Software - INC and DEC techniques

Typical 16-bit increment

Overflow (incrementing to $0000) sets the Z flag is set (i.e. BEQ branches).

;
      INC NUML
      BNE LABEL
      INC NUMH
LABEL

Typical 16-bit decrement

;
      LDA NUML
      BNE LABEL
      DEC NUMH
LABEL DEC NUML

16-bit decrement, test for zero

;
      LDA NUML
      BNE LABEL
      LDA NUMH
      BEQ ZERO ; branch when NUM = $0000 (NUM is not decremented in that case)
      DEC NUMH
LABEL DEC NUML

Add 255

;
      LDA NUML
      BEQ LABEL
      INC NUMH
LABEL DEC NUML

Subtract 255

;
      INC NUML
      BEQ LABEL
      DEC NUMH
LABEL

Constant time increment

6 cycles

A = high byte, X = low byte

Overflow (incrementing to $0000) sets the carry

CPX #$FF
INX
ADC #$00

Constant time decrement

6 cycles

A = high byte, X = low byte

Underflow (decrementing to $FFFF) clears the carry

CPX #$01
DEX
SBC #$00