# 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 ¶

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 undefined.

## Protected Mode Exceptions ¶

 #UD If the LOCK prefix is used.

 #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.