| Opcode | Instruction | Op/En | 64-Bit Mode | Compat/Leg Mode | Description |
|---|---|---|---|---|---|
| 27 | DAA | ZO | Invalid | Valid | Decimal adjust AL after addition. |
| Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4 |
|---|---|---|---|---|
| ZO | N/A | N/A | N/A | N/A |
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.
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;
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
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.
| #UD | If the LOCK prefix is used. |
| #UD | If the LOCK prefix is used. |
| #UD | If the LOCK prefix is used. |
| #UD | If the LOCK prefix is used. |
| #UD | If in 64-bit mode. |