PRT_LOW = $FD
PTR_HIGH = $FE
* = $1000

start
    lda #$28       ; Load value, add with carry and store the result
    adc PTR_LOW
    sta PRT_LOW
    bvc end        ; Branch on overflow clear, the V flag is set if A >= #$80
                   ;  instead of a "regular" overflow (more than #$FF)
    inc PTR_HIGH   ; Increase the "higher address value"

end
    ;[Unrelated code...]
    jmp start