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https://github.com/sehugg/8bitworkshop.git
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110 lines
2.3 KiB
NASM
110 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 word ; used during NMI
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;;;;; NES CARTRIDGE HEADER
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NES_HEADER 0,2,1,1 ; 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
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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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PPU_SETADDR $2000
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ldy #$10 ; set $10 pages ($1000 bytes)
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.loop:
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tya ; Y -> A
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ora #$40 ; A = A OR $40
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sta PPU_DATA ; X -> PPU data port
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inx ; X = X + 1
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bne .loop ; repeat until 256 bytes
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dey ; Y = Y - 1
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bne .loop ; repeat until Y is 0
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rts ; return to caller
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; set palette colors
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SetPalette: subroutine
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PPU_SETADDR $3f00
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.loop:
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lda Palette,y ; lookup byte in ROM
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sta PPU_DATA ; store byte to PPU data
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iny ; Y = Y + 1
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cpy #32 ; is Y equal to 32?
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bne .loop ; not yet, loop
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rts ; return to caller
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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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SAVE_REGS
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; update scroll position (must be done after VRAM updates)
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inc ScrollPos ; increment low byte
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bne .noinc ; Z flag set if wrapped to 0
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inc ScrollPos+1 ; increment high byte
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.noinc
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; store X and Y scroll position
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lda ScrollPos ; A -> low byte
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sta PPU_SCROLL ; set horiz scroll
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lda #0 ; A -> zero
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sta PPU_SCROLL ; set vert scroll
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; store 8th bit into name table selector
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; name table A or B ($2000 or $2400)
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lda ScrollPos+1 ; load high byte
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and #1 ; select its low bit
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ora #CTRL_NMI ; set rest of bits
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sta PPU_CTRL
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; restore registers, return from interrupt
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RESTORE_REGS
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rti
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;;;;; CONSTANT DATA
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Palette:
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hex 1f ;screen color
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hex 09092c00 ;background 0
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hex 09091900 ;background 1
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hex 09091500 ;background 2
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hex 09092500 ;background 3
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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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