i386/i386.reference/s03_03.htm

   1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
   2 <HTML>
   3 <HEAD>
   4 <TITLE>80386 Programmer's Reference Manual -- Section 3.3</TITLE>
   5 </HEAD>
   6 <BODY>
   7 <B>up:</B> <A HREF="c03.htm">
   8 Chapter 3 -- Applications Instruction Set</A><BR>
   9 <B>prev:</B> <A HREF="s03_02.htm">3.2  Binary Arithmetic Instructions</A><BR>
  10 <B>next:</B> <A HREF="s03_04.htm">3.4  Logical Instructions</A>
  11 <P>
  12 <HR>
  13 <P>
  14 <H1>3.3  Decimal Arithmetic Instructions</H1>
  15 Decimal arithmetic is performed by combining the binary arithmetic
  16 instructions (already discussed in the prior section) with the decimal
  17 arithmetic instructions. The decimal arithmetic instructions are used in one
  18 of the following ways:
  19 <UL>
  20 <LI> To adjust the results of a previous binary arithmetic operation to
  21 produce a valid packed or unpacked decimal result.
  22 <LI> To adjust the inputs to a subsequent binary arithmetic operation so
  23 that the operation will produce a valid packed or unpacked decimal
  24 result.
  25 </UL>
  26 These instructions operate only on the AL or AH registers. Most utilize the
  27 AF flag.
  28
  29 <H2>3.3.1  Packed BCD Adjustment Instructions</H2>
  30 <A HREF="DAA.htm">DAA</A> (Decimal Adjust after Addition)
  31 adjusts the result of adding two valid
  32 packed decimal operands in AL.
  33 <A HREF="DAA.htm">DAA</A> must always follow the addition of two
  34 pairs of packed decimal numbers (one digit in each half-byte) to obtain a
  35 pair of valid packed decimal digits as results. The carry flag is set if
  36 carry was needed.
  37 <P>
  38 <A HREF="DAS.htm">DAS</A> (Decimal Adjust after Subtraction)
  39 adjusts the result of subtracting
  40 two valid packed decimal operands in AL.
  41 <A HREF="DAS.htm">DAS</A> must always follow the
  42 subtraction of one pair of packed decimal numbers (one digit in each half-
  43 byte) from another to obtain a pair of valid packed decimal digits as
  44 results. The carry flag is set if a borrow was needed.
  45
  46 <H2>3.3.2  Unpacked BCD Adjustment Instructions</H2>
  47 <A HREF="AAA.htm">AAA</A> (ASCII Adjust after Addition)
  48 changes the contents of register AL to a
  49 valid unpacked decimal number, and zeros the top 4 bits.
  50 <A HREF="AAA.htm">AAA</A> must always
  51 follow the addition of two unpacked decimal operands in AL. The carry flag
  52 is set and AH is incremented if a carry is necessary.
  53 <P>
  54 <A HREF="AAS.htm">AAS</A> (ASCII Adjust after Subtraction)
  55 changes the contents of register AL to
  56 a valid unpacked decimal number, and zeros the top 4 bits.
  57 <A HREF="AAS.htm">AAS</A> must always
  58 follow the subtraction of one unpacked decimal operand from another in AL.
  59 The carry flag is set and AH decremented if a borrow is necessary.
  60 <P>
  61 <A HREF="AAM.htm">AAM</A> (ASCII Adjust after Multiplication)
  62 corrects the result of a
  63 multiplication of two valid unpacked decimal numbers.
  64 <A HREF="AAM.htm">AAM</A> must always follow
  65 the multiplication of two decimal numbers to produce a valid decimal result.
  66 The high order digit is left in AH, the low order digit in AL.
  67 <P>
  68 <A HREF="AAD.htm">AAD</A> (ASCII Adjust before Division)
  69 modifies the numerator in AH and AL to
  70 prepare for the division of two valid unpacked decimal operands so that the
  71 quotient produced by the division will be a valid unpacked decimal number.
  72 AH should contain the high-order digit and AL the low-order digit. This
  73 instruction adjusts the value and places the result in AL. AH will contain
  74 zero.
  75 <P>
  76 <HR>
  77 <P>
  78 <B>up:</B> <A HREF="c03.htm">
  79 Chapter 3 -- Applications Instruction Set</A><BR>
  80 <B>prev:</B> <A HREF="s03_02.htm">3.2  Binary Arithmetic Instructions</A><BR>
  81 <B>next:</B> <A HREF="s03_04.htm">3.4  Logical Instructions</A>
  82 </BODY>