millfork/examples/a8/vertical_scroll.mfk

const array text align(64) = "...MILLFORK RULEZ..." atariscr

const array(byte) dl align(16) = [
  $70,$70,$70,$70,
  $67,@word[text.addr],
  $41,@word[dl.addr]
]

noinline asm void wait(byte register(a) f) {
  clc
  adc os_RTCLOK.b2
  .rt_check:
  cmp os_RTCLOK.b2
  bne .rt_check
  rts
}

void main(){
  byte i0B @ $80

  i0B = $f
  os_SDLST = dl.addr

  while true {
    while (i0B != 0){
      i0B -= 1
      antic_vscrol = i0B
      wait(3)
    }
    wait(50)
    while (i0B < $f){
      i0B += 1
      antic_vscrol = i0B
      wait(2)
    }
  }
}