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Inverse Subtraction

White Flame (aka David Holz)

To subtract A from a number, or “number - .A”, we transform it to the doable “-.A + number”:

 eor #$ff
 adc number

and that's it.

Further elaboration

by Frantic

One may think that the following two pieces of code would produce exactly the same result:

  ;Variant 1 - Subtract XX by YY using clc/adc
  lda #XX
  adc #YY  ;E.g. to subtract with 1, use $ff here
  ;Variant 2 - Subtract $XX by $YY using sec/adc
  lda #XX
  adc #YY-1  ;E.g. to subtract with 1, use $ff-1=$fe here

The resulting byte in the A register will indeed be the same for all possible values (0-255) of value XX and YY in each of the two variants. However, the carry flag will be set differently after executing these two pieces of code for various values of XX and YY.

This means that if the value of the carry after the “subtraction” matters to you, you can't use standard addition form (clc/adc) to do the “subtraction”. White Flame's code above is obviously using the correct version of inverse subtraction (e.g. sec/adc).

base/inverse_subtraction.txt · Last modified: 2015-04-17 04:32 (external edit)