738 lines
20 KiB
Plaintext
738 lines
20 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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; We've got collisions working, but now we'd like some more
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; interaction. We can make a little "breakout" style game
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; where the ball knocks out rows of bricks. We'll need to
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; draw several rows of bricks, any or all of which might be
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; missing.
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; We'll use a technique called "asychronous playfields".
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; Remember that the playfield is either symmetric (20 pixels
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; followed by the same 20 pixels reversed) or repeated (20 pixels
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; repeated twice). But if we change the playfield registers
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; *during* the scanline, we can make the second half a
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; different bitmap than the first half.
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; We're going to move away from the HMPx/HMOVE method of
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; setting object position and use the SetHorizPos method, since
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; we really need to know the X position of both player and ball
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; at all times. The way the subroutine is written, this takes
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; two scanlines per object. But we do it during the overscan
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; period at the end of the frame, and we've got the cycles
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; to spare.
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; Also, we're going to keep score and have a rudimentary
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; scoreboard, which makes this sort of an actual game!
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; Fun fact: Messing with the HMOVE register causes a "comb"
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; effect on the left 8 pixels of the screen, which can be seen
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; at the bottom of the screen in the overscan region.
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seg.u Variables
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org $80
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XPlyr byte ; player x pos
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YPlyr byte ; player y pos
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XBall byte ; ball x pos
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YBall byte ; ball y pos
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SpritePtr word ; sprite pointer
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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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Score byte ; current BCD-encoded score
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Temp byte ; temporary storage
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Bricks ds 36 ; brick bitmap (6x6 bytes)
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ScoreHeight equ 20 ; height of top scoreboard
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BrickYStart equ 32 ; starting Y coordinate of bricks
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BrickHeight equ 16 ; height of each brick in pixels
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NBrickRows equ 6 ; number of lines of bricks
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NBL equ NBrickRows ; abbreviation for number of brick rows
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BytesPerRow equ 6 ; number of bytes for each row of bricks
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BricksPerRow equ 40 ; number of bricks in each row
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; (two bytes have only 4 active pixels)
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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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; Takes 13 cycles if ball is present, 12 if absent.
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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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; Set player 0 vertical position
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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 #70
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sta XPlyr
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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 #0
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sta XBall
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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 #129
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sta YBall
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lda #$40
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sta XBallVel
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; Set up initial bricks
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ldx #0
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lda #$ff
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SetupBricks
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;txa ; uncomment for a sparse brick pattern
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sta Bricks,x
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inx
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cpx #BytesPerRow*NBrickRows
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bne SetupBricks
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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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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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TIMER_SETUP 37
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; Set up sprite pointer depending on button press
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lda #<Frame0
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sta SpritePtr
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lda #>Frame0
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sta SpritePtr+1 ; normal sprite bitmap
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lda INPT4 ;read button input
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bmi ButtonNotPressed2 ;skip if button not pressed
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lda #<Frame1
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sta SpritePtr
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lda #>Frame1
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sta SpritePtr+1 ; alternate sprite bitmap
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ButtonNotPressed2
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TIMER_WAIT
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; Draw 192 scanlines.
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ldx #0 ; X will contain the frame Y coordinate
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; First, we'll draw the scoreboard.
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; Put the playfield into score mode (bit 2) which gives
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; two different colors for the left/right side of
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; the playfield (given by COLUP0 and COLUP1).
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lda #%00010010 ; score mode + 2 pixel ball
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sta CTRLPF
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lda #$48
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sta COLUP0 ; set color for left
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lda #$a8
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sta COLUP1 ; set color for right
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; We need to extract each digit of the BCD-coded score
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; (there are two digits, each 4 bits) and find the appropriate
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; entry in the DigitsBitmap bitmap table.
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; We'll just draw one digit to keep it simple.
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ScanLoop1a
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clc ; clear carry flag
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; Digits are 5 pixels high, so we need to multiply each
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; digit by 5 to find our digit in the bitmap table.
