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https://github.com/sehugg/8bitworkshop.git
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122 lines
2.4 KiB
NASM
122 lines
2.4 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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ScrollX byte ; used during NMI
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ScrollY 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, 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
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jsr WaitSync
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jsr ClearRAM
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jsr WaitSync ;wait for VSYNC
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jsr SetPalette ;set colors
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jsr FillVRAM ;set PPU 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 = 0
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sta PPU_SCROLL
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sta PPU_SCROLL ;scroll = 0
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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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PPU_SETADDR $2000
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ldy #$10
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ldx #0
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.loop:
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stx PPU_DATA
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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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PPU_SETADDR $3f00
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ldy #0
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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_REGS ; save registers
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; update scroll position (must be done after VRAM updates)
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jsr ReadJoypad0 ; read first controller
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pha ; push joypad bitmask
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and #$03 ; only keep first 2 bits
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tay ; A -> Y
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lda ScrollDirTab,y ; lookup table
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clc
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adc ScrollX ; A = A + ScrollX
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sta ScrollX ; -> ScrollX
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sta PPU_SCROLL ; -> first scroll byte
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pla ; pop joypad bitmask
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lsr
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lsr
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and #$03 ; take next two bits
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tay
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lda ScrollDirTab,y ; do another lookup
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clc
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adc ScrollY ; A = A + ScrollY
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sta ScrollY ; -> ScrollY
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sta PPU_SCROLL ; -> second scroll byte
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RESTORE_REGS ; restore registers
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rti
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; Scroll direction lookup table
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ScrollDirTab:
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hex 00 01 ff 00 ; 0,1,-1,0
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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 09090000 ;bg0
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hex 09090c00 ;bg1
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hex 09091c00 ;bg2
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hex 09092c00 ;bg3
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hex 14243400 ;sp0
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hex 15253500 ;sp1
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hex 16263600 ;sp2
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hex 17273700 ;sp3
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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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