mirror of
https://github.com/sehugg/8bitworkshop.git
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103 lines
1.9 KiB
NASM
103 lines
1.9 KiB
NASM
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;;;;; CONSTANTS
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PPU_CTRL equ $2000
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PPU_MASK equ $2001
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PPU_STATUS equ $2002
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PPU_SCROLL equ $2005
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PPU_ADDR equ $2006
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PPU_DATA equ $2007
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DMC_FREQ equ $4010
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;;;;; CARTRIDGE FILE HEADER
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processor 6502
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seg Header
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org $7FF0
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NES_MAPPER equ 0 ;mapper number
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NES_PRG_BANKS equ 2 ;number of 16K PRG banks, change to 2 for NROM256
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NES_CHR_BANKS equ 1 ;number of 8K CHR banks (0 = RAM)
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NES_MIRRORING equ 1 ;0 horizontal, 1 vertical, 8 four screen
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.byte $4e,$45,$53,$1a ; header
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.byte NES_PRG_BANKS
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.byte NES_CHR_BANKS
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.byte NES_MIRRORING|(NES_MAPPER<<4)
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.byte NES_MAPPER&$f0
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;;;;; CODE
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seg Code
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org $8000
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start:
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_exit:
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sei ;disable IRQs
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cld ;decimal mode not supported
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ldx #$ff
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txs ;set up stack pointer
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inx ;increment X to 0
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stx PPU_MASK ;disable rendering
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stx DMC_FREQ ;disable DMC interrupts
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stx PPU_CTRL ;disable NMI interrupts
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jsr WaitSyncSafe ;wait for VSYNC
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; clear RAM -- not a subroutine because we clear the stack too
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lda #0
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tax
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.clearRAM
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sta $0,x
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sta $100,x
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; skip $200-$2FF, used for OAM display list
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sta $300,x
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sta $400,x
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sta $500,x
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sta $600,x
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sta $700,x
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inx
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bne .clearRAM
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; wait for PPU warmup
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jsr WaitSync
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; set palette background
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ldy #$0
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lda #$3f
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sta PPU_ADDR
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sty PPU_ADDR
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lda #$1c
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sta PPU_DATA
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; enable PPU rendering
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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 #$90
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sta PPU_CTRL ;enable NMI
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lda #$1e
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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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;;;;; SUBROUTINES
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; wait for VSYNC to start
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WaitSyncSafe: subroutine
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bit PPU_STATUS
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WaitSync:
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bit PPU_STATUS
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bpl WaitSync
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rts
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;;;;; INTERRUPT HANDLERS
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nmi:
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irq:
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rti
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;;;;; CPU VECTORS
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org $fffa
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.word nmi ;$fffa vblank nmi
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.word start ;$fffc reset
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.word irq ;$fffe irq / brk
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