Meta [
	Title:   "Rainbow Atari 8-bit demo"
	Author:  "Kaj de Vos"
	Rights:  "Copyright (c) 2020-2022 Kaj de Vos"
	License: {
		PD/CC0
		http://creativecommons.org/publicdomain/zero/1.0/
	}
]

; Define fixed memory addresses

Unsafe!!! constant reference volatile byte! [
	; Operating system software addresses

	RTCLOK3= ~14
	SDMCTL=  ~022F

	; ANTIC hardware addresses

	COLBK=   ~D01A
	WSYNC=   ~D40A
	VCOUNT   ; ~D40B
]

SDMCTL: 0  ; Switch off display DMA

Forever [
  COLBK:             ; Paint the background
    WSYNC:           ; Wait a scanline
      overflow       ; Wrap around
        VCOUNT << 1  ; Pick rainbow colour
        + RTCLOK3    ; Scroll the rainbow
]