User's Manual

THE 80286 INSTRUCTION SET
AAA - ASCII Adjust AL After Addition
Opcode
Instruction
Clocks
Description
37 AAA 3
ASCII adjust AL after addition
FLAGS MODIFIED
Auxiliary carry, carry
FLAGS UNDEFINED
Overflow, sign, zero, parity
OPERATION
AAA should 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
8-15