mirror of
https://github.com/sehugg/8bitworkshop.git
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142 lines
2.2 KiB
NASM
142 lines
2.2 KiB
NASM
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include "nesdefs.asm"
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;;;;; ZERO-PAGE VARIABLES
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seg.u ZEROPAGE
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org $0
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;;;;; OTHER VARIABLES
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seg.u RAM
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org $300
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LineXLo ds 224
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LineXHi ds 224
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;;;;; NES CARTRIDGE HEADER
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NES_HEADER 0,2,1,0 ; mapper 0, 2 PRGs, 1 CHR, horiz. mirror
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;;;;; START OF CODE
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Start:
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NES_INIT ; set up stack pointer, turn off PPU
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jsr WaitSync ; wait for VSYNC
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jsr ClearRAM ; clear RAM
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jsr WaitSync ; wait for VSYNC (and PPU warmup)
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jsr SetPalette ; set palette colors
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jsr FillVRAM ; set PPU video RAM
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lda #0
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sta PPU_ADDR
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sta PPU_ADDR ; PPU addr = $0000
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sta PPU_SCROLL
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sta PPU_SCROLL ; scroll = $0000
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lda #CTRL_NMI
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sta PPU_CTRL ; enable NMI
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lda #MASK_BG|MASK_SPR
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sta PPU_MASK ; enable rendering
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.endless
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jmp .endless ; endless loop
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; fill video RAM
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FillVRAM: subroutine
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txa
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ldy #$20
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sty PPU_ADDR
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sta PPU_ADDR
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ldy #$10
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.loop:
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sta PPU_DATA
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adc #7
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inx
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bne .loop
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dey
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bne .loop
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rts
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; set palette colors
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SetPalette: subroutine
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ldy #$00
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lda #$3f
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sta PPU_ADDR
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sty PPU_ADDR
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ldx #32
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.loop:
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lda Palette,y
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sta PPU_DATA
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iny
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dex
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bne .loop
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rts
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; set sprite 0
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SetSprite0: subroutine
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sta $200 ;y
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lda #1 ;code
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sta $201
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lda #0 ;flags
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sta $202
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lda #8 ;xpos
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sta $203
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rts
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;;;;; COMMON SUBROUTINES
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include "nesppu.asm"
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;;;;; INTERRUPT HANDLERS
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NMIHandler: subroutine
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SAVE_REGS
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lda #112
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jsr SetSprite0
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; load sprites
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lda #$02
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sta PPU_OAM_DMA
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; wait for sprite 0
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.wait0 bit PPU_STATUS
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bvs .wait0
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.wait1 bit PPU_STATUS
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bvc .wait1
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ldy #0
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.loop
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; alter horiz. scroll position for each scanline
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tya
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sec
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adc LineXLo,y
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sta LineXLo,y
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lda LineXHi,y
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adc #0
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sta LineXHi,y
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sta PPU_SCROLL ; horiz byte
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lda #0
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sta PPU_SCROLL ; vert byte
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; pad out rest of scanline
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SLEEP 72
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iny
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cpy #224
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bne .loop
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RESTORE_REGS
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rti
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;;;;; CONSTANT DATA
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align $100
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Palette:
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hex 1f ;background
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hex 09092c00 ;bg0
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hex 09091900 ;bg1
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hex 09091500 ;bg2
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hex 09092500 ;bg3
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;;;;; CPU VECTORS
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NES_VECTORS
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;;;;; TILE SETS
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org $10000
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incbin "jroatch.chr"
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