processor 6502
	include "vcs.h"
	include "macro.h"
	include "xmacro.h"

	org  $f000

; To output a PAL signal, you need the following:
;
; - 3 scanlines of VSYNC
; - 45 blank lines
; - 228 visible scanlines
; - 36 blank lines
;
; Total = 312 lines (vs 262 for NTSC)

; PAL runs at 50 Hz (vs 60 Hz for NTSC)
; so your game will run more slowly.
; Since you have extra cycles to play with, you could just
; call your position update routine twice every five frames.

; Note also that PAL has different colors than NTSC.
; (See http://www.randomterrain.com/atari-2600-memories-tia-color-charts.html)

; The TIMER_SETUP macro only goes up to 215 lines,
; so for the PAL visible frame you would need to use
; multiple sections (say 215 + 13 = 228) or count manually
; like we do in this example.

; Many VCS games use conditional defines
; (IFCONST and IFNCONST in DASM)
; to switch between NTSC and PAL constants.

; background color
BGColor	equ $81

; Start of program, init registers and clear memory
Start	CLEAN_START

NextFrame
; 01 + 3 lines of VSYNC
	VERTICAL_SYNC
; Now we need 44 lines of VBLANK...
	TIMER_SETUP 44
        TIMER_WAIT
; Re-enable output (disable VBLANK)
	lda #0
        sta VBLANK
; 228 PAL scanlines are visible
; We'll draw some rainbows
	ldx #228
	lda BGColor	; load the background color out of RAM
ScanLoop
	adc #1		; add 1 to the current background color in A
	sta COLUBK	; set the background color
	sta WSYNC	; WSYNC doesn't care what value is stored
	dex
	bne ScanLoop

; 36 lines of overscan to complete the frame
	TIMER_SETUP 36
; Enable VBLANK again
	lda #2
        sta VBLANK
        TIMER_WAIT
	
; The next frame will start with current color value - 1
; to get a downwards scrolling effect
	dec BGColor

; Go back and do another frame
	jmp NextFrame

; 6502 vectors
	org $fffc
	.word Start
	.word Start