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8bitworkshop/presets/nes/xyscroll.dasm
2019-08-03 17:23:38 -04:00

152 lines
2.5 KiB
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include "nesdefs.dasm"
;;;;; ZERO-PAGE VARIABLES
seg.u ZEROPAGE
org $0
ScrollX ds 1
ScrollY ds 1
Temp ds 1
;;;;; OTHER VARIABLES
seg.u RAM
org $300
;;;;; NES CARTRIDGE HEADER
NES_HEADER 0,2,1,0 ; mapper 0, 2 PRGs, 1 CHR, horiz. mirror
;;;;; START OF CODE
Start:
NES_INIT ; set up stack pointer, turn off PPU
jsr WaitSync ; wait for VSYNC
jsr ClearRAM ; clear RAM
jsr WaitSync ; wait for VSYNC (and PPU warmup)
jsr SetPalette ; set palette colors
jsr FillVRAM ; set PPU video RAM
lda #0
sta PPU_ADDR
sta PPU_ADDR ; PPU addr = $0000
sta PPU_SCROLL
sta PPU_SCROLL ; scroll = $0000
lda #MASK_BG|MASK_SPR
sta PPU_MASK ; enable rendering
lda #CTRL_NMI
sta PPU_CTRL ; enable NMI
.endless
jmp .endless ; endless loop
; fill video RAM
FillVRAM: subroutine
PPU_SETADDR $2000
ldy #$10
.loop:
sta PPU_DATA
adc #7
inx
bne .loop
dey
bne .loop
rts
; set palette colors
SetPalette: subroutine
PPU_SETADDR $3f00
ldx #32
.loop:
lda Palette,y
sta PPU_DATA
iny
dex
bne .loop
rts
; set sprite 0
SetSprite0: subroutine
sta $200 ;y
lda #$01 ;code
sta $201
lda #$20 ;flags
sta $202
lda #$fe ;xpos
sta $203
rts
;;;;; COMMON SUBROUTINES
include "nesppu.dasm"
;;;;; INTERRUPT HANDLERS
NMIHandler: subroutine
SAVE_REGS
lda #0
sta PPU_SCROLL
sta PPU_SCROLL
lda #111
jsr SetSprite0
; load sprites
lda #$02
sta PPU_OAM_DMA
; set XY scroll
; compute second PPU_ADDR write
lda ScrollX
tax ; ScrollX -> X
lsr
lsr
lsr
sta Temp
lda ScrollY
tay ; ScrollY -> Y
and #$38
asl
asl
ora Temp ; (ScrollX >> 3) | ((ScrollY & 0x38) << 2)
pha ; push onto stack
; wait for sprite 0
.wait0 bit PPU_STATUS
bvs .wait0
.wait1 bit PPU_STATUS
bvc .wait1
; set PPU_ADDR.1
lda #0
sta PPU_ADDR
; set PPU_SCROLL.1
sty PPU_SCROLL
; set PPU_SCROLL.2
stx PPU_SCROLL
; set PPU_ADDR.2
pla
sta PPU_ADDR
; modify scroll positions
inc ScrollX
inc ScrollY
; restore registers and return
RESTORE_REGS
rti
;;;;; CONSTANT DATA
align $100
Palette:
hex 1f ;background
hex 09092c00 ;bg0
hex 09091900 ;bg1
hex 09091500 ;bg2
hex 09092500 ;bg3
;;;;; CPU VECTORS
NES_VECTORS
;;;;; TILE SETS
org $10000
incbin "jroatch.chr"
incbin "jroatch.chr"