DAS (Decimal Adjust AL after Subtraction)

Opcodes

Opcode Instruction Op/En 64-Bit Mode Compat/Leg Mode Description
2F DAS NP Invalid Valid Decimal adjust AL after subtraction.

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
NP NA NA NA NA

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

#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#UD If the LOCK prefix is used.

Compatibility Mode Exceptions

#UD If the LOCK prefix is used.

64-Bit Mode Exceptions

#UD If in 64-bit mode.