the inputs if necessary. The
      integer conversion functions contained in CEL287.LIB fully meet the
      Standard in this respect; the 80287 FIST instruction alone does not
      perform this normalization.

  5.  The Standard specifies the remainder function which is provided by
      mqerRMD in CEL287.LIB. The 80287 FPREM instruction returns answers
      within a different range.


Additional Software to Meet the Standard
There are two cases in which additional software is required in conjunction
with the 80287 Support Library in order to meet the standard. The 80287
Support Library does not provide this software in the interest of saving
space and because the vast majority of applications will never encounter
these cases.

  1.  When the Invalid Operation exception is masked, Nontrapping NaNs are
      not implemented fully. Likewise, the Standard's equality test for
      "unordered" operands is not implemented when the Invalid Operation
      exception is masked. Programmers can simulate the Standard notion of
      a masked Invalid Operation exception by unmasking the 80287 Invalid
      Operation exception, and providing an Invalid Operation exception
      handler that supports nontrapping NaNs and the equality test, but
      otherwise acts just as if the Invalid Operation exception were
      masked. The 80287 Support Library Reference Manual contains examples
      for programming this handler in both ASM286 andPL/M-286.

  2.  In Normalizing Mode, Denormal operands in the TEMP_REAL format are
      converted to 0 by EH287.LIB, giving sharp Underflow to 0. The Standard
      specifies that the operation be performed on the real numbers
      represented by the denormals, giving gradual underflow. To correctly
      perform such arithmetic while in Normalizing Mode, programmers would
      have to normalize the operands into a format identical to TEMP_REAL
      except for two extra exponent bits, then perform the operation on
      those numbers. Thus, software must be written to handle the 17-bit
      exponent explicitly.

In designing the EH287.LIB, it was felt that it would be a disadvantage to
most users to increase the size of the Normalizing routine by the amount
necessary to provide this expanded arithmetic. Because the TEMP_REAL
exponent field is so much larger than the LONG_REAL exponent field, it is
extremely unlikely that TEMP_REAL underflow will be encountered in most
applications.

If meeting the Standard is a more important criterion for your application
than the choice between Normalizing and warning modes, then you can select
warning mode (Denormal operand exceptions masked), which fully meets the
Standard.

If you do wish to implement the Normalization of denormal operands in
TEMP_REAL format using extra exponent bits, the list below indicates some
useful pointers about handling Denormal operand exceptions:

  1.  TEMP_REAL numbers are considered Denormal by the NPX whenever the
      Biased Exponent is 0 (minimum exponent). This is true even if the
      explicit integer bit of the significand is 1. Such numbers can occur
      as the result of Underflow.

  2.  The 80287 FLD instruction can cause a Denormal Operand error if a
      number is being loaded from memory. It will not cause this exception
      if the number is being loaded from elsewhere in the 80287 stack.

  3.  The 80287 FCOM and FTST instructions will cause a Denormal Operand
      exception for unnormal operands as well as for denormal operands.

  4.  In cases where both the Denormal Operand and Invalid Operation
      exceptions occur, you will want to know which is signalled first. When
      a comparison instruction operates between a nonexistent stack element
      and a denormal number in 80286 memory, the D and I exceptions are
      issued simultaneously. In all other situations, a Denormal Operand
      exception takes precedence over a nonstack Invalid operation
      exception, while a stack Invalid Operation exception takes precedence
      over a Denormal Operand exception.


Glossary of 80287 and Floating-Point Terminology

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

This glossary defines many terms that have precise technical meanings as
specified in the IEEE 754 Standard. Where these terms are used, they have
been capitalized to emphasize the precision of their meanings.

Affine Mode:
  a state of the 80287, selected in the 80287 Control Word, in which
  infinities are treated as having a sign. Thus, the values +INFINITY and
  -INFINITY are considered different; they can be compared with finite
  numbers and with each other.

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:
  the difference between the unsigned Integer that appears in the Exponent
  field of a Floating-Point Number and the true Exponent that it
  represents. To obtain the true Exponent, you must subtract the Bias from
  the given Exponent. For example, the Short 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 Number, 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 hex
  values A through F (1010 through 1111) are not used. The 80287 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 80287 Status Word. These bits are
  set to certain values by the compare, test, examine, and remainder
  functions of the 80287.

Characteristic:
  a term used for some non-Intel computers, meaning the Exponent field of
  a Floating-Point Number.

Chop:
  to set the fractional part of a real number to zero, yielding the
  nearest integer in the direction of zero.

