aaa

ASCII Adjust After Addition

AAA

Adjusts AL after addition for unpacked BCD.

Pseudocode Operation

if ((AL & 0x0F) > 9 || AF == 1) { AL ← (AL + 6) & 0xFF; AH ← AH + 1; AF ← 1; CF ← 1 } else { AF ← 0; CF ← 0 }; AL ← AL & 0x0F

Encoding

Binary Layout

Format Legacy

Opcode 37

Extension Base (Legacy)

Operands

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
37	AAA	ZO	Invalid Valid		ASCII adjust AL 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 addition) 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
AX := AX + 106H;
AF := 1;
CF := 1;
ELSE
AF := 0;
CF := 0;
FI;
AL := AL AND 0FH;
FI;

Flags Affected

The AF and CF flags are set to 1 if the adjustment results in a decimal carry; otherwise they are set to 0. The OF, SF, ZF, and PF flags are undefined.

Exceptions

Protected Mode Exceptions

#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Same exceptions as protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as protected mode. AAA-ASCII Adjust After Addition Vol. 2A 3-1

Compatibility Mode Exceptions