564 lines
12 KiB
NASM
564 lines
12 KiB
NASM
.INCLUDE "header.asm"
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.INCLUDE "init.asm"
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.INCLUDE "registers.asm"
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; Assumes 16-bit X & 8-bit A.
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; Stores result to A.
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; Modifies X.
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; Updates $0018-$0019 to point at the next available random byte.
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.MACRO GetRandomByte
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ldx $18
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lda $028000, X ; $028000: beginning of ROM bank 2.
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inx
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cpx #$8000 ; This is the size of the entire ROM bank.
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bne +
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ldx #0
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+
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stx $18
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.ENDM
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.BANK 0 SLOT 0
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.ORG 0
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.SECTION "MainCode"
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; Memory layout:
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; 0000-000F: scratch space for functions.
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; 0010-0011: controller state of joypad #1.
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; 0012-0013: controller state of joypad #2.
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; 0014-0017: 32-bit counter of vblanks.
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; 0018-0019: 16-bit pointer to next random byte.
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; 0020-0021: (x, y) coordinates of player.
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; 0022-0024: RGB color values to use for background color, from [0-31].
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;
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; Sprite table buffers -- copied each frame to OAM during VBlank, using DMA.
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; 0100-02FF: table 1 (4 bytes each: x/y coord, tile #, flip/priority/palette)
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; 0300-031F: table 2 (2 bits each: high x-coord bit, size)
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Start:
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InitializeSNES
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jsr LoadPaletteAndTileData
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jsr InitializeSpriteTables
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; Set screen mode: 16x16 tiles for backgrounds, mode 1.
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lda #%11000001
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sta SCREENMODE
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; Set sprite size to 16x16 (small) and 32x32 (large).
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lda #%01100000
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sta OAMSIZE
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; Main screen: enable sprites & BG3.
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lda #%00010100
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sta MSENABLE
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; Turn on the screen.
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; Format: x000bbbb
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; x: 0 = screen on, 1 = screen off, bbbb: Brightness ($0-$F)
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lda #%00001111
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sta INIDISP
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; Enable NMI interrupt & joypad.
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; n-vh---j n: NMI interrupt enable v: vertical counter enable
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; h: horizontal counter enable j: joypad enable
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lda #%10000001
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sta NMITIMEN
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; Store zeroes to the controller status registers.
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; TODO(mcmillen): is this needed? I think the system will overwrite these
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; automatically.
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stz JOY1H
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stz JOY1L
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; Write something recognizable into our scratch space.
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jsr FillScratch
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; Start the background color as a dark blue.
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lda #4
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sta $24
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; Player's initial starting location.
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lda #(256 / 4)
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sta $20
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lda #((224 - 16) / 2)
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sta $21
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MainLoop:
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wai ; Wait for interrupt.
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jmp MainLoop
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LoadPaletteAndTileData:
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; For more details on DMA, see:
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; http://wiki.superfamicom.org/snes/show/Grog%27s+Guide+to+DMA+and+HDMA+on+the+SNES
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; http://wiki.superfamicom.org/snes/show/Making+a+Small+Game+-+Tic-Tac-Toe
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;
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; A lot of the graphics-related registers are explained in Qwertie's doc:
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; http://emu-docs.org/Super%20NES/General/snesdoc.html
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; ... but be careful, because there are some errors in this doc.
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;
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; bazz's tutorial (available from http://wiki.superfamicom.org/snes/) is
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; quite helpful with palette / sprites / DMA, especially starting at
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; http://wiki.superfamicom.org/snes/show/Working+with+VRAM+-+Loading+the+Palette
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; 16-bit X/Y registers. Used for DMA source address & transfer size, both of
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; which want 16-bit values.
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rep #%00010000
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; 8-bit A/B registers. Used for DMA source bank & destination address.
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sep #%00100000
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; Initialize the palette memory in a loop.
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; We could also do this with a DMA transfer (like we do with the tile data
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; below), but it seems overkill for just a few bytes. :)
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; TODO(mcmillen): do it with a DMA transfer.
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; First, sprite palette data:
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ldx #0
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lda #128 ; Palette entries for sprites start at 128.
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sta CGADDR
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-
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lda.l SpritePalette, X
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sta CGDATA
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inx
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cpx #32 ; 32 bytes of palette data.
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bne -
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; Now, BG3 palette data.
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; Palette entries for BG3 start at 0.
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; TODO(mcmillen): BG2 started at 32, but maybe that's only in mode 0?
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ldx #0
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lda #0
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sta CGADDR
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-
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lda.l TilePalette, X
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sta CGDATA
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inx
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cpx #8 ; 8 bytes of palette data.
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bne -
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; TODO(mcmillen): make the "DMA stuff into VRAM" a macro or function.
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; Set VMADDR to where we want the DMA to start. We'll store sprite data
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; at the beginning of VRAM.
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ldx #$0000
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stx VMADDR
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; DMA 0 source address & bank.
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ldx #SpriteData
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stx DMA0SRC
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lda #:SpriteData
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sta DMA0SRCBANK
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; DMA 0 transfer size. Equal to the size of sprites32.pic.
