AAA

ASCII Adjust After Addition

stableVMJITAOTinstruction

Encodings

Measured cost

Not measured: it does not run on 64-bit processors.

Flags named

• CF Carry
• PF Parity
• AF Auxiliary carry
• ZF Zero
• SF Sign
• OF Overflow

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

Sources

• This instruction in the Intel manual