477 lines
12 KiB
Plaintext
477 lines
12 KiB
Plaintext
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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seg.u Variables
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org $80
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; In this example, we're going to tackle collision detection,
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; which is one thing in the VCS that's easier than expected.
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; The TIA has 15 different collision flags that can detect a
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; collision between any of the 2 players, 2 missiles, ball,
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; and playfield. You can check these flags at any time (at the
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; end of the frame is pretty common). When you're done checking
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; you clear them all at once by writing to CXCLR.
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; For this example we'll use the ball object, and detect collisions
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; between it and the playfield and the player. We only know
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; the Y position of the ball and player (the X position is in
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; the TIA chip) so we'll base our bounce decisions on the Y position
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; of the ball (for playfield bounces) or the relative Y position of
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; ball and player (for player bounces).
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; Note: You can press the button to capture the ball.
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; We're going to also include sound, which is generated by writing
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; a volume register, a frequency register, and a mode register for
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; one of two channels.
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counter byte ; increments each frame
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yplyr byte ; player y pos
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yball byte ; ball y pos
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animofs byte ; sprite data offset, used for animation
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ysprofs byte ; temp sprite offset
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yballvel byte ; ball Y velocity
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xballvel byte ; ball X velocity
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xballerr byte ; ball X fractional error
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captured byte ; ball capture flag
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avol0 byte ; shadow register for AVOL0
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; Color constants
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BGCOLOR equ $80
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PLCOLOR equ $6c
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GNDCOLOR equ $c0
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Enable ball if it is on this scanline (in X register)
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; Modifies A.
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MAC DRAW_BALL
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lda #%00000000
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cpx yball
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bne .noball
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lda #%00000010 ; for ENAM0 the 2nd bit is enable
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.noball
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sta ENABL ; enable ball
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ENDM
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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seg Code
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org $f000
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; Initialize and set initial offsets of objects.
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Start CLEAN_START
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lda #185-SpriteHeight
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sta yplyr ; player Y position, top to bottom
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; Set player 0 horizontal position
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lda #80
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ldx #0
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jsr SetHorizPos2
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; Set ball horizontal position
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lda #84
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ldx #4
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jsr SetHorizPos2
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sta WSYNC
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sta HMOVE
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; Set ball initial velocity
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lda #1
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sta yballvel
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lda #$30
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sta xballvel
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; Next frame loop
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NextFrame
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VERTICAL_SYNC
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; in case the ball is on screen
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lda ColorFrame0 ; load 1st entry of color data
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sta COLUP0 ; set sprite 0 color
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; Set up playfield
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lda #BGCOLOR ; set the background color
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sta COLUBK
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lda #PLCOLOR ; set the playfield color
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sta COLUPF
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lda #%00010101 ; playfield reflection and ball size/priority
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sta CTRLPF
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lda #0 ; blank out the playfield
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sta PF0
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sta PF1
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sta PF2
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; 37 lines of VBLANK
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ldx #37
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Underscan
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sta WSYNC
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dex
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bne Underscan
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; Draw 192 scanlines
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; First, 20 lines at top of screen
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ldx #0
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ScanLoop1
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sta WSYNC
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stx PF1
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inx
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cpx #20
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bne ScanLoop1
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; Top border of screen (8 lines)
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ScanLoop2
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; Fetch 1st part of bitmap data from table (we start on 20th line)
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lda #TopBorder0-20,x
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sta WSYNC
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sta PF0
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DRAW_BALL ; draw the ball on this line?
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; (only for collision purposes)
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; Fetch 2nd and 3rd parts of bitmap
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lda #TopBorder1-20,x
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sta PF1
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lda #TopBorder2-20,x
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sta PF2
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inx
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cpx #28
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bne ScanLoop2
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; Top half of screen (100 pixels)
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lda #0
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sta PF0
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sta PF1
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sta PF2
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ScanLoop3
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sta WSYNC
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DRAW_BALL ; draw the ball on this line?
