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

	org  $f000

; We're going to try to animate sprites horizontally.
; Remember, we have to pause the CPU until the exact moment the
; scanline hits the desired horizontal position of the sprite.
; Since we can't hard-code the SLEEP macro we'll have to do it
; dynamically somehow. But since the TIA beam is racing so much
; faster than the CPU clock, we'll have to be clever.

counter	equ $81

start	CLEAN_START

nextframe
	VERTICAL_SYNC
	
; 37 lines of VBLANK
	ldx #35
lvblank	sta WSYNC
	dex
	bne lvblank

; This will be the 36th VBLANK. We'll use up some of this scanline's
; time to set up for the next line, where we'll set the sprite position.
; For now the position will be in CPU clocks, not TIA clocks.
; We're going to use 'counter' as the horiz. position, load into A.

	lda counter
	ror		; divide frame counter by 4 to slow animation
	ror
	and #$3f	; now in range (0-63)
	sta WSYNC	; wait for next line

; This is the 37th VBLANK where we'll set the sprite position.
; We've got our desired horizontal position in A.
; First we divide it by 4. We'll need it later in our delay loop.
; But we'll use the remainder bits to add cycles.

; The first bit will add either 0 or 1 CPU cycles.
	lsr		; shift right, bit 0 goes into carry flag
	bcs delay1	; branch to next insn if carry set - adds +1 cycle
delay1
; The second bit will add either 0 or 2 CPU cycles.
	lsr
	bcc delay2	; branch if carry clear - subtract -1 cycle
	bcs delay2	; guaranteed to succeed - adds +3 cycles
delay2
; So now we've used the remainder of our divide-by-4 operations to
; add between 0 and 3 CPU cycles (0-9 TIA clocks).
; The next loop takes 5 CPU cycles per iteration (15 TIA clocks).
	tax		; transfer A to X
delayx  dex		; decrement X
	bpl delayx	; branch while X is non-negative
	sta RESP0	; set position of sprite #1
	sta WSYNC	; end of 37th line

; The TIA clocks 3 pixels for every 1 CPU clock.
; So our final sprite position is C + (X%4)*3 + (X/4)*15
; (C is the fixed # of instruction clocks)

; We've lost a bit of resolution in our positioning, since we
; divided the horizontal position by 4 but our tightest loop takes
; 5 cycles per iteration. We'll achieve finer control later using
; some additional TIA registers.

; Now draw the 192 scanlines, drawing the sprite.
; We've already set its horizontal position for the entire frame.
	ldx #192
	lda #0		; changes every scanline
	ldy counter	; changes every frame
lvscan
	sta WSYNC	; wait for next scanline
	sta COLUBK	; set the background color
	sta GRP0	; set sprite 0 pixels
	adc #1		; increment A to cycle through colors and bitmaps
	dex
	bne lvscan
	
; Clear the background color and sprites before overscan
	stx COLUBK
	stx GRP0
; 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
	jmp nextframe
	
	org $fffc
	.word start
	.word start