Control Word:
  a 16-bit 80287 register that the user can set, to determine the modes of
  computation the 80287 will use, and the error interrupts that will be
  enabled.

Denormal:
  a special form of Floating-Point Number, produced when an Underflow
  occurs. On the 80287, a Denormal is defined as a number with a Biased
  Exponent that is 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 80287 Temporary Real format, with more
  Exponent and Significand bits than the Double (Long Real) format, and an
  explicit Integer bit in the Significand.

Double Floating Point Number:
  the Standard's term for the 80287's 64-bit Long Real format.

Environment:
  the 14 bytes of 80287 registers affected by the FSTENV and FLDENV
  instructions. It encompasses the entire state of the 80287, except for
  the 8 Temporary Real numbers of the 80287 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 error conditions (I, D, O, U, Z, P) signalled by the
  80287.

Exponent:
  (1) any power that is raised by an exponential function. For example,
  the operand to the function mqerEXP is an Exponent. The Integer operand
  to mqerYI2 is an Exponent.

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.

Floating-Point Number:
  a sequence of data bytes that, when interpreted in a standardized way,
  represents a Real number. Floating-Point Numbers 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 Short Real and Long 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 1, except in one
  case. When the Exponent is the minimum (Biased Exponent is 0), the
  Implicit Integer Bit is 0.

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 Nontrapping 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
  80287 Packed Decimal format, the Indefinite value contains all 1's in
  the sign byte and the uppermost digits byte.

Infinity:
  a value that has greater magnitude than any Integer or any Real number.
  The existence of Infinity is subject to heated philosophical debate.
  However, 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 +INFINITY and -INFINITY. They
  support two ways of dealing with Infinity: Projective (unsigned) and
  Affine (signed).

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 80287 does whenever an Integer is
  pushed onto the 80287 stack.

Invalid Operation:
  the error condition for the 80287 that covers all cases not covered by
  other errors. Included are 80287 stack overflow and underflow, NaN
  inputs, illegal infinite inputs, out-of-range inputs, and illegal
  unnormal inputs.

Long Integer:
  an Integer format supported by the 80287 that consists of a 64-bit Two's
  Complement quantity.

Long Real:
  a Floating-Point Format supported by the 80287 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.

Mantissa:
  a term used for some non-Intel computers, meaning the Significand of a
  Floating-Point Number.

Masked:
  a term that applies to each of the six 80287 Exceptions I,D,Z,O,U,P. An
  exception is Masked if a corresponding bit in the 80287 Control Word is
  set to 1. If an exception is Masked, the 80287 will not generate an
  interrupt when the error condition occurs; it will instead provide its
  own error recovery.

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.

Nontrapping NaN:
  a NaN in which the most significant bit of the fractional part of the
  Significand is 1. By convention, these NaNs can undergo certain
  operations without visible error. Nontrapping NaNs are implemented for
  the 80287 via the software in EH87.LIB.

Normal:
  the representation of a number in a Floating-Point format in which the
  Significand has an Integer bit 1 (either explicit or Implicit).

Normalizing Mode:
  a state in which nonnormal inputs are automatically converted to normal
  inputs whenever they are used in arithmetic. Normalizing Mode is
  implemented for the 80287 via the software in EH87.LIB.

NPX:
  Numeric Processor Extension. This is the 80287.

Overflow:
  an error condition in which the correct answer is finite, but has
  magnitude too great to be represented in the destination format.

Packed Decimal:
  an Integer format supported by the 80287. 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 Control:
  an option, programmed through the 80287 Control Word, that allows all
  80287 arithmetic to be performed with reduced precision. Because no
  speed advantage results from this option, its only use is for strict
  compatibility with the IEEE Standard, and with other computer
  systems.

Precision Exception:
  an 80287 error 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.

Projective Mode:
  a state of the 80287, selected in the 80287 Control Word, in which
  infinities are treated as not having a sign. Thus the values +INFINITY
  and -INFINITY are considered the same. Certain operations, such as
  comparison to finite numbers, are illegal in Projective Mode but legal
  in Affine Mode. Thus Projective Mode gives you a greater degree of error
  control over infinite inputs.

Pseudo Zero:
  a special value of the Temporary Real format. It is a number with a zero
  significand and an Exponent that is neither all zeros or all ones.
  Pseudo zeros can come about as the result of multiplication of two
  Unnormal numbers; but they are very rare.

