153 lines
3.4 KiB
Plaintext
153 lines
3.4 KiB
Plaintext
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include "nesdefs.dasm"
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;;;;; VARIABLES
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seg.u ZEROPAGE
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org $0
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ScrollPos word ; used during NMI
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Rand byte ; random number
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Temp1 byte ; temporary
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; OAM sprite buffer
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SpriteBuf equ $200
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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
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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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jsr InitSprites
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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 #MASK_BG|MASK_SPR
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sta PPU_MASK ; enable rendering
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lda #CTRL_NMI
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sta PPU_CTRL ; enable NMI
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.endless
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jmp .endless ;endless loop
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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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; 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 ; 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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; initialize OAM data
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InitSprites: subroutine
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lda #1 ; A = 1
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ldx #0 ; X = 0
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.loop
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sta SpriteBuf,x ; store to OAM buffer
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jsr NextRandom ; get next random number
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inx ; X = X + 1
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bne .loop ; loop until X wraps
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rts ; return to caller
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; move sprite data
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MoveSprites: subroutine
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lda #1 ; A = 1
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ldx #0 ; X = 0
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.loop
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sta Temp1 ; save A
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and #3 ; keep lower 2 bits
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clc ; clear carry before add
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adc SpriteBuf,x ; add to sprite buffer
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sta SpriteBuf,x ; store in sprite buffer
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lda Temp1 ; restore A
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jsr NextRandom ; get next random number
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inx ; X = X + 1
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bne .loop ; loop until X wraps
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rts ; return to caller
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;;;;; COMMON SUBROUTINES
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include "nesppu.dasm"
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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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; load sprites
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lda #$02 ; page 2 ($0200)
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sta PPU_OAM_DMA ; start DMA transfer
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; move sprites
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jsr MoveSprites ; move sprites
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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 01112100 ;background 0
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hex 02122200 ;background 1
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hex 02112100 ;background 2
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hex 01122200 ;background 3
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hex 19293900 ;sprite 0
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hex 1a2a3a00 ;sprite 1
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hex 1b2b3b00 ;sprite 2
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hex 1c2c3c ;sprite 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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incbin "jroatch.chr"
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