AAS             ASCII Adjust after Subtraction       Flags: O D I T S Z A P C
                                                            ? - - - ? ? * ? *
AAS

        Logic   if ((AL AND 0Fh) > 9) OR (AF = 1)
                   AL ← AL - 6
                   AH ← AH - 1
                   AF ← 1;  CF ← 1
                else
                   AF ← 0;  CF ← 0
                endif
                AL ← AL AND 0Fh

    Execute AAS following a SUB or SBB operation on two unpacked BCD
    digits that leaves a byte result in the AL register. AAS converts
    the number in the lower 4 bits (nibble) of AL to an unpacked BCD
    number (high-order nibble of AL is zeroed). If the subtraction
    produced a decimal carry, the AH register is decremented, and the
    carry and auxiliary carry flags are set to 1. If the subtraction
    produced no decimal carry, the carry and auxiliary carry flags are
    set to 0 and AH is unchanged.


    Opcode      Format
    3F          AAS


    Length and timing
    Operands    Bytes   8088    186     286     386     486     Pentium
    -            1       8       7       3       4       3       3   NP