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lda Score ; grab the BCD score
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and #$0F ; mask out the least significant digit
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sta Temp
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asl
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asl
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adc Temp
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sta Temp ; tmp = score * 5
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; Now we divide our current Y coordinate by 2
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; to get the index into the digit bitmap.
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txa
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ror ; A = Ycoord / 2
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adc Temp ; A += tmp
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tay
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lda DigitsBitmap,y ; A = DigitsBitmap[offset]
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and #$0F ; mask out the rightmost digit
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sta WSYNC
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sta PF1 ; store digit to playfield 1 register
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DRAW_BALL ; draw the ball on this line?
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; (only for collision purposes)
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inx
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cpx #10 ; digits are 5 pixels high * 2 lines per pixel
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bcc ScanLoop1a
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; Clear the playfield
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lda #0
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sta PF1
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; Turn playfield reflection off, since our brick field
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; will be drawn asymetrically (and turn score mode off)
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lda #%00010100 ; no reflection + ball priority + 2 pixel ball
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sta CTRLPF
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; Continue until the bricks start on line 32.
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ScanLoop1b
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sta WSYNC
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DRAW_BALL ; draw the ball on this line?
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; (only for collision purposes)
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inx
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cpx #BrickYStart
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bne ScanLoop1b
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; Next we'll draw the brick field, which is asymmetrical.
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; We use two loops: the inner loop draws a row of bricks
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; and the outer loop sets up for the next row.
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; Timing is very important here! Note that we skip
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; drawing the ball if it falls on a line after we start a
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; new row. This will cause a little flicker but it is not
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; very noticable.
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SLEEP 44 ; make sure we start near the end of scanline
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ldy #$ff ; start with row = -1
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ScanLoop3b
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iny ; go to next brick row
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lda #BrickHeight ; for the outer loop, we count
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sta Temp ; 'brickheight' scan lines for each row
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cpy #NBrickRows ; done drawing all brick rows?
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bcc ScanSkipSync ; no -- but don't have time to draw ball!
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jmp DoneBrickDraw ; exit outer loop
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ScanLoop3a
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; These instructions are skipped on lines after the brick row changes.
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; We need the extra cycles.
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DRAW_BALL ; draw the ball on this line?
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ScanSkipSync
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sta WSYNC
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< stx COLUPF ; change colors for bricks
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; Load the first byte of bricks
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; Bricks are stored in six contiguous arrays (row-major)
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lda Bricks+NBL*0,y
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sta PF0 ; store first playfield byte
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; Store the next two bytes
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lda Bricks+NBL*1,y
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sta PF1
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lda Bricks+NBL*2,y
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sta PF2
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; Here's the asymmetric part -- by this time the TIA clock
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; is far enough that we can rewrite the same PF registers
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; and display new data on the right side of the screen
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inx ; good place for INX b/c of timing
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nop ; yet more timing
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lda Bricks+NBL*3,y
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sta PF0
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lda Bricks+NBL*4,y
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sta PF1
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lda Bricks+NBL*5,y
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sta PF2
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dec Temp
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beq ScanLoop3b ; all lines in current brick row done?
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bne ScanLoop3a ; branch always taken
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; Clear playfield from bricks loop
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DoneBrickDraw
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SLEEP 6
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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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; Draw bottom half of screen with player 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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lda (SpritePtr),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, and then we'll just leave it
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; on for the overscan region.
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ldy #$c8 ; set the playfield color
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ScanLoop5
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dey ; make a nice gradient
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lda #$ff
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sta WSYNC
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sty COLUPF ; set the playfield color
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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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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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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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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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; 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 #$ff ; top half, bounce up
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bne StoreVel
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PlayfieldCollision
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; Which brick do we break?
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; try the one nearest to us
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lda YBall
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ldx XBall
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jsr BreakBrick
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bmi CollisionNoBrick ; return -1 = no brick found
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; Did we hit the top or the bottom of a brick?
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; If top, bounce up, otherwise down
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ldx #$ff ; ball velocity = up
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cmp #BrickHeight/2 ; top half of brick?
