0 - template.dasm

; A simple VCS program template by Adam Drew
; The goal here was to take everything I learned about VCS timing and build a 
; template that has all the important stuff done already. This template gives
; you clearly demarcated and cycle counted VSync, VBlank, HBlank, Scanline, and OverScan
; Building this template helped me learn the timing, and it gives me a good platform to start from

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

;Set up some memory locations we'll use
ScanlineCount = $80
VBlankTimer = $81
OverscanTimer = $82

Start
	;From macro.h gets memory and other stuff initialized
	CLEAN_START
	;Set background color
	lda #$AA
	sta COLUBK
	;Set some variables
	lda #192
	sta ScanlineCount
	lda #43
	sta VBlankTimer
	lda #35
	sta OverscanTimer

;Tell the TV we're starting a new frame
VSync
	;Wait until the electron hun is back to origin
	sta WSYNC
	;Enable VSYNC
	lda #%0010
	sta VSYNC
	;Wait for 3 scanlines. This is a hardware thing and we have to do it
	sta WSYNC
	sta WSYNC
	sta WSYNC
	;Disable VSYNC
	lda #0
	sta VSYNC

;We have 37 scanlines of VBlank time before we start drawing scan lines
;This is a great place to do logic. The tightest code would cycle count here
;but we use a timer instead. Not as tight, but still good. 
VBlankStart
	;Enable VBlank
	lda #%0010
	sta VBLANK
	;Set the timer
	lda VBlankTimer
	sta TIM64T

VBlankLogic
	;Do vblank stuff here

VBlankEnd
	;Check the timer
	lda INTIM
	;If its not zero loop
	bne VBlankEnd
	;Disable VBlank
	lda #0
	sta VBLANK
	;Wait until the electron gun is back at origin
	sta WSYNC
	;Load the count of scanlines into the Y register
	ldy ScanlineCount

;There's a little bit of time before each scanline where you can do some logic
;Only around 10-15 CPU cycles or so, but still something
HBlank
	;Do pre-line stuff here
	sta WSYNC

DrawScanLine
	;Do scanline stuff here

FinishScanline
	;Reduce our scanline counter
	dey
	;If we're not on scanline 0 loop back to HBlank
	bne HBlank

;We've drawn all our scanlines and now we have around 30 scanlines
;worth of overscan. Another great place for logic
OverscanStart
	;Enable VBlank
	lda #%0010
	sta VBLANK
	;Set up the timer
	lda OverscanTimer
	sta TIM64T

OverscanLogic
	;Do overscan stuff here

OverscanEnd
	;Check the timer
	lda INTIM
	;If the timer isn't 0 loop
	bne OverscanEnd
	;Disable VBlank
	lda #0
	sta VBLANK
	;Finish out any line we might have in flight
	sta WSYNC
	;Loop back to vsync
	jmp VSync
	
	org $FFFC
	.word Start
	.word Start