DAS              Decimal Adjust after Subtraction    Flags: O D I T S Z A P C
?       * * * * *
DAS

Logic:    If  (AL & 0Fh) > 9 or (AF = 1) then
AL   AL - 6
AF   1
else AF   0
If (AL > 9Fh) or (CF = 1) then
AL   AL - 60h
CF   1
else CF   0

DAS corrects the result of a previous subtraction of two valid packed
decimal operands (note that this result must be in AL). This
instruction changes the content of AL so that it will contain a pair
of valid packed decimal digits.

                                                                          
Operands                  Clocks   Transfers  Bytes   Example
no operands                 4          -        1     DAS
                                                                          

Note:          Packed BCD stores one digit per nibble (4 bits); the
least significant digit is in the lower nibble.

It is not possible to apply an adjustment after
division or multiplication of packed BCD numbers.
If you need to use multiplication and division, it
is better to use unpacked BCD numbers.  See, for
example, the description of AAM (ASCII Adjust after
Multiply).

See also: AAS
See also: DAA
See also: SUB
See also: SBB
See also: DEC
See also: NEG
See also: Flags

DAS Decimal Adjust after Subtraction