User Guide

48 General-Purpose Programming

AMD64 Technology 24592—Rev. 3.15—November 2009

Although the base of the numeration for ASCII-adjust instructions is assumed to be 10, the AAM and

AAD instructions can be used to correct multiplication and division with other bases.

BCD Adjust

• DAA—Decimal Adjust after Addition

• DAS—Decimal Adjust after Subtraction

The DAA and DAS instructions perform corrections of addition and subtraction operations on packed

BCD values. (Packed BCD values have two decimal digits stored in a byte register, with the higher

digit in the higher four bits, and the lower one in the lower four bits.) There are no instructions for

correction of multiplication and division with packed BCD values.

DAA should be applied after addition of two packed-BCD numbers. DAS should be applied after

subtraction of two packed-BCD numbers.

DAA and DAS can be used in a loop to perform addition or subtraction of two multiple-precision

decimal numbers stored in packed-BCD format. Each loop cycle would operate on corresponding

bytes (containing two decimal digits) of operands.

Endian Conversion

• BSWAP—Byte Swap

The BSWAP instruction changes the byte order of a doubleword or quadword operand in a register, as

shown in Figure 3-8. In a doubleword, bits 7–0 are exchanged with bits 31–24, and bits 15–8 are

exchanged with bits 23–16. In a quadword, bits 7–0 are exchanged with bits 63–56, bits 15–8 with bits

55–48, bits 23–16 with bits 47–40, and bits 31–24 with bits 39–32. See the following illustration.

Figure 3-8. BSWAP Doubleword Exchange

A second application of the BSWAP instruction to the same operand restores its original value. The

result of applying the BSWAP instruction to a 16-bit register is undefined. To swap bytes of a 16-bit

The BSWAP instruction is used to convert data between little-endian and big-endian byte order.

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