x86 Instruction Set Reference


Decimal Adjust AL after Subtraction

Opcode Mnemonic Description
2F DAS Decimal adjust AL after subtraction.

Adjusts the result of the subtraction of two packed BCD values to create a packed BCD result.

The AL register is the implied source and destination operand. The DAS instruction is only useful when it follows a SUB instruction that subtracts (binary subtraction) one 2-digit, packed BCD value from another and stores a byte result in the AL register. The DAS instruction then adjusts the contents of the AL register to contain the correct 2-digit, packed BCD result. If a decimal borrow is detected, the CF and AF flags are set accordingly.

OldAL = AL;
OldCF = CF;
CF = 0;

if(AL & 0xF > 9 || AF == 1) {
	CF = OldCF | GetBorrow(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 borrow 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.