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 addi- tion) 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.

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.