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bcc BounceBallUp ; yofs < brickheight/2
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ldx #1 ; ball velocity = down
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BounceBallUp
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; Go to BCD mode and increment the score.
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; This treats 'score' as two decimal digits,
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; one in each nibble, for ADC and SBC operations.
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sed
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lda Score
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clc
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adc #1
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sta Score
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cld
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jmp StoreVel
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CollisionNoBrick
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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 DoneMovement
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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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bne NoBallHitTop
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ldx #1
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stx YBallVel
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NoBallHitTop
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sta YBall
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; Move ball horizontally
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lda XBallVel ; signed X velocity
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bmi BallMoveLeft ; < 0? move left
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clc
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adc XBallErr
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sta XBallErr ; XBallErr += XBallVel
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bcc DoneMovement ; no wrap around? done
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inc XBall ; XBall += 1
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lda XBall
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cmp #160 ; moved off right side?
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bcc DoneMovement ; no, done
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lda #0
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sta XBall ; wrap around to left
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beq DoneMovement ; always taken
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BallMoveLeft
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clc
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adc XBallErr
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sta XBallErr
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bcs DoneMovement
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dec XBall ; decrement xball
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lda XBall
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cmp #160
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bcc DoneMovement
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lda #159
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sta XBall
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DoneMovement
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; Joystick player 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 ; bit set? skip move
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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 YPlyr
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SkipMoveUp
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lda #%00100000 ;Down?
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bit SWCHA
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bne SkipMoveDown ; bit set? skip move
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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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; We'll test the left/right flags using a special feature of
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; the BIT instruction, which sets the N and V flags to the
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; 7th and 6th bit of the target.
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bit SWCHA
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bvs SkipMoveLeft ; V flag set?
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lda XPlyr
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beq SkipMoveLeft ; don't allow move left of screen
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dec XPlyr
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lda Captured
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beq SkipMoveLeft
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dec XBall ; if captured, also move the ball
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SkipMoveLeft
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bit SWCHA
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bmi SkipMoveRight ; N flag set?
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lda XPlyr
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cmp #150
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bcs SkipMoveRight ; don't allow move right of screen
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inc XPlyr
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lda Captured
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beq SkipMoveRight
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inc XBall ; if captured, also move the ball
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SkipMoveRight
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; Set ball position using SetHorizPos
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lda XBall
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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 player position using SetHorizPos
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lda XPlyr
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ldx #0
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jsr SetHorizPos2
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sta WSYNC
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sta HMOVE
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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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;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
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; Subroutine to try to break a brick at a given X-Y coordinate.
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; X contains the X coordinate.
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; A contains the Y coordinate.
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; On return, A = -1 if no brick was present,
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; otherwise A = Y offset (0-brickheight-1) of brick hit.
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BreakBrick
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ldy #$ff
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sec
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sbc #BrickYStart ; subtract top Y of brick field
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; Divide by brick height
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DivideRowLoop
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iny
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sbc #BrickHeight
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bcs DivideRowLoop ; loop until < 0
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cpy #NBrickRows
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bcs NoBrickFound
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; Now that we have the line, get byte and bit offset for brick
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clc
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adc #BrickHeight
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pha ; save the remainder to return as result
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txa
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clc
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adc #3 ; adjust because SetHorizPos is off by a few pixels
|
|
lsr
|
|
lsr ; divide X coordinate by 4
|
|
tax ; transfer brick column to X
|
|
tya ; load brick row # in A
|
|
clc
|
|
adc PFOfsTable,x ; add offset
|
|
tay
|
|
lda PFMaskTable,x
|
|
eor #$ff
|
|
and Bricks,y
|
|
cmp Bricks,y ; was there a change?
|
|
beq NoBrickFound2 ; no, so return -1 as result
|
|
sta Bricks,y
|
|
pla ; return remainder as result
|
|
rts
|
|
NoBrickFound2
|
|
pla ; pull the remainder, but ignore it
|
|
NoBrickFound
|
|
lda #$FF ; return -1 as result
|
|
rts
|
|
|
|
; SetHorizPos2 - Sets the horizontal position of an object.
