background.asm

.segment "HEADER"
	
	.byte	"NES", $1A	; iNES header identifier
	.byte	2		; 2x 16KB PRG code
	.byte	1		; 1x  8KB CHR data
	.byte	0, 0		; mapper 0, vertical mirroring

;;;;;;;;;;;;;;;

;;; "nes" linker config requires a STARTUP section, even if it's empty
.segment "STARTUP"

.segment "CODE"

reset:
	sei			; disable IRQs
	cld			; disable decimal mode
	ldx	#$40
	stx	$4017		; dsiable APU frame IRQ
	ldx	#$ff		; Set up stack
	txs			;  +
	inx			; now X = 0
	stx	$2000		; disable NMI
	stx	$2001		; disable rendering
	stx	$4010		; disable DMC IRQs

@vblankwait1:			; First wait for vlbank to make sure PPU is ready
	bit	$2002
	bpl	@vblankwait1

@clrmem:
	lda	#$00
	sta	$0000, x
	sta	$0100, x
	sta	$0200, x
	sta	$0400, x
	sta	$0500, x
	sta	$0600, x
	sta	$0700, x
	lda	#$fe
	sta	$0300, x
	inx
	bne	@clrmem

@vblankwait2:			; Second wait for vblank, PPU is ready after this
	bit	$2002
	bpl	@vblankwait2

	
.if 0 ; Enable this to work on Nestopia
	lda	#$3f		; Set PPU address to BG palette RAM ($3F00)
	sta	$2006
	lda	#$00
	sta	$2006

	lda	#$00
	sta	$2007
	sta	$2007
	
@vblankwait3:
	bit	$2002
	bpl	@vblankwait3
.endif
	
	lda	#%10000000	; intensify blues
	sta	$2001

@forever:
	jmp	@forever

nmi:
	rti
 
;;;;;;;;;;;;;;  
  
.segment "VECTORS"

	.word	0, 0, 0		; Unused
	;; When an NMI happens (once per frame if enabled) the label nmi:
	.word	nmi
	;; When the processor first turns on or is reset, it will jump to the
	;; label reset:
	.word	reset
	;; External interrupt IRQ is not used in this tutorial 
	.word	0
  
;;;;;;;;;;;;;;  
  
.segment "CHARS"

	.incbin	"mario.chr"	; includes 8KB graphics from SMB1