DAA

Decimal Adjust AL after Addition

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
27 DAA A Invalid Valid Decimal adjust AL after addition.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA NA NA

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.

This instruction executes as described above in compatibility mode and legacy mode. It is not valid in 64-bit mode.

Pseudo Code

IF 64-Bit Mode
	#UD;
ELSE
	old_AL = AL;
	old_CF = CF;
	CF = 0;
	IF (((AL AND 0FH) > 9) or AF = 1)
		AL = AL + 6;
		CF = old_CF or (Carry from AL = AL + 6);
		AF = 1;
	ELSE
		AF = 0;
	FI;
	IF ((old_AL > 99H) or (old_CF = 1))
		AL = AL + 60H;
		CF = 1;
	ELSE
		CF = 0;
	FI;
FI;
Example
ADD
AL, BL
Before: AL = 79H BL = 35H EFLAGS(OSZAPC) = XXXXXX
After: AL = AEH BL = 35H EFLAGS(0SZAPC) = 110000
DAA
Before: AL = AEH BL = 35H EFLAGS(OSZAPC) = 110000
After: AL = 14H BL = 35H EFLAGS(0SZAPC) = X00111
DAA
Before: AL = 2EH BL = 35H EFLAGS(OSZAPC) = 110000
After: AL = 34H BL = 35H EFLAGS(0SZAPC) = X00101

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 "Operation" section above). The SF, ZF, and PF flags are set according to the result. The OF flag is undefined.

Exceptions

64-Bit Mode Exceptions

Exception Description
#UD If in 64-bit mode.

Compatibility Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.