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INSTRUCTION SET REFERENCE
DASDecimal Adjust AL after Subtraction
Description
This instruction adjusts the result of the subtraction of two packed BCD values to create a
packed BCD result. The AL register is the implied source and destination operand. The DAS
instruction is only useful when it follows a SUB instruction that subtracts (binary subtraction)
one 2-digit, packed BCD value from another and stores a byte result in the AL register. The DAS
instruction then adjusts the contents of the AL register to contain the correct 2-digit, packed
BCD result. If a decimal borrow is detected, the CF and AF flags are set accordingly.
Operation
IF (AL AND 0FH) > 9 OR AF = 1
THEN
AL
<
AL
?
6;
CF
<
CF OR BorrowFromLastSubtraction; (* CF OR borrow from AL
<
AL
?
6 *)
AF
<
1;
ELSE AF
<
0;
FI;
IF ((AL > 9FH) or CF = 1)
THEN
AL
<
AL
?
60H;
CF
<
1;
ELSE CF
<
0;
FI;
Example
SUB AL, BLBefore: AL=35H BL=47H EFLAGS(OSZAPC)=XXXXXX
After: AL=EEH BL=47H EFLAGS(0SZAPC)=010111
DAA
Before: AL=EEH BL=47H EFLAGS(OSZAPC)=010111
After: AL=88H BL=47H EFLAGS(0SZAPC)=X10111
Flags Affected
The CF and AF flags are set if the adjustment of the value results in a decimal borrow in either
digit of the result (refer to the Operation section above). The SF, ZF, and PF flags are set
according to the result. The OF flag is undefined.
Exceptions (All Operating Modes)
None.
Opcode
Instruction
Description
2F
DAS
Decimal adjust AL after subtraction
