3-18
INSTRUCTION SET REFERENCE
AADASCII Adjust AX Before Division
Description
This instruction adjusts two unpacked BCD digits (the least-significant digit in the AL register
and the most-significant digit in the AH register) so that a division operation performed on the
result will yield a correct unpacked BCD value. The AAD instruction is only useful when it
precedes a DIV instruction that divides (binary division) the adjusted value in the AX register
by an unpacked BCD value.
The AAD instruction sets the value in the AL register to (AL + (10 * AH)), and then clears the
AH register to 00H. The value in the AX register is then equal to the binary equivalent of the
original unpacked two-digit (base 10) number in registers AH and AL.
The generalized version of this instruction allows adjustment of two unpacked digits of any
number base (refer to the Operation section below), by setting the imm8 byte to the selected
number base (for example, 08H for octal, 0AH for decimal, or 0CH for base 12 numbers). The
AAD mnemonic is interpreted by all assemblers to mean adjust ASCII (base 10) values. To
adjust values in another number base, the instruction must be hand coded in machine code (D5
imm8).
Operation
tempAL
<
AL;
tempAH
<
AH;
AL
<
(tempAL + (tempAH
?
imm8)) AND FFH; (* imm8 is set to 0AH for the AAD mnemonic *)
AH
<
0
The immediate value (imm8) is taken from the second byte of the instruction.
Flags Affected
The SF, ZF, and PF flags are set according to the result; the OF, AF, and CF flags are undefined.
Exceptions (All Operating Modes)
None.
Opcode
Instruction
Description
D5 0A
AAD
ASCII adjust AX before division
D5 ib
(No mnemonic)
Adjust AX before division to number base imm8
