DAA — Decimal Adjust AL After Addition

Opcode	Instruction	Op/En	64-Bit Mode	Compat/Leg Mode	Description
27	DAA	ZO	Invalid	Valid	Decimal adjust AL after addition.

Instruction Operand Encoding ¶

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
ZO	N/A	N/A	N/A	N/A

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.

Operation ¶

IF 64-Bit Mode
    THEN
        #UD;
    ELSE
        old_AL := AL;
        old_CF := CF;
        CF := 0;
        IF (((AL AND 0FH) > 9) or AF = 1)
                THEN
                    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))
            THEN
                    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.

Protected Mode Exceptions ¶

#UD	If the LOCK prefix is used.

Real-Address Mode Exceptions ¶

#UD	If the LOCK prefix is used.

Virtual-8086 Mode Exceptions ¶

#UD	If the LOCK prefix is used.

Compatibility Mode Exceptions ¶

#UD	If the LOCK prefix is used.

64-Bit Mode Exceptions ¶

#UD	If in 64-bit mode.