To add a signed 8-bit delta to a 16-bit value, we need to sign-extend the delta to a full 16 bits. The low byte can be added as normal, but the upper byte needs to be $00 or $ff based on the sign of the low byte.
; Precalculate the sign-extended high byte in .X ldx #$00 lda delta bpl :+ dex ; decrement high byte to $ff for a negative delta : ; Normal 16-bit addition clc adc value ; .A still holds delta sta value txa ; .X is the high byte adc value+1 sta value+1
The following version uses only the accumulator, adding 2 bytes and 1 cycle:
; Standard low byte addition clc lda delta adc value sta value ; Sign extend the high byte lda delta and #$80 ; Extract the sign bit beq :+ ; If zero, add #$00 (+ carry) lda #$ff ; Else, add $ff (+ carry) :adc value+1 sta value+1