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(nasm.info)Section B.4.1

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B.4.1. `AAA', `AAS', `AAM', `AAD': ASCII Adjustments
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     AAA                           ; 37                   [8086]

     AAS                           ; 3F                   [8086]

     AAD                           ; D5 0A                [8086]
     AAD imm                       ; D5 ib                [8086]

     AAM                           ; D4 0A                [8086]
     AAM imm                       ; D4 ib                [8086]

   These instructions are used in conjunction with the add, subtract,
multiply and divide instructions to perform binary-coded decimal
arithmetic in _unpacked_ (one BCD digit per byte - easy to translate to
and from `ASCII', hence the instruction names) form. There are also
packed BCD instructions `DAA' and `DAS': see *Note Section B.4.57::.

   * `AAA' (ASCII Adjust After Addition) should be used after a one-byte
     `ADD' instruction whose destination was the `AL' register: by
     means of examining the value in the low nibble of `AL' and also the
     auxiliary carry flag `AF', it determines whether the addition has
     overflowed, and adjusts it (and sets the carry flag) if so. You
     can add long BCD strings together by doing `ADD'/`AAA' on the low
     digits, then doing `ADC'/`AAA' on each subsequent digit.

   * `AAS' (ASCII Adjust AL After Subtraction) works similarly to
     `AAA', but is for use after `SUB' instructions rather than `ADD'.

   * `AAM' (ASCII Adjust AX After Multiply) is for use after you have
     multiplied two decimal digits together and left the result in
     `AL': it divides `AL' by ten and stores the quotient in `AH',
     leaving the remainder in `AL'. The divisor 10 can be changed by
     specifying an operand to the instruction: a particularly handy use
     of this is `AAM 16', causing the two nibbles in `AL' to be
     separated into `AH' and `AL'.

   * `AAD' (ASCII Adjust AX Before Division) performs the inverse
     operation to `AAM': it multiplies `AH' by ten, adds it to `AL', and
     sets `AH' to zero. Again, the multiplier 10 can be changed.