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https://github.com/sehugg/8bitworkshop.git
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114 lines
2.3 KiB
NASM
114 lines
2.3 KiB
NASM
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include "nesdefs.asm"
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;;;;; VARIABLES
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seg.u RAM
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org $0
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ScrollPos byte ; used during NMI
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;;;;; NES CARTRIDGE HEADER
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NES_HEADER 0,2,1,0 ; mapper 0, 2 PRGs, 1 CHR, vertical
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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 SetPalette ; set palette colors
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jsr FillVRAM ; set PPU video RAM
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jsr WaitSync ; wait for VSYNC (and PPU warmup)
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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_SPR|MASK_BG|MASK_SPR_CLIP|MASK_BG_CLIP
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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 ; PPU addr = $2000
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ldy #$10 ; $10 (16) 256-byte pages
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.loop:
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sta PPU_DATA ; write to VRAM
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adc #7 ; add 7 to make randomish pattern
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inx
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bne .loop
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dey ; next page
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bne .loop ; out of pages?
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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 ; PPU addr = 0x3f00
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ldx #32 ; 32 palette colors
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.loop:
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lda Palette,y ; load from ROM
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sta PPU_DATA ; store to palette
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iny
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dex
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bne .loop ; loop until 32 colors stored
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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:
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; save registers
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pha ; save A
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; update scroll position (must be done after VRAM updates)
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inc ScrollPos
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lda ScrollPos
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sta PPU_SCROLL ; X scroll position
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lda #0
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sta PPU_SCROLL ; Y scroll position
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; reload registers
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pla ; reload A
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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 09091900 ;bg0
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hex 09091900 ;bg1
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hex 09091900 ;bg2
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hex 09091900 ;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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REPEAT 64
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hex 003c6666766e663c007e181818381818
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hex 007e60300c06663c003c66061c06663c
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hex 0006067f661e0e06003c6606067c607e
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hex 003c66667c60663c00181818180c667e
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hex 003c66663c66663c003c66063e66663c
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hex 01010101010101010000000000000000
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hex ff000000000000000000000000000000
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hex 01020408102040800000000000000000
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REPEND
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