Real:
  any finite value (negative, positive, or zero) that can be represented
  by a decimal expansion. The fractional part of the decimal expansion can
  contain an infinite number of digits. 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 80287 that consists of a 32-bit Two's
  Complement quantity. Short Integer is not the shortest 80287 Integer
  format‘‘the 16-bit Word Integer is.

Short Real:
  a Floating-Point Format supported by the 80287, 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.

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 alone is considered to
  have a Binary Point after the first (possibly Implicit) 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 Significand
  bit. The 80287's Temporary Real format meets the Single Extended
  requirement as well as the Double Extended requirement.

Single Floating-Point Number:
  the Standard's term for the 80287's 32-bit Short Real format.

Standard:
  "a Proposed Standard for Binary Floating-Point Arithmetic," Draft 10.0
  of IEEE Task P754, December 2, 1982.

Status Word:
  A 16-bit 80287 register that can be manually set, but which is usually
  controlled by side effects to 80287 instructions. It contains condition
  codes, the 80287 stack pointer, busy and interrupt bits, and error
  flags.

Tag Word:
  a 16-bit 80287 register that is automatically maintained by the 80287.
  For each space in the 80287 stack, it tells if the space is occupied by
  a number; if so, it gives information about what kind of number.

Temporary Real:
  the main Floating-Point Format used by the 80287. It consists of a sign,
  a 15-bit Biased Exponent, and a Significand with an explicit Integer bit
  and 63 fractional-part bits.

Transcendental:
  one of a class of functions for which polynomial formulas are always
  approximate, never exact for more than isolated values. The 80287
  supports trigonometric, exponential, and logarithmic functions; all are
  Transcendental.

Trapping NaN:
  a NaN that causes an I error whenever it enters into a calculation or
  comparison, even a nonordered comparison.

Two's Complement:
  a method of representing Integers. If the uppermost bit is 0, the number
  is considered positive, with the value given by the rest of the bits. If
  the uppermost bit is 1, 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 Short Real 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 error 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.

Unmasked:
  a term that applies to each of the six 80287 Exceptions: I,D,Z,O,U,P. An
  exception is Unmasked if a corresponding bit in the 80287 Control Word
  is set to 0. If an exception is Unmasked, the 80287 will generate an
  interrupt when the error condition occurs. You can provide an interrupt
  routine that customizes your error recovery.

Unnormal:
  a Temporary Real representation in which the explicit Integer bit of the
  Significand is zero, and the exponent is nonzero. We consider Unnormal
  numbers distinct from Denormal numbers.

Word Integer:
  an Integer format supported by both the 80286 and the 80287 that
  consists of a 16-bit Two's Complement quantity.

Zero divide:
  an error condition in which the inputs are finite, but the correct
  answer, even with an unlimited exponent, has infinite magnitude.


Index
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘


A
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Address Modes
Architecture
Arithmetic Instructions
ASM 286
Automatic Exception Handling


B
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Binary Integers


C
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Comparison Instructions
Compatibility Between the 80287 and 8087
Computation Fundamentals
Concurrent (80286 and 80287) Processing
Condition Codes Interpretation
Constant Instructions
Control Word


D
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Data Synchronization
Data Transfer Instructions
Data Types and Formats
 Binary Integers
 Decimal Integers
 Encoding of Data Type
 Infinity Control
 Precision Control
 Real Numbers
 Rounding Control
Decimal Integers
Denormalization
Denormalized Operand
Denormals
Destination Operands


E
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
EM (Emulation Mode) Bit in 80286
Emulation of 80287
Encoding of Data Types
Error Synchronization
Exception Handling Examples
Exception Handling, Numeric Processing
Exceptions, Numeric
 Automatic Exception Handling
 Handling Numeric Errors
 Inexact Result
 Invalid Operation
 Masked Response
 Numeric Overflow and Underflow
 Software Exception Handling
 Zero Divisor
Exponent Field


