Intel 80286, 80287 manual AAA Ascii Adjust AL After Addition

THE 80286 INSTRUCTION SET

AAA- ASCII Adjust AL After Addition

OpcodeInstruction ClocksDescription

37 AAA 3 ASCII adjust AL after addition

FLAGS MODIFIED

Auxiliary carry, carry

FLAGS UNDEFINED

Overflow, sign, zero, parity

OPERATION

AAAshould be executed only after an ADD instruction which leaves a byte result in the AL register. The lower nibbles of the operands to the ADD instruction should be in the range 0 through 9 (BCD digits). In this case, the AAA instruction will adjust AL to contain the correct decimal digit result. If the addition produced a decimal carry, the AH register is incremented, 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 AAA instruction with OR AL,30H.

The precise definition of AAA is as follows: if the lower 4 bits of AL are greater than nine, or if the auxiliary carry flag is 1, then increment AL by 6, AH by 1, and set the carry and auxiliary carry flags. Otherwise, reset the carry and auxiliary carry flags. In any case, conclude the AAA operation by setting the upper four bits of AL to zero.

PROTECTED MODE EXCEPTIONS

None

REAL ADDRESS MODE EXCEPTIONS

None

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