|
|
; The X register contains the index of the desired object:
|
|
; X=0: player 0
|
|
; X=1: player 1
|
|
; X=2: missile 0
|
|
; X=3: missile 1
|
|
; X=4: ball
|
|
; This routine does a WSYNC both before and after, followed by
|
|
; a HMOVE and HMCLR. So it takes two scanlines to complete.
|
|
SetHorizPos2
|
|
sta WSYNC ; start a new line
|
|
sec ; set carry flag
|
|
DivideLoop
|
|
sbc #15 ; subtract 15
|
|
bcs DivideLoop ; branch until negative
|
|
eor #7 ; calculate fine offset
|
|
asl
|
|
asl
|
|
asl
|
|
asl
|
|
sta HMP0,x ; set fine offset
|
|
sta RESP0,x ; fix coarse position
|
|
sta WSYNC
|
|
sta HMOVE ; apply the previous fine position(s)
|
|
sta HMCLR ; reset the old horizontal position(s)
|
|
rts ; return to caller
|
|
|
|
; Height of our sprite in lines
|
|
SpriteHeight equ 17
|
|
|
|
; Bitmap data "standing" position
|
|
Frame0
|
|
.byte #0
|
|
.byte #%01101100;$F6
|
|
.byte #%00101000;$86
|
|
.byte #%00101000;$86
|
|
.byte #%00111000;$86
|
|
.byte #%10111010;$C2
|
|
.byte #%10111010;$C2
|
|
.byte #%01111100;$C2
|
|
.byte #%00111000;$C2
|
|
.byte #%00111000;$16
|
|
.byte #%01000100;$16
|
|
.byte #%01111100;$16
|
|
.byte #%01111100;$18
|
|
.byte #%01010100;$18
|
|
.byte #%01111100;$18
|
|
.byte #%11111110;$F2
|
|
.byte #%00111000;$F4
|
|
|
|
; Bitmap data "throwing" position
|
|
Frame1
|
|
.byte #0
|
|
.byte #%01101100;$F6
|
|
.byte #%01000100;$86
|
|
.byte #%00101000;$86
|
|
.byte #%00111000;$86
|
|
.byte #%10111010;$C2
|
|
.byte #%10111101;$C2
|
|
.byte #%01111101;$C2
|
|
.byte #%00111001;$C2
|
|
.byte #%00111000;$16
|
|
.byte #%01101100;$16
|
|
.byte #%01111100;$16
|
|
.byte #%01111100;$18
|
|
.byte #%01010100;$18
|
|
.byte #%01111100;$18
|
|
.byte #%11111110;$F2
|
|
.byte #%00111000;$F4
|
|
|
|
; Color data for each line of sprite
|
|
ColorFrame0
|
|
.byte #$FF ; ball color if not sharing line with player sprite
|
|
.byte #$F6;
|
|
.byte #$86;
|
|
.byte #$86;
|
|
.byte #$86;
|
|
.byte #$C2;
|
|
.byte #$C2;
|
|
.byte #$C2;
|
|
.byte #$C2;
|
|
.byte #$16;
|
|
.byte #$16;
|
|
.byte #$16;
|
|
.byte #$18;
|
|
.byte #$18;
|
|
.byte #$18;
|
|
.byte #$F2;
|
|
.byte #$F4;
|
|
|
|
; Bitmap pattern for digits
|
|
|
|
DigitsBitmap .byte $0E ; | XXX | $F5C5 Leading zero is not drawn
|
|
.byte $0A ; | X X | $F5C6 because it's never used.
