mirror of
https://github.com/sehugg/8bitworkshop.git
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298 lines
6.2 KiB
Plaintext
298 lines
6.2 KiB
Plaintext
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processor 6502
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include "vcs.h"
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include "macro.h"
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include "xmacro.h"
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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;
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; This example demonstrates a scene with a full-screen
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; playfield, and two sprites overlapping it. This takes more
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; CPU time, so our kernel operates in 4-line chunks.
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;
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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seg.u Variables
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org $80
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PFPtr word ; pointer to playfield data
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PFIndex byte ; offset into playfield array
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PFCount byte ; lines left in this playfield segment
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YPos0 byte ; Y position of player 0 sprite
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XPos0 byte ; X position of player 0 sprite
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YPos1 byte ; Y position of player 1 sprite
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XPos1 byte ; X position of player 1 sprite
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; pointers to bitmap and color tables
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SpritePtr0 word
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ColorPtr0 word
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SpritePtr1 word
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ColorPtr1 word
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; temporary values for kernel
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YP0 byte ; Y counter for player 0
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YP1 byte ; Y counter for player 1
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Temp byte
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Bitp0 byte
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Bitp1 byte
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Colp0 byte
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Colp1 byte
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tmpPF0 byte
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tmpPF1 byte
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tmpPF2 byte
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SpriteHeight equ 16
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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seg Code
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org $f000
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Start
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CLEAN_START
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Data0
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; set up pointers to playfield and sprites
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lda #<Data0
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sta PFPtr
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lda #>Data0
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sta PFPtr+1
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lda #<Frame0
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sta SpritePtr0
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lda #>Frame0
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sta SpritePtr0+1
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lda #<ColorFrame0
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sta ColorPtr0
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lda #>ColorFrame0
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sta ColorPtr0+1
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lda #<Frame0
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sta SpritePtr1
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lda #>Frame0
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sta SpritePtr1+1
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lda #<ColorFrame0
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sta ColorPtr1
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lda #>ColorFrame0
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sta ColorPtr1+1
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; set up initial positions
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lda #242
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sta YPos0
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lda #200
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sta YPos1
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lda #38
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sta XPos0
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sta XPos1
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lda #1
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sta VDELP0 ; updates to GRP0 will be delayed
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NextFrame
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VERTICAL_SYNC
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; Set up VBLANK timer
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TIMER_SETUP 37
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lda #$88
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sta COLUBK ; bg color
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lda #$5b
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sta COLUPF ; fg color
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lda #$68
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sta COLUP0 ; player color
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lda #1
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sta CTRLPF ; symmetry
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lda #72
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sta PFIndex ; reset playfield offset
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; Set temporary Y counter and set horizontal position
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lda YPos0
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sta YP0 ; yp0 = temporary counter
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lda YPos1
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sta YP1 ; yp0 = temporary counter
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lda XPos0
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ldx #0
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jsr SetHorizPos
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lda XPos1
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ldx #1
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jsr SetHorizPos
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sta WSYNC
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sta HMOVE ; gotta apply HMOVE
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; Wait for end of VBLANK
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TIMER_WAIT
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lda #0
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sta VBLANK
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SLEEP 31 ; so timing analysis works out
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KernelLoop
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; Phase 0: Fetch PF0 byte
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jsr DrawSprites
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ldy PFIndex ; no more playfield?
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beq NoMoreLines ; exit loop
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dey
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lda (PFPtr),y ; load PF0
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sty PFIndex
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sta tmpPF0
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; Phase 1: Fetch PF1 byte
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jsr DrawSprites
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ldy PFIndex
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dey
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lda (PFPtr),y ; load PF1
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sty PFIndex
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sta tmpPF1
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; Phase 2: Fetch PF2 byte
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jsr DrawSprites
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ldy PFIndex
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dey
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lda (PFPtr),y ; load PF2
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sty PFIndex
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sta tmpPF2
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; Phase 3: Write PF0/PF1/PF2 registers
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jsr DrawSprites
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lda tmpPF0
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sta PF0
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lda tmpPF1
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sta PF1
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lda tmpPF2
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sta PF2
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; Go to next scanline
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jmp KernelLoop
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NoMoreLines
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; Set up overscan timer
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TIMER_SETUP 27
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lda #2
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sta VBLANK
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jsr MoveJoystick
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TIMER_WAIT
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jmp NextFrame
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; DrawSprite subroutine called by kernel
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DrawSprites
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; Fetch sprite 0 values
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lda #SpriteHeight ; height in 2xlines
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sec
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isb YP0 ; INC yp0, then SBC yp0
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bcs DoDraw0 ; inside bounds?