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ldx #2048
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stx DMA0SIZE
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; DMA 0 control register.
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; Transfer type 001 = 2 addresses, LH.
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lda #%00000001
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sta DMA0CTRL
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; DMA 0 destination.
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lda #$18 ; The upper byte is assumed to be $21, so this is $2118 & $2119.
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sta DMA0DST
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; Enable DMA channel 0.
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lda #%00000001
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sta DMAENABLE
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; Store background tile data at byte $2000 of VRAM.
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; (VMADDR is a word address, so multiply by 2 to get the byte address.)
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ldx #$1000
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stx VMADDR
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; DMA 0 source address & bank.
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ldx #TileData
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stx DMA0SRC
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lda #:TileData
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sta DMA0SRCBANK
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; DMA 0 transfer size. Equal to the size of tiles.pic.
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ldx #512
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stx DMA0SIZE
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; DMA 0 control register.
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; Transfer type 001 = 2 addresses, LH.
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lda #%00000001
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sta DMA0CTRL
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; DMA 0 destination.
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lda #$18 ; The upper byte is assumed to be $21, so this is $2118 & $2119.
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sta DMA0DST
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; Enable DMA channel 0.
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lda #%00000001
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sta DMAENABLE
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; Tell the system that the BG3 tilemap starts at $4000.
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lda #%00100000
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sta BG3TILEMAP
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; ... and that the background tile data for BG3 starts at $2000.
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lda #%00000001
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sta BG34NBA
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; Set up the BG3 tilemap.
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; VRAM write mode: increments the address every time we write a word.
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lda #%10000000
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sta VMAIN
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; Set word address for accessing VRAM.
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ldx #$2000 ; BG 3 tilemap starts here. (Byte address $4000.)
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stx VMADDR
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; Now write entries into the tile map.
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ldy #0
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-
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GetRandomByte
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sta $00
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ldx #$0000 ; This is a blank tile.
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; 1 in 8 chance that we choose a non-blank tile.
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and #%00000111
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cmp #%00000111
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bne +
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ldx #$0002
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lda $00
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and #%10000000
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cmp #%10000000
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bne +
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ldx #$8002 ; Flip vertically.
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+
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stx VMDATA
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iny
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; The tile map is 32x32 (1024 entries).
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cpy #1024
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bne -
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rts
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InitializeSpriteTables:
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; This page is a good reference on SNES sprite formats:
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; http://wiki.superfamicom.org/snes/show/SNES+Sprites
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; It uses the same approach we're using, in which we keep a buffer of the
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; sprite tables in RAM, and DMA the sprite tables to the system's OAM
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; during VBlank.
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rep #%00110000 ; 16-bit A/X/Y.
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ldx #$0000
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; Fill sprite table 1. 4 bytes per sprite, laid out as follows:
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; Byte 1: xxxxxxxx x: X coordinate
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; Byte 2: yyyyyyyy y: Y coordinate
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; Byte 3: cccccccc c: Starting tile #
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; Byte 4: vhoopppc v: vertical flip h: horizontal flip o: priority bits
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; p: palette #
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lda #$01
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-
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sta $0100, X ; We keep our sprite table at $0100 and DMA it to OAM later.
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inx
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inx
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inx
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inx
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cpx #$0200
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bne -
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; Fill sprite table 2. 2 bits per sprite, like so:
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; bits 0,2,4,6 - High bit of the sprite's x-coordinate.
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; bits 1,3,5,7 - Toggle Sprite size: 0 - small size 1 - large size
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; Setting all the high bits keeps the sprites offscreen.
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lda #%0101010101010101
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-
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sta $0100, X
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inx
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inx
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cpx #$0220
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bne -
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sep #%00100000 ; 8-bit A.
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rts
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VBlankHandler:
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jsr VBlankCounter ; DEBUG
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jsr JoypadHandler
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jsr SetBackgroundColor
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jsr UpdateGraphics
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jsr DMASpriteTables
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rti
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VBlankCounter:
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; Increment a counter of how many VBlanks we've done.
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inc $14
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lda $14
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cmp #$00
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bne VBlankCounterDone
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inc $15
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lda $15
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cmp #$00
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bne VBlankCounterDone
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inc $16
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lda $16
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cmp #$00
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bne VBlankCounterDone
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inc $17
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VBlankCounterDone:
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rts
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JoypadHandler:
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jsr JoypadDebug ; DEBUG
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JoypadUp:
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lda JOY1H
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and #$08 ; Up
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cmp #$08
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bne JoypadDown ; Button not pressed.
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lda $21
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cmp #0
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beq JoypadDown ; Value saturated.
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dec $21
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dec $21
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JoypadDown:
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lda JOY1H
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and #$04
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cmp #$04
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bne JoypadLeft ; Button not pressed.
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lda $21
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cmp #(224 - 32)
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beq JoypadLeft ; Value saturated.
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inc $21
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inc $21
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JoypadLeft:
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lda JOY1H
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and #$02 ; Left
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cmp #$02
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bne JoypadRight ; Button not pressed.
