212 lines
3.8 KiB
NASM
212 lines
3.8 KiB
NASM
.INCLUDE "header.inc"
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.INCLUDE "InitSNES.asm"
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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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; 00-0F: scratch space for functions.
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; 10-13: controller state.
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; 14-17: 32-bit counter of vblanks.
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; 20-22: [rgb] color values to use for background color.
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Start:
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InitSNES ; Initialize SNES.
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; Set the background color.
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; $2121 is the color palette selection register [CGADD].
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; Storing 0 because that's the SNES background color.
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stz $2121
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; Turn on the screen.
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; $2100: Screen display register [INIDISP]
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;
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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 $2100
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; Enable NMI interrupt & joypad.
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; Register $4200: Counter enable [NMITIMEN]
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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 #$81
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sta $4200
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; Store zeroes to the controller status registers.
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; TODO(mcmillen): is this needed?
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stz $4218
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stz $4219
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; Write something recognizable into our scratch space.
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jsr FillScratch
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MainLoop:
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wai ; Wait for interrupt.
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jmp MainLoop
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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 FillScratch ; DEBUG
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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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; $4218: Joypad #1 status [JOY1L]
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; Format: AXLR0000
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; $4219: Joypad #1 status [JOY1H]
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; Format: BYsSudlr (s=select, S=start, udlr = joypad)
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; Joypad #2 status would be $421A [JOY2L] and $421B [JOY2H].
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jsr JoypadDebug ; DEBUG
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; TODO(mcmillen): read joypad from local memory instead of registers?
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JoypadUp:
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lda $4219
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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 $20
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cmp #31
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beq JoypadDown ; Value saturated.
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inc $20
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JoypadDown:
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lda $4219
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and #$04 ; Down
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cmp #$04
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bne JoypadLeft ; Button not pressed.
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lda $20
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cmp #0
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beq JoypadLeft ; Value saturated.
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dec $20
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JoypadLeft:
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lda $4219
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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 $22
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cmp #0
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beq JoypadRight ; Value saturated.
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dec $22
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JoypadRight:
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lda $4219
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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 $22
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cmp #31
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beq JoypadB ; Value saturated.
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inc $22
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JoypadB:
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lda $4219
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and #$80 ; B
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cmp #$80
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bne JoypadY ; Button not pressed.
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lda $21
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cmp #31
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beq JoypadY ; Value saturated.
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inc $21
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JoypadY:
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lda $4219
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and #$40 ; Y
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cmp #$40
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bne JoypadDone ; Button not pressed.
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lda $21
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cmp #0
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beq JoypadDone
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dec $21
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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 $4218
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sta $10
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lda $4219
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sta $11
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lda $421A
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sta $12
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lda $421B
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sta $13
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rts
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SetBackgroundColor:
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; $20 $21 $22 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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; Compute and store the low-order byte.
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lda $21 ; Green.
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.rept 5
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asl
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.endr
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ora $20 ; Red.
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sta $2122
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; Compute and store the high-order byte.
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lda $22 ; 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 $21 ; 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 $2122
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rts
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FillScratch:
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lda #$42 ; B
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ldx #0
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FillScratchLoop:
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sta $00,X
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inx
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cpx #$10
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bne FillScratchLoop
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rts
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.ENDS
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