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Vol. 2A 3-19
INSTRUCTION SET REFERENCE, A-M
AAA—ASCII Adjust After Addition
AAA—ASCII Adjust After Addition
Description
Adjusts the sum of two unpacked BCD values to create an unpacked BCD result. The
AL register is the implied source and destination operand for this instruction. The AAA
instruction is only useful when it follows an ADD instruction that adds (binary addi-
tion) two unpacked BCD values 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 addition produces a decimal carry, the AH register increments by 1, and the CF
and AF flags are set. If there was no decimal carry, the CF and AF flags are cleared
and the AH register is unchanged. In either case, bits 4 through 7 of the AL register
are 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
AF ← 0;
CF ← 0;
AL ← AL AND 0FH;
FI;
FI;
Flags Affected
The AF and CF flags are set to 1 if the adjustment results in a decimal carry; other-
wise they are set to 0. The OF, SF, ZF, and PF flags are undefined.
Opcode Instruction 64-Bit Mode Compat/
Leg Mode
Description
37 AAA Invalid Valid ASCII adjust AL after addition.