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Some undocumented instruction updates

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Jeff Parsons 2016-02-16 12:21:58 -08:00
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---
layout: page
title: "x86 Instructions: AAD"
permalink: /docs/x86/ops/AAD/
---
AAD (0xD5)
---
### Description
From [http://www.rcollins.org/secrets/opcodes/AAD.html](http://www.rcollins.org/secrets/opcodes/AAD.html):
Undocumented: Available to all Intel x86 processors
Useful in production source code.
AAD
Flags: ASCII Adjust before Division
+-+-+-+-+-+-+-+-+-+ +----------+----------+
|O|D|I|T|S|Z|A|P|C| | 11010101 | DATA |
+-+-+-+-+-+-+-+-+-+ +----------+----------+
|+| | | |+|+|+|+|+| | D5 | IMM8 |
+-+-+-+-+-+-+-+-+-+ +----------+----------+
This instruction is the multiplication counterpart to AAM. As is the case with AAM, AAD uses the second byte
as an operand. This operand is the multiplicand for AAD. Like AAM, AAD provides a way to execute a MUL IMM8 that
is unavailable through any other means in the CPU. Unlike MUL, or IMUL, AAD sets all of the CPU status flags
according to the result. Intel states that the Overflow Flag (OF), Auxiliary carry Flag (AF), and Carry Flag (CF)
are undefined. This assertion is incorrect. These flags are fully defined, and are set consistently with respect
to any other integer operations. And again, like AMM, beginning with the Pentium, Intel has finally acknowledged
the existence of the second byte of this instruction as its operand. Intel says:
Note: imm8 has the value of the instruction's second byte. The second byte under normally assembly [sic] of
this instruction will be 0A, however, explicit modification of this byte will result in the operation described
above and may alter results.
This instruction exists in this form on all Intel x86 processors. See the file AAD.ASM for diagnostics source code
for this instruction.
See [AAD.ASM](AAD.ASM) for the source code mentioned above.