aas

ASCII Adjust After Subtraction

AAS

Adjusts AL after subtraction for unpacked BCD.

Details

Adjusts the value in AL after a subtraction operation to make it a valid unpacked BCD digit (0–9 in the lower nibble). If the lower nibble of AL is greater than 9 or the auxiliary carry flag is set, AL is decremented by 6 and AH is decremented by 1; otherwise no adjustment occurs. Sets or clears AF and CF based on whether an adjustment was made; other flags are undefined.

Pseudocode Operation

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

Example

AAS

Encoding

Binary Layout
3F
+0
 
Format Legacy
Opcode 3F
Extension Base (Legacy)

Operands

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
3F AAS ZO Invalid Valid 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 instruction 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 1digit 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 four bits 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 – 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.

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. AAS—ASCII Adjust AL After Subtraction Vol. 2A 3-7

Compatibility Mode Exceptions

Same exceptions as protected mode.

64-Bit Mode Exceptions

#UD If in 64-bit mode. AAS—ASCII Adjust AL After Subtraction Vol. 2A 3-8