DAA

Decimal Adjust AL After Addition

stableVMJITAOTinstruction

Encodings

Measured cost

Not measured: it does not run on 64-bit processors.

Flags named

• CF Carry
• PF Parity
• AF Auxiliary carry
• ZF Zero
• SF Sign
• OF Overflow

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     Before: AL=79H BL=35H EFLAGS(OSZAPC)=XXXXXX
ADD AL, BL  After: AL=AEH BL=35H EFLAGS(0SZAPC)=110000
            Before: AL=AEH BL=35H EFLAGS(OSZAPC)=110000
DAA         After: AL=14H BL=35H EFLAGS(0SZAPC)=X00111
            Before: AL=2EH BL=35H EFLAGS(OSZAPC)=110000
DAA         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

Sources

• This instruction in the Intel manual