3-17
INSTRUCTION SET REFERENCE
AAA—ASCII Adjust After Addition
Description
This instruction 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 is incremented 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 cleared to 0.
Operation
IF ((AL AND 0FH) > 9) OR (AF = 1)
THEN
AL
<
(AL + 6);
AH
<
AH + 1;
AF
<
1;
CF
<
1;
ELSE
AF
<
0;
CF
<
0;
FI;
AL
<
AL AND 0FH;
Flags Affected
The AF and CF flags are set to 1 if the adjustment results in a decimal carry; otherwise they are
cleared to 0. The OF, SF, ZF, and PF flags are undefined.
Exceptions (All Operating Modes)
None.
Opcode
Instruction
Description
37
AAA
ASCII adjust AL after addition
