DAS—Decimal Adjust AL after Subtraction

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 DAA

AL, BL

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

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.