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Vol. 2A 3-25
INSTRUCTION SET REFERENCE, A-M
AAS—ASCII Adjust AL After Subtraction
AAS—ASCII Adjust AL After Subtraction
Description
Adjusts the result of the subtraction of two unpacked BCD values to create a
unpacked BCD result. The AL register is the implied source and destination operand
for this instruction. The AAS instruction is only useful when it follows a SUB instruc-
tion that subtracts (binary subtraction) one unpacked BCD value from another and
stores a byte result in the AL register. The AAA instruction then adjusts the contents
of the AL register to contain the correct 1-digit unpacked BCD result.
If the subtraction produced a decimal carry, the AH register decrements by 1, and the
CF and AF flags are set. If no decimal carry occurred, the CF and AF flags are cleared,
and the AH register is unchanged. In either case, the AL register is left with its top
nibble set to 0.
This instruction executes as described in compatibility mode and legacy mode. It is
not valid in 64-bit mode.
Operation
IF 64-bit mode
THEN
#UD;
ELSE
IF ((AL AND 0FH) > 9) or (AF
= 1)
THEN
AL ← AL – 6;
AH ← AH – 1;
AF ← 1;
CF ← 1;
AL ← AL AND 0FH;
ELSE
CF ← 0;
AF ← 0;
AL ← AL AND 0FH;
FI;
FI;
Flags Affected
The AF and CF flags are set to 1 if there is a decimal borrow; otherwise, they are
cleared to 0. The OF, SF, ZF, and PF flags are undefined.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
3F AAS Invalid Valid ASCII adjust AL after subtraction.