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inx
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cpx #128
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bne ScanLoop3
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; Bottom half of screen with sprite
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; Setup 'ysprofs' which is the calculated offset into
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; sprite lookup tables (it can exceed bounds, we'll test)
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; Since the sprite table is reversed, the starting offset is
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; Yplyr - Ystart - SpriteHeight
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lda yplyr
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sec
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sbc #128-SpriteHeight
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sta ysprofs
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ScanLoop4
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; Is this scanline within sprite bounds?
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dec ysprofs
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lda ysprofs
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cmp #SpriteHeight ; sprite is 16 pixels high + padding
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bcc InSprite
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lda #0 ; no sprite, draw the padding
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InSprite
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tay
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lda ColorFrame0,y ; load color data
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pha ; push color data onto stack
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tya
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clc ; clear carry flag
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adc animofs ; add animation offset (not for color though)
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tay
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lda Frame0,y ; load bitmap data
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sta WSYNC ; wait for next scanline (as late as possible!)
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sta GRP0 ; set sprite 0 pixels
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pla ; pull bitmap data from stack
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sta COLUP0 ; set sprite 0 color
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DRAW_BALL ; draw the ball on this line?
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inx
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cpx #184
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bne ScanLoop4 ; repeat next scanline until finished
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; 8 more pixels for bottom border
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lda #GNDCOLOR ; set the background color
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sta COLUPF
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ScanLoop5
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lda #$ff
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sta WSYNC
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sta PF0
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sta PF1
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sta PF2
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lda #0
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sta GRP0
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inx
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cpx #192
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bne ScanLoop5
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; Disable ball
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lda #0
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sta ENABL
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; 30 lines of overscan needed, but we have lots of logic to do.
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; So we're going to use the PIA timer to let us know when
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; almost 30 lines of overscan have passed.
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; This handy macro does a WSYNC and then sets up the timer.
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TIMER_SETUP 30
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; Check for collisions
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lda #%01000000
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bit CXP0FB ; collision between player 0 and ball?
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bne PlayerCollision
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lda #%10000000
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bit CXBLPF ; collision between playfield and ball?
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bne PlayfieldCollision
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beq NoCollision
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; Now we bounce the ball depending on where it is
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PlayerCollision
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; Is the button pressed? if so, just capture the ball
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lda INPT4 ;read button input
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bmi ButtonNotPressed ;skip if button not pressed
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inc captured ; set capture flag
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lda #SpriteHeight
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sta animofs ; use different bitmap when captured
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bne NoCollision
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ButtonNotPressed
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lda #0
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sta captured ; clear capture flag
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sta animofs ; use regular bitmap
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; See if we bounce off of top half or bottom half of player
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; (yplyr + height/2 - yball)
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ldx #1
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lda yplyr
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clc
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adc #SpriteHeight/2
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sec
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sbc yball
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bmi StoreVel ; bottom half, bounce down
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ldx #$ff ; top half, bounce up
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bne StoreVel
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PlayfieldCollision
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; If bouncing off top of playfield, bounce down
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ldx #1
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lda yball
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bpl StoreVel
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; Otherwise bounce up
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ldx #$ff
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StoreVel
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; Store final velocity
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stx yballvel
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; Make a little sound
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txa
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adc #45
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sta AUDF0 ; frequency
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lda #6
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sta avol0 ; shadow register for volume
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NoCollision
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; Clear collision registers for next frame
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sta CXCLR
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; Ball captured? if so, no motion
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lda captured
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bne ballCaptured
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; Move ball vertically
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lda yball
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clc
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adc yballvel
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sta yball
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; Move ball horizontally
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; We use an fractional counter for the ball, and we have to
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; set a different HMOVE value depending on if it's left or right
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lda xballvel
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bmi ballMoveLeft
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clc
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adc xballerr
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sta xballerr
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bcc ballCaptured
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lda #$f0
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sta HMBL
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bne ballCaptured
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ballMoveLeft
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sec
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sbc xballerr
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sta xballerr
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bcs ballCaptured
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lda #$10
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sta HMBL
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ballCaptured
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sta WSYNC
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sta HMOVE ; apply the move(s)
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sta HMCLR
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; Joystick movement
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; For up and down, we INC or DEC the Y Position
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lda #%00010000 ;Up?
