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DAS

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Henk-Jan Lebbink edited this page Jun 5, 2018 · 13 revisions

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 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

        AL, BL  Before: AL = 35H, BL = 47H, EFLAGS(OSZAPC) = XXXXXX
SUB 
                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.

Source: Intel® Architecture Software Developer's Manual (May 2018)
Generated: 5-6-2018

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