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Vol. 2A 3-257
INSTRUCTION SET REFERENCE, A-M
DAA—Decimal Adjust AL after Addition
DAA—Decimal Adjust AL after Addition
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;
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
27 DAA Invalid Valid Decimal adjust AL after addition.