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

	org  $f000
	
; Sprites are a tricky beast on the 2600.
; You only have two of them.
; They are 8 bits wide and 1 bit high.
; There's no way to position the X or Y coordinate directly.
; To position X, you have to wait until the TIA clock hits a
; given X position and then write to a register to lock it in.
; To position Y, you simply wait until the desired scanline and
; set the bits of your sprite to a non-zero value.
; Having fun yet? :)

Counter	equ $81

Start	CLEAN_START

NextFrame
; This macro efficiently gives us 3 lines of VSYNC
	VERTICAL_SYNC
	
; 37 lines of VBLANK
	ldx #37
LVBlank	sta WSYNC
	dex
	bne LVBlank

; For the last VBLANK line, we'll lock in our sprites' X position.
; The first one will be 20 TIA clocks from the left, plus whatever the
; DEX and BNE instructions in the previous loop took.
; There are 68 TIA clocks *before* the scanline starts, and there are
; 3 TIA clocks per CPU cycle, so counting clocks (or trial-and-error)
; is neccessary to get exact positioning.

	SLEEP 20	; this macro hard-codes a 20 cycle delay (60 TIA clocks)
	sta RESP0	; set position of sprite #1
	SLEEP 35	; wait 35 more cycles (105 TIA clocks)
	sta RESP1	; set position of sprite #2

; Draw the 192 scanlines
; We'll draw both sprites all the way down the screen
	ldx #192
	lda #0		; changes every scanline
	ldy Counter	; changes every frame
ScanLoop
	sta WSYNC	; wait for next scanline
	sta COLUBK	; set the background color
	sta GRP0	; set sprite 0 pixels
	sta GRP1	; set sprite 1 pixels
	adc #1		; increment A to cycle through colors and bitmaps
	dex
	bne ScanLoop
	
; Clear the background color and sprites before overscan
; (We don't need to use VBLANK neccessarily...)
	stx COLUBK
	stx GRP0
	stx GRP1
; 30 lines of overscan
	ldx #30
LVOver	sta WSYNC
	dex
	bne LVOver
	
; Cycle the sprite colors for the next frame
	inc Counter
	lda Counter
	sta COLUP0
	sta COLUP1
	jmp NextFrame
	
	org $fffc
	.word Start
	.word Start