DAS

Decimal Adjust AL After Subtraction

stableVMJITAOTinstruction

Encodings

Measured cost

Not measured: it does not run on 64-bit processors.

Flags named

• CF Carry
• PF Parity
• AF Auxiliary carry
• ZF Zero
• SF Sign
• OF Overflow

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     Before: AL = 35H, BL = 47H, EFLAGS(OSZAPC) = XXXXXX
SUB AL, BL  After: AL = EEH, BL = 47H, EFLAGS(0SZAPC) = 010111
            Before: AL = EEH, BL = 47H, EFLAGS(OSZAPC) = 010111
DAA         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 undefined.

Exceptions

Sources

• This instruction in the Intel manual