F
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
F2XM1 (Exponentiation)
FADD (Add Real)
FADDP (Add Real and POP)
FABS (Absolute Value)
FBLD (Packed Decimal‘‘BCD‘‘Load)
FBSTP (Packed Decimal‘‘BCD‘‘Store and Pop)
FCHS (Change Signs)
FCLEX/FNCLEX (Clear Exceptions)
FCOM (Compare Real)
FCOMP (Compare Real and Pop)
FCOMPP (Compare Real and Pop Twice)
FDECSTP (Decrement Stack Pointer)
FDISI/FNDISI
FDIV (Divide Real)
FDIV DWORD PTR (Division, Single Precision)
FDIVP (Divide Real and Pop)
FDIVR (Divide Real Reversed)
FDIVRP (Divide Real Reversed and Pop)
FENI/FNENI
FFREE (Free Register)
FIADD (Integer Add)
FICOM (Integer Compare)
FICOMP (Integer Compare and Pop)
FIDIV (Integer Divide)
FIDIVR (Integer Divide Reversed)
FILD (Integer Load)
FIMUL (Integer Multiply)
FINCSTP (Increment Stack Pointer)
FINIT/FNINIT (Initialize Processor)
FIST (Integer Store)
FISTP (Integer Store and Pop)
FISUB (Integer Subtract)
FISUBR (Integer Subtract Reversed)
FLD (Load Real)
FLD1 (Load One)
FLDCW (Load Control Word)
FLDENV (Load Environment)
FLDL2E (Load Log Base 2 of e)
FLDL2T (Load Log Base 2 of 10)
FLDLG2 (Load Log Base 3 10 of 2)
FLDLN2 (Load Log Base e of 2)
FLDPI (Load PI)
FLDZ (Load Zero)
FMUL (Multiply Real)
FMULP (Multiply Real and Pop)
FNOP (No Operation)
FPATAN (Partial Arctangant)
FPREM (Partial Remainder)
FPTAN (Partial Tangent)
FRNDINT (Round to Integer)
FRSTOR (Restore State)
FSAVE, FNSAVE (Save State)
FSCALE (Scale)
FSETPM (Set Protected Mode)
FSQRT (Square Root)
FST (Store Real)
FSTCW/FNSTCW (Store Control Word)
FSTENV/FNSTENV (Store Environment)
FSTP (Store Real and Pop)
FSTSW/FNSTSW (Store Status Word)
FSTSW AX, FNSTSW AX (Store Status Word in AX)
FSUB (Subtract Real)
FSUBP (Subtract Real and Pop)
FSUBR (Subtract Real Reversed)
FSUBRP (Subtract Real Reversed and Pop)
FTST (Test)
FWAIT (CPU Wait)
FXAM (Examine)
FXCH (Exchange Registers)
FXTRACT (Extract Exponent and Significand)
FYL2X (Logarithm‘‘of x)
FYL2XP1 (Logarithm‘‘of x+1)


G
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
GET$REAL$ERROR (Store, then Clear, Exception Flags)


H
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Handling Numeric Errors
Hardware Interface


I
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
I/O Locations (Dedicated and Reserved)
IEEE P754 Standard, Implementation
Indefinite
Inexact Result
Infinity
Infinity Control
INIT$REAL$MATH$UNIT (Initialize Processor Procedure)
Initialization and Control
Instruction Coding and Decoding
Instruction Execution Times
Instruction Length
Integer Bit
Introduction to Numeric Processor 80287
Invalid Operation


L
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Long Integer Format
Long Real Format


M
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Machine Instruction Encoding and Decoding
Masked Response
MP (Math Present) Flag


N
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
NaN (Not a Number)
NO-WAIT FORM
Nonnormal Real Numbers
Number System
Numeric Exceptions
Numeric Operands
Numeric Overflow and Underflow
Numeric Processor Overview


O
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Output Format
Overflow


P
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Packed Decimal Notation
Precision Control
PLM-286
Pointers (INstruction/Data)
Processor Control Instructions
Programming Examples,
 Comparative
 Conditional Branching
 Exception Handling
 Floating Point to ASCII Conversion
 Function Partitioning
 Special Instructions
Programming Interface
Pseudo zeros and zeros


R
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Real Number Range
Real Numbers
Recognizing the 80287
Register Stack
RESTORE$REAL$STATUS (Restore Processor State)
Rounding Control


S
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
SAVE$REAL$STATUS (Save Processor State)
Scaling
SET$REAL$MODE (Set Exception Masks,Rounding Precision, and Infinity
   Controls)
Short Integer Format
Short Real Format
Significand
Software Exception Handling
Source Operands
Status Word


T
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Tag Word
Temporary Real Format
Transcendental Instructions
Trigonometric Calculation Examples


U
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Underflow
Unnormals
Upgradability


W
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
WAIT Form
Word Integer Format


Z
‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘‘
Zero Divisor


99""*-\98]PR_:*pRDWFL\x03^AWJU\x06FIL\bSLXEX\x0elnb\x17\x0e\x0e\x0f\x1e  U\x01\br\x17\x16szxu\x10<\x13wytx\x7f3\x1fzuw|\x197\x18~\f\x1a