x86 Instruction Set Reference

DAA

Decimal Adjust AL after Addition

Opcode Mnemonic Description
27 DAA Decimal adjust AL after addition.
Description

Adjusts the sum of two packed BCD values to create a packed BCD result. The AL register is the implied source and destination operand. The DAA instruction is only useful when it follows an ADD instruction that adds (binary addition) two 2-digit, packed BCD values and stores a byte result in the AL register. The DAA instruction then adjusts the contents of the AL register to contain the correct 2-digit, packed BCD result. If a decimal carry is detected, the CF and AF flags are set accordingly.

Operation
if(AL & 0xF > 9 || AF == 1) {
	CF = OldCF | GetCarry(AL = AL + 6);
	AF = 1;
}
else AF = 0;

if(OldAL > 0x99 || OldCF == 1) {
	AL = AL + 0x60;
	CF = 1;
}
else CF = 0;
Flags affected

The CF and AF flags are set if the adjustment of the value results in a decimal carry in either digit of the result (see the "Operations" section above). The SF, ZF, and PF flags are set according to the result. The OF flag is undefined.

Exceptions
None.