DAS—Decimal Adjust AL after Subtraction

Instruction Operand Encoding

Description

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.

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 (Borrow from AL ← AL − 6);
        AF ← 1;
        ELSE
        AF ← 0;
    FI;
    IF ((old_AL > 99H) or (old_CF = 1))
        THEN
        AL ← AL − 60H;
        CF ← 1;
    FI;
FI;

Example

SUB

AL, BL

Before: 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 (see the “Operation” section above). The SF, ZF, and PF flags are set according to the result. The OF flag is unde-fined.

Protected Mode Exceptions

Real-Address Mode Exceptions

Virtual-8086 Mode Exceptions

Compatibility Mode Exceptions

64-Bit Mode Exceptions