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AAS -- ASCII Adjust AL after Subtraction

Opcode    Instruction    Clocks    Description

3F        AAS            4         ASCII adjust AL after subtraction

Operation

IF (AL AND 0FH) > 9 OR AF = 1
THEN
   AL := AL - 6;
   AL := AL AND 0FH;
   AH := AH - 1;
   AF := 1;
   CF := 1;
ELSE
   CF := 0;
   AF := 0;
FI;

Description

Execute AAS only after a SUB instruction that leaves the byte result in the AL register. The lower nibbles of the operands of the SUB instruction must have been in the range 0 through 9 (BCD digits). In this case, AAS adjusts AL so it contains the correct decimal digit result. If the subtraction produced a decimal carry, the AH register is decremented, and the carry and auxiliary carry flags are set to 1. If no decimal carry occurred, the carry and auxiliary carry flags are set to 0, and AH is unchanged. In either case, AL is left with its top nibble set to 0. To convert AL to an ASCII result, follow the AAS with OR AL, 30H.

Flags Affected

AF and CF as described above; OF, SF, ZF, and PF are undefined

Protected Mode Exceptions

None

Real Address Mode Exceptions

None

Virtual 8086 Mode Exceptions

None

up: Chapter 17 -- 80386 Instruction Set
prev: AAM ASCII Adjust AX after Multiply
next: ADC Add with Carry