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DAS - Decimal Adjust AL After Subtraction

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

Instruction Operand Encoding

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
ZO	N/A	N/A	N/A	N/A

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 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.