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bit SWCHA
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bne SkipMoveUp
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ldx yplyr
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cpx #129
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bcc SkipMoveUp
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dex
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stx yplyr
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lda captured ; captured? move the ball too
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beq SkipMoveUp
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dec yball
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SkipMoveUp
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lda #%00100000 ;Down?
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bit SWCHA
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bne SkipMoveDown
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ldx yplyr
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cpx #185-SpriteHeight
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bcs SkipMoveDown
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inx
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stx yplyr
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lda captured ; captured? move the ball too
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beq SkipMoveDown
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inc yball
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SkipMoveDown
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; Note that the horizontal position is not contained in RAM,
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; but inaccessibly inside the TIA's registers! Some games can
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; get away with this if they use the collision registers.
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ldx #0 ; assume speed is 0 if no movement
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lda #%01000000 ;Left?
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bit SWCHA
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bne SkipMoveLeft
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ldx #$10 ;a 1 in the left nibble means go left
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SkipMoveLeft
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lda #%10000000 ;Right?
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bit SWCHA
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bne SkipMoveRight
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ldx #$F0 ;a -1 in the left nibble means go right...
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SkipMoveRight
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stx HMP0 ; set the move for player 0
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lda captured ; captured? move the ball too
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beq NoCaptureMove
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stx HMBL ; set ball move register
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NoCaptureMove
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sta WSYNC
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sta HMOVE ; apply the move(s)
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; Play audio from shadow register
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ldx avol0
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beq NoAudio
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dex ; decrement volume every frame
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stx AUDV0 ; store in volume hardware register
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stx avol0 ; store in shadow register
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lda #3
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sta AUDC0 ; shift counter mode 3 for weird bounce sound
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NoAudio
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; Wait until our timer expires and then WSYNC, so then we'll have
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; passed 30 scanlines. This handy macro does this.
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TIMER_WAIT
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; Goto next frame
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jmp NextFrame
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; SetHorizPos2 - Sets the horizontal position of an object.
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; The X register contains the index of the desired object:
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; X=0: player 0
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; X=1: player 1
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; X=2: missile 0
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; X=3: missile 1
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; X=4: ball
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; This routine does a WSYNC both before and after, followed by
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; a HMOVE and HMCLR. So it takes two scanlines to complete.
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SetHorizPos2
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sta WSYNC ; start a new line
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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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sta WSYNC
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sta HMOVE ; apply the previous fine position(s)
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sta HMCLR ; reset the old horizontal position(s)
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rts ; return to caller
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; Height of our sprite in lines
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SpriteHeight equ 17
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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;$F4
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; Bitmap data "throwing" position
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Frame1
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.byte #0
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.byte #%01101100;$F6
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.byte #%01000100;$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 #%10111101;$C2
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.byte #%01111101;$C2
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.byte #%00111001;$C2
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.byte #%00111000;$16
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.byte #%01101100;$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 ; ball color if not sharing line with player sprite
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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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; Playfield top border bitmap
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TopBorder0
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.byte #%11111111
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.byte #%11111111
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.byte #%11111111
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.byte #%11111111
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.byte #%11110000
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.byte #%11000000
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.byte #%10000000
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.byte #%00000000
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TopBorder1
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.byte #%11111111
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.byte #%11111111
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.byte #%11111111
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.byte #%11111111
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.byte #%11111000
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.byte #%11100000
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.byte #%11000000
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.byte #%10000000
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TopBorder2
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.byte #%11111111
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.byte #%11111111
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.byte #%11111111
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.byte #%00000000
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.byte #%00000000
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.byte #%00000000
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.byte #%00000000
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.byte #%00000000
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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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