4 - sprite buffering.dasm

; Scan Line Buffering
; I am again leaning on a great tutorial https://alienbill.com/2600/101/07buffer.html
; I noticed in my previous example that weird stuff happened to the sprite at the left edge of the screen. If you got close to the left 
; edge the player sprite would partially and then fully disappear. I figured it had to be a timing problem, that I had too much going on 
; for the sprite to fit in HBlank. I did some cycle counting and yeah, for sure. I then found the tutorial above. 
; I read the intro of that page and then came up with my own implementation before looking at his. Works great. So I guess buffering
; is going to be a big deal

	processor 6502
	include vcs.h
	include macro.h
	org $F000

ScanlineCount = $80
VBlankTimer = $81
OverscanTimer = $82

PlayerSpriteY = $83
SpriteSliceToDraw = $84

InputUp = $85
InputDown = $86
InputLeft = $87
InputRight = $88

HorizMotionLeft = $89
HorizMotionRight = $90

PlayerSpriteBuffer = $91


Start
	CLEAN_START
	lda #$AA
	sta COLUBK
	lda #$34
	sta COLUP0

	lda #192
	sta ScanlineCount
	lda #43
	sta VBlankTimer
	lda #35
	sta OverscanTimer
	lda #75
	sta PlayerSpriteY
	lda #0
	sta SpriteSliceToDraw

	lda #%00100000
	sta InputUp
	lda #%00010000
	sta InputDown
	lda #%01000000
	sta InputLeft
	lda #%10000000
	sta InputRight

	lda #$10
	sta HorizMotionLeft
	lda #$F0
	sta HorizMotionRight

	lda #0
	sta PlayerSpriteBuffer

VSync
	sta WSYNC
	lda #%0010
	sta VSYNC
	sta WSYNC
	sta WSYNC
	sta WSYNC
	lda #0
	sta VSYNC

VBlankStart
	lda #%0010
	sta VBLANK
	lda VBlankTimer
	sta TIM64T

VBlankLogic
	;Do vblank stuff here
	ldy ScanlineCount
	;Set the number of lines of player sprite to draw to 0
	;this ensures we skip drawing until the right time
	lda #0
	sta SpriteSliceToDraw

	lda InputUp
	bit SWCHA
	bne CheckMoveDown
	dec PlayerSpriteY

CheckMoveDown
	lda InputDown
	bit SWCHA
	bne CheckMoveLeft
	inc PlayerSpriteY

CheckMoveLeft
	;We'll use register X for our horizontal movement speed
	;Default horizontal speed is 0
	ldx #0

	lda InputLeft
	bit SWCHA
	bne CheckMoveRight
	ldx HorizMotionLeft

CheckMoveRight
	lda InputRight
	bit SWCHA
	bne SetHorizontalMovement
	ldx HorizMotionRight	

SetHorizontalMovement
	stx HMP0	


VBlankEnd
	lda INTIM
	bne VBlankEnd
	lda #0
	sta VBLANK
	sta WSYNC
	sta HMOVE


HBlank
	;Do pre-line stuff here
	sta WSYNC

DrawAndClearPlayerBuffer
	;Draw the player buffer to the screen and then clear it out
	;This is our only draw call, so this is the only thing that would
	;add any cycle overhead to the currently rendering line
	lda PlayerSpriteBuffer
	sta GRP0
	lda #0
	sta PlayerSpriteBuffer
	;Now we have a whole glorious scanline worth of CPU to do whatever we want for the next line!


DrawScanLine

CheckPlayerOnScanline
	;See if this is the line we should draw at
	cpy PlayerSpriteY
	;If it isn't the line we should start drawing the player sprite at move on
	bne BufferPlayerSprite
	;If it is the line we should start drawing at set the lines of sprite to draw to 8
	lda #8
	sta SpriteSliceToDraw

BufferPlayerSprite
	;Load the sprite line count
	ldx SpriteSliceToDraw
	cpx #0
	;If the line count is 0 there's nothing to draw so skip drawing
	beq SkipPlayerSprite
	;Load the sprite line and buffer it 
	lda SpriteFace-1,X
	sta PlayerSpriteBuffer
	;Decrement and store the sprite draw line
	dex
	stx SpriteSliceToDraw
SkipPlayerSprite

FinishScanline
	dey
	bne HBlank

OverscanStart
	lda #%0010
	sta VBLANK
	lda OverscanTimer
	sta TIM64T

OverscanLogic
	;Do overscan stuff here

OverscanEnd
	lda INTIM
	bne OverscanEnd
	lda #0
	sta VBLANK
	sta WSYNC
	jmp VSync
	
SpriteFace
	.byte %01111110
	.byte %11000011
	.byte %10111101
	.byte %11111111
	.byte %10011001
	.byte %10011001
	.byte %11111111
	.byte %01111110

	org $FFFC
	.word Start
	.word Start