8bitworkshop/presets/nes/ex2.asm

	include "nesdefs.asm"

;;;;; VARIABLES

	seg.u RAM
	org $0

ScrollPos	byte	; used during NMI
Rand		byte
Temp1		byte

SpriteBuf	equ	$200

;;;;; NES CARTRIDGE HEADER

	NES_HEADER 0,2,1,0 ; mapper 0, 2 PRGs, 1 CHR, vertical

;;;;; START OF CODE

Start:
	NES_INIT		; set up stack pointer, turn off PPU
        jsr WaitSync
        jsr WaitSync
        jsr ClearRAM
        jsr WaitSync		;wait for VSYNC
	jsr SetPalette		;set colors
        jsr FillVRAM		;set PPU RAM
        jsr WaitSync		;wait for VSYNC (and PPU warmup)
        jsr InitSprites
        lda #0
        sta PPU_ADDR
        sta PPU_ADDR		;PPU addr = 0
        sta PPU_SCROLL
        sta PPU_SCROLL		;scroll = 0
        lda #$90
        sta PPU_CTRL		;enable NMI
        lda #$1e
        sta PPU_MASK		;enable rendering
.endless
	jmp .endless		;endless loop

; fill video RAM
FillVRAM: subroutine
	txa
	ldy #$20
	sty PPU_ADDR
	sta PPU_ADDR
	ldy #$10
.loop:
	sta PPU_DATA
        adc #7
	inx
	bne .loop
	dey
	bne .loop
        rts

;
InitSprites: subroutine
	lda #1
        ldx #0
.loop
	sta SpriteBuf,x
        jsr NextRandom
        inx
        bne .loop
        rts

;
MoveSprites: subroutine
	lda #1
        ldx #0
.loop
	sta Temp1
	lda Temp1
        and #3
        clc
        adc SpriteBuf,x
	sta SpriteBuf,x
        lda Temp1
        jsr NextRandom
        inx
        bne .loop
        rts

; set palette colors

SetPalette: subroutine
        ldy #$00
	lda #$3f
	sta PPU_ADDR
	sty PPU_ADDR
	ldx #32
.loop:
	lda Palette,y
	sta PPU_DATA
        iny
	dex
	bne .loop
        rts


;;;;; COMMON SUBROUTINES

	include "nesppu.asm"

;;;;; INTERRUPT HANDLERS

NMIHandler:
; save registers
	pha	; save A
; load sprites
	lda #$02
        sta PPU_OAM_DMA
; update scroll position (must be done after VRAM updates)
	inc ScrollPos
        lda ScrollPos
        sta PPU_SCROLL
        lda #0
        sta PPU_SCROLL
; TODO: write high bits to PPUCTRL
	lda ScrollPos
        and #0
	ora #$90	; enable NMI
        sta PPU_CTRL
; move sprites
	jsr MoveSprites
; reload registers
        pla	; reload A
	rti

;;;;; CONSTANT DATA

	align $100
Palette:
	hex 1f		;background
	hex 09091900	;bg0
        hex 09091900	;bg1
        hex 09091900	;bg2
        hex 09091900	;bg3
        hex 14243400	;sp0
        hex 15253500	;sp1
        hex 16263600	;sp2
        hex 17273700	;sp3

;;;;; CPU VECTORS

	NES_VECTORS

;;;;; TILE SETS

	org $10000
	REPEAT 64
	hex 003c6666766e663c007e181818381818
        hex 007e60300c06663c003c66061c06663c
        hex 0006067f661e0e06003c6606067c607e
        hex 003c66667c60663c00181818180c667e
        hex 003c66663c66663c003c66063e66663c
        hex 01010101010101010000000000000000
        hex ff000000000000000000000000000000
        hex 01020408102040800000000000000000
	REPEND