AAS is the subtraction counterpart of AAA. After SUB (or SBB) of one unpacked BCD digit from another, with the result in AL, it brings AL back to a digit 0–9 and takes a decimal borrow from AH when one was needed.

Exactly what it does

If the low four bits of AL are greater than 9, or AF is 1, then AAS subtracts 6 from AL, subtracts 1 from AH, and sets AF and CF to 1. Otherwise it clears AF and CF. In both cases it then clears the upper four bits of AL. OF, SF, ZF and PF are undefined.

A borrow shows up in AF: subtracting a larger digit from a smaller one borrows across bit 4, and the low four bits then hold 16 plus the smaller digit minus the larger (2 − 5 leaves Dh, 13). Taking 6 more away leaves 7, the right digit for ten plus 2, minus 5.

Prints 37.
MOV     AX, 0402h  ; unpacked 42: AH = 4, AL = 2
SUB     AL, 5      ; AL = FDh, a borrow (AF = 1)
AAS                ; AL = 7, AH = 3: unpacked 37
OR      AX, 3030h  ; both digits -> characters
MOV     BX, AX
MOV     DL, BH
MOV     AH, 02h    ; DOS: print DL
INT     21h
MOV     DL, BL
MOV     AH, 02h
INT     21h

Multi-digit differences

For long numbers held as digit strings, go from the last digit to the first: SBB the two digits (CF holds the borrow from the previous column), AAS, store AL, repeat. Clear CF with CLC before the first column. When the final CF is 1 the second number was larger and the stored digits are the ten's complement of the difference.

Pitfalls

AAS corrects AL only. It is for unpacked digits; for two digits packed in a byte use DAS. Remember that AAS changes AH, which may be holding something else.

Later CPUs

Unchanged in 16- and 32-bit code; not available in 64-bit mode.