Intel 80287, 80286 manual AAS-ASCII Adjust AL After Subtraction, Aas

THE 80286 INSTRUCTION SET

AAS-ASCII Adjust AL After Subtraction

Opcode Instruction ClocksDescription

3F AAS 3 ASCII adjust AL after subtraction

FLAGS MODIFIED

Auxiliary carry, carry

FLAGS UNDEFINED

Overflow, sign, zero, parity

OPERATION

AASshould be executed only after a subtraction instruction which left the byte result in the AL register. The lower nibbles of the operands to the SUB instruction should have been in the range 0 through 9 (BCD digits). In this case, the AAS instruction will adjust AL to contain 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 there was no decimal carry, the carry and auxiliary carry flags are set to 0, and AH is unchanged. In any case, AL is left with its top nibble set to O. To convert.AL to an ASCII result, you can follow the AAS instruction with OR AL,30H.

The precise definition of AAS is as follows: if the lower four bits of AL are greater than 9, or if the auxiliary carry flag is 1, then decrement AL by 6, AH by I, and set the carry and auxiliary carry flags. Otherwise, reset the carry and auxiliary carry flags. In any case, conclude the AAS operation by setting the upper four bits of AL to zero.

PROTECTED MODE EXCEPTIONS

None

REAL ADDRESS MODE EXCEPTIONS

None

B-18