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lda #0 ; no, load the padding offset (0)
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DoDraw0
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tay ; -> Y
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lda (ColorPtr0),y ; color for both lines
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sta Colp0 ; -> colp0
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lda (SpritePtr0),y ; bitmap for first line
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sta GRP0 ; -> [GRP0] (delayed due to VDEL)
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; Fetch sprite 1 values
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lda #SpriteHeight ; height in 2xlines
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sec
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isb YP1 ; INC yp0, then SBC yp0
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bcs DoDraw1 ; inside bounds?
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lda #0 ; no, load the padding offset (0)
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DoDraw1
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tay ; -> Y
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lda (ColorPtr1),y ; color for both lines
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tax
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lda (SpritePtr1),y ; bitmap for first line
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tay
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; WSYNC and store sprite values
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lda Colp0
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sta WSYNC
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sty GRP1 ; GRP0 is also updated due to VDELP0 flag
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stx COLUP1
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sta COLUP0
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; Return to caller
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rts
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SetHorizPos
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sta WSYNC ; start a new line
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bit 0 ; waste 3 cycles
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sec ; set carry flag
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DivideLoop
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sbc #15 ; subtract 15
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bcs DivideLoop ; branch until negative
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eor #7 ; calculate fine offset
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asl
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asl
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asl
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asl
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sta RESP0,x ; fix coarse position
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sta HMP0,x ; set fine offset
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rts ; return to caller
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; Read joystick movement and apply to object 0
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MoveJoystick
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; Move vertically
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; (up and down are actually reversed since ypos starts at bottom)
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ldx YPos0
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lda #%00100000 ;Up?
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bit SWCHA
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bne SkipMoveUp
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cpx #175
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bcc SkipMoveUp
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dex
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SkipMoveUp
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lda #%00010000 ;Down?
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bit SWCHA
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bne SkipMoveDown
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cpx #254
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bcs SkipMoveDown
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inx
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SkipMoveDown
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stx YPos0
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; Move horizontally
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ldx XPos0
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lda #%01000000 ;Left?
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bit SWCHA
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bne SkipMoveLeft
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cpx #1
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bcc SkipMoveLeft
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dex
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SkipMoveLeft
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lda #%10000000 ;Right?
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bit SWCHA
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bne SkipMoveRight
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cpx #153
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bcs SkipMoveRight
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inx
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SkipMoveRight
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stx XPos0
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rts
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; Bitmap data "standing" position
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Frame0
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.byte #0
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.byte #%01101100;$F6
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.byte #%00101000;$86
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.byte #%00101000;$86
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.byte #%00111000;$86
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.byte #%10111010;$C2
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.byte #%10111010;$C2
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.byte #%01111100;$C2
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.byte #%00111000;$C2
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.byte #%00111000;$16
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.byte #%01000100;$16
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.byte #%01111100;$16
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.byte #%01111100;$18
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.byte #%01010100;$18
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.byte #%01111100;$18
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.byte #%11111110;$F2
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.byte #%00111000;$F4
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; Color data for each line of sprite
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ColorFrame0
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.byte #$FF;
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.byte #$F6;
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.byte #$86;
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.byte #$86;
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.byte #$86;
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.byte #$C2;
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.byte #$C2;
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.byte #$C2;
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.byte #$C2;
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.byte #$16;
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.byte #$16;
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.byte #$16;
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.byte #$18;
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.byte #$18;
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.byte #$18;
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.byte #$F2;
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.byte #$F4;
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; Epilogue
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org $fffc
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.word Start
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.word Start
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