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lda $20
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cmp #0
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beq JoypadRight ; Value saturated.
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dec $20
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dec $20
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JoypadRight:
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lda JOY1H
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and #$01
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cmp #$01 ; Right
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bne JoypadB ; Button not pressed.
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lda $20
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cmp #(256 - 32)
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beq JoypadB ; Value saturated.
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inc $20
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inc $20
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JoypadB:
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lda JOY1H
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and #$80 ; B
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cmp #$80
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bne JoypadA ; Button not pressed.
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lda $22
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cmp #0
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beq JoypadA ; Value saturated.
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dec $22
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JoypadA:
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lda JOY1L
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and #$80 ; A
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cmp #$80
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bne JoypadY ; Button not pressed.
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lda $22
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cmp #31
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beq JoypadY ; Value saturated.
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inc $22
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JoypadY:
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lda JOY1H
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and #$40 ; Y
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cmp #$40
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bne JoypadX ; Button not pressed.
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lda $23
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cmp #0
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beq JoypadX ; Value saturated.
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dec $23
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JoypadX:
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lda JOY1L
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and #$40 ; X
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cmp #$40
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bne JoypadL ; Button not pressed.
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lda $23
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cmp #31
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beq JoypadL ; Value saturated.
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inc $23
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JoypadL:
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lda JOY1L
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and #$20 ; L
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cmp #$20
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bne JoypadR ; Button not pressed.
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lda $24
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cmp #0
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beq JoypadR ; Value saturated.
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dec $24
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JoypadR:
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lda JOY1L
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and #$10 ; R
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cmp #$10
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bne JoypadDone ; Button not pressed.
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lda $24
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cmp #31
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beq JoypadDone ; Value saturated.
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inc $24
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; TODO(mcmillen): have Start and Select do something too.
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JoypadDone:
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rts
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JoypadDebug:
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; Load joypad registers into RAM for easier inspection.
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lda JOY1L
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sta $10
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lda JOY1H
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sta $11
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lda JOY2L
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sta $12
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lda JOY2H
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sta $13
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rts
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SetBackgroundColor:
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; $22 $23 $24 are (R, G, B), each ranging from [0-31].
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; The palette color format is 15-bit: [0bbbbbgg][gggrrrrr]
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; Set the background color.
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; Entry 0 corresponds to the SNES background color.
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stz CGADDR
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; Compute and the low-order byte and store it in CGDATA.
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lda $23 ; Green.
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.rept 5
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asl
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.endr
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ora $22 ; Red.
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sta CGDATA
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; Compute the high-order byte and store it in CGDATA.
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lda $24 ; Blue.
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.rept 2
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asl
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.endr
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sta $00
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lda $23 ; Green.
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.rept 3
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lsr
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.endr
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ora $00
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sta CGDATA
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rts
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UpdateGraphics:
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; Copy player coords into sprite table.
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lda $0020
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sta $0100
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lda $0021
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sta $0101
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; Choose which sprite based on frame count.
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; Set priority bits so that the sprite is drawn in front.
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lda #%00110000
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sta $0103
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; Clear x-MSB so that the sprite is displayed.
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lda $0300
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and #%11111110
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ora #%00000010 ; ... and make it the large size. (32x32)
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sta $0300
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; Make the background scroll. Horizontal over time; vertical depending on
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; player's y-coordinate.
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lda $14
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sta BG3HOFS
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lda $15
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sta BG3HOFS
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lda $21
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.rept 3
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lsr
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.endr
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sta BG3VOFS
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stz BG3VOFS
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rts
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DMASpriteTables:
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rep #%00010000 ; 16-bit X/Y.
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sep #%00100000 ; 8-bit A.
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; Store at the base OAM address.
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ldx #$0000
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stx OAMADDR
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; Default DMA control; destination $2104 (OAM data register).
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stz DMA0CTRL
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lda #$04
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sta DMA0DST
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; Our sprites start at $0100 in bank 0 and are #$220 bytes long.
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ldx #$0100
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stx DMA0SRC
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stz DMA0SRCBANK
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ldx #$0220
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stx DMA0SIZE
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; Kick off the DMA transfer.
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lda #%00000001
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sta DMAENABLE
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rts
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FillScratch:
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lda #$42 ; ASCII "B"
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ldx #0
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-
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sta $00, X
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inx
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cpx #$10
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bne -
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rts
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.ENDS
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; Bank 1 is used for our graphics assets.
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.BANK 1 SLOT 0
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.ORG 0
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.SECTION "GraphicsData"
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SpriteData:
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.INCBIN "sprites32.pic"
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SpritePalette:
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.INCBIN "sprites32.clr"
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TileData:
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.INCBIN "tiles.pic"
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TilePalette:
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.INCBIN "tiles.clr"
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.ENDS
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; Fill an entire bank with random numbers.
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.SEED 1
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.BANK 2 SLOT 0
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.ORG 0
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.SECTION "RandomBytes"
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.DBRND 32 * 1024, 0, 255
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.ENDS |