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56 lines
1.2 KiB
NASM
56 lines
1.2 KiB
NASM
; Create an iNES header
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.ines {"prog": 1, "char": 1, "mapper": 0, "mirror": 1}
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; Main code segment
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.org $C000
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start:
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SEI ; disable IRQs
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CLD ; disable decimal mode
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LDX #$40
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STX $4017 ; disable APU frame IRQ
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LDX #$FF
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TXS ; Set up stack
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INX ; now X = 0
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STX $2000 ; disable NMI
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STX $2001 ; disable rendering
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STX $4010 ; disable DMC IRQs
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vblankwait1: ; First wait for vblank to make sure PPU is ready
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BIT $2002
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BPL vblankwait1
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clrmem:
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LDA #$00
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STA $0000, X
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STA $0100, X
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STA $0200, X
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STA $0400, X
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STA $0500, X
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STA $0600, X
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STA $0700, X
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LDA #$FE
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STA $0300, X
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INX
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BNE clrmem
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vblankwait2: ; Second wait for vblank, PPU is ready after this
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BIT $2002
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BPL vblankwait2
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LDA #$60 ;intensify blues
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STA $2001
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forever:
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JMP forever ;jump back to Forever, infinite loop
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nmi:
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RTI
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.org $FFFA ;first of the three vectors starts here
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.dw nmi ;when an NMI happens (once per frame if enabled) the processor will jump to the label NMI:
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.dw start ;when the processor first turns on or is reset, it will jump to the label RESET:
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.dw $0 ;external interrupt IRQ is not used in this tutorial
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