343 lines
6.4 KiB
Plaintext
343 lines
6.4 KiB
Plaintext
;
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; atari-2600-example.oph
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; Skeleton code for an Atari 2600 ROM,
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; plus an example of reading the joystick.
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; By Chris Pressey, November 2, 2012.
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;
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; SPDX-FileCopyrightText: Chris Pressey, the author of this work, has dedicated it to the public domain.
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; For more information, please refer to <https://unlicense.org/>
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; SPDX-License-Identifier: Unlicense
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;
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; Based on Chris Cracknell's Atari 2600 clock (also in the public domain):
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; http://everything2.com/title/An+example+of+Atari+2600+source+code
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;
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; to build and run in Stella:
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; ophis atari-2600-example.oph -o example.bin
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; stella example.bin
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;
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; More useful information can be found in the Stella Programmer's Guide:
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; http://alienbill.com/2600/101/docs/stella.html
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;
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;
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; Useful system addresses (TODO: briefly describe each of these.)
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;
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.alias VSYNC $00
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.alias VBLANK $01
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.alias WSYNC $02
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.alias NUSIZ0 $04
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.alias NUSIZ1 $05
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.alias COLUPF $08
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.alias COLUBK $09
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.alias PF0 $0D
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.alias PF1 $0E
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.alias PF2 $0F
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.alias SWCHA $280
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.alias INTIM $284
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.alias TIM64T $296
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.alias CTRLPF $0A
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.alias COLUP0 $06
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.alias COLUP1 $07
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.alias GP0 $1B
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.alias GP1 $1C
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.alias HMOVE $2a
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.alias RESP0 $10
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.alias RESP1 $11
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;
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; Cartridge ROM occupies the top 4K of memory ($F000-$FFFF).
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; Thus, typically, the program will occupy all that space too.
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;
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; Zero-page RAM we can use with impunity starts at $80 and goes
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; upward (at least until $99, but probably further.)
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;
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.alias colour $80
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.alias luminosity $81
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.alias joystick_delay $82
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.org $F000
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;
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; Standard prelude for Atari 2600 cartridge code.
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;
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; Get various parts of the machine into a known state:
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;
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; - Disable interrupts
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; - Clear the Decimal flag
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; - Initialize the Stack Pointer
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; - Zero all bytes in Zero Page memory
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;
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start:
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sei
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cld
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ldx #$FF
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txs
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lda #$00
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zero_loop:
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sta $00, x
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dex
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bne zero_loop
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; and fall through to...
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;
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; Initialization.
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;
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; - Clear the Playfield Control register.
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; - Set the player (sprite) colour to light green (write to COLUP0.)
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; - Set the player (sprite) size/repetion to normal (write to NUSIZ0.)
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;
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lda #$00
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sta CTRLPF
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lda #$0c
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sta colour
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lda #$0a
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sta luminosity
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lda #$00
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sta NUSIZ0
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; and fall through to...
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;
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; Main loop.
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;
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; A typical main loop consists of:
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; - Waiting for the frame to start (vertical blank period)
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; - Displaying stuff on the screen (the _display kernel_)
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; - Doing any processing you like (reading joysticks, updating program state,
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; etc.), as long as you get it all done before the next frame starts!
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;
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main:
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jsr vertical_blank
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jsr display_frame
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jsr read_joystick
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jmp main
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;
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; Vertical blank routine.
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;
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; In brief: wait until it is time for the next frame of video.
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; TODO: describe this in more detail.
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;
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vertical_blank:
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ldx #$00
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lda #$02
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sta WSYNC
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sta WSYNC
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sta WSYNC
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sta VSYNC
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sta WSYNC
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sta WSYNC
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lda #$2C
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sta TIM64T
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lda #$00
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sta WSYNC
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sta VSYNC
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rts
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;
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; Display kernal.
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;
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; First, wait until it's time to display the frame.
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;
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.scope
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display_frame:
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lda INTIM
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bne display_frame
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;
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; (After that loop finishes, we know the accumulator must contain 0.)
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; Wait for the next scanline, zero HMOVE (for some reason; TODO discover
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; this), then turn on the screen.
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;
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sta WSYNC
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sta HMOVE
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sta VBLANK
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;
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; Actual work in the display kernal is done here.
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;
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; This is a pathological approach to writing a display kernal.
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; This wouldn't be how you'd do things in a game. So be it.
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; One day I may improve it. For now, be happy that it displays
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; anything at all!
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;
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;
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; Wait for $3f (plus one?) scan lines to pass, by waiting for
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; WSYNC that many times.
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;
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ldx #$3F
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_wsync_loop:
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sta WSYNC
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dex
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bpl _wsync_loop
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sta WSYNC
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;
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; Delay while the raster scans across the screen. The more
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; we delay here, the more to the right the player will be when
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; we draw it.
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;
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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nop
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;
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; OK, *now* display the player.
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;
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sta RESP0
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;
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; Loop over the rows of the sprite data, drawing each to the screen
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; over four scan lines.
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;
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; TODO understand this better and describe it!
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;
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ldy #$07
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_image_loop:
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lda image_data, y
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sta GP0
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sta WSYNC
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sta WSYNC
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sta WSYNC
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sta WSYNC
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dey
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bpl _image_loop
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lda #$00
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sta GP0
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;
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; Turn off screen display and clear display registers.
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;
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lda #$02
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sta WSYNC
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sta VBLANK
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lda #$00
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sta PF0
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sta PF1
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sta PF2
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sta COLUPF
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sta COLUBK
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rts
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.scend
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;
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; Read the joystick and use it to modify the colour and luminosity
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; of the player.
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;
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.scope
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read_joystick:
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lda joystick_delay
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beq _continue
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dec joystick_delay
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rts
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_continue:
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lda SWCHA
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and #$f0
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cmp #$e0
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beq _up
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cmp #$d0
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beq _down
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cmp #$b0
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beq _left
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cmp #$70