|
|
.byte $0A ; | X X | $F5C7
|
|
.byte $0A ; | X X | $F5C8
|
|
.byte $0E ; | XXX | $F5C9
|
|
|
|
.byte $22 ; | X X | $F5CA
|
|
.byte $22 ; | X X | $F5CB
|
|
.byte $22 ; | X X | $F5CC
|
|
.byte $22 ; | X X | $F5CD
|
|
.byte $22 ; | X X | $F5CE
|
|
|
|
.byte $EE ; |XXX XXX | $F5CF
|
|
.byte $22 ; | X X | $F5D0
|
|
.byte $EE ; |XXX XXX | $F5D1
|
|
.byte $88 ; |X X | $F5D2
|
|
.byte $EE ; |XXX XXX | $F5D3
|
|
|
|
.byte $EE ; |XXX XXX | $F5D4
|
|
.byte $22 ; | X X | $F5D5
|
|
.byte $66 ; | XX XX | $F5D6
|
|
.byte $22 ; | X X | $F5D7
|
|
.byte $EE ; |XXX XXX | $F5D8
|
|
|
|
.byte $AA ; |X X X X | $F5D9
|
|
.byte $AA ; |X X X X | $F5DA
|
|
.byte $EE ; |XXX XXX | $F5DB
|
|
.byte $22 ; | X X | $F5DC
|
|
.byte $22 ; | X X | $F5DD
|
|
|
|
.byte $EE ; |XXX XXX | $F5DE
|
|
.byte $88 ; |X X | $F5DF
|
|
.byte $EE ; |XXX XXX | $F5E0
|
|
.byte $22 ; | X X | $F5E1
|
|
.byte $EE ; |XXX XXX | $F5E2
|
|
|
|
.byte $EE ; |XXX XXX | $F5E3
|
|
.byte $88 ; |X X | $F5E4
|
|
.byte $EE ; |XXX XXX | $F5E5
|
|
.byte $AA ; |X X X X | $F5E6
|
|
.byte $EE ; |XXX XXX | $F5E7
|
|
|
|
.byte $EE ; |XXX XXX | $F5E8
|
|
.byte $22 ; | X X | $F5E9
|
|
.byte $22 ; | X X | $F5EA
|
|
.byte $22 ; | X X | $F5EB
|
|
.byte $22 ; | X X | $F5EC
|
|
|
|
.byte $EE ; |XXX XXX | $F5ED
|
|
.byte $AA ; |X X X X | $F5EE
|
|
.byte $EE ; |XXX XXX | $F5EF
|
|
.byte $AA ; |X X X X | $F5F0
|
|
.byte $EE ; |XXX XXX | $F5F1
|
|
|
|
.byte $EE ; |XXX XXX | $F5F2
|
|
.byte $AA ; |X X X X | $F5F3
|
|
.byte $EE ; |XXX XXX | $F5F4
|
|
.byte $22 ; | X X | $F5F5
|
|
.byte $EE ; |XXX XXX | $F5F6
|
|
|
|
; Playfield bitmasks for all 40 brick columns
|
|
PFMaskTable
|
|
REPEAT 2
|
|
.byte #$10,#$20,#$40,#$80
|
|
.byte #$80,#$40,#$20,#$10,#$08,#$04,#$02,#$01
|
|
.byte #$01,#$02,#$04,#$08,#$10,#$20,#$40,#$80
|
|
REPEND
|
|
; Brick array byte offsets for all 40 brick columns
|
|
PFOfsTable
|
|
.byte NBL*0,NBL*0,NBL*0,NBL*0
|
|
.byte NBL*1,NBL*1,NBL*1,NBL*1, NBL*1,NBL*1,NBL*1,NBL*1
|
|
.byte NBL*2,NBL*2,NBL*2,NBL*2, NBL*2,NBL*2,NBL*2,NBL*2
|
|
.byte NBL*3,NBL*3,NBL*3,NBL*3
|
|
.byte NBL*4,NBL*4,NBL*4,NBL*4, NBL*4,NBL*4,NBL*4,NBL*4
|
|
.byte NBL*5,NBL*5,NBL*5,NBL*5, NBL*5,NBL*5,NBL*5,NBL*5
|
|
|
|
; Epilogue
|
|
org $fffc
|
|
.word Start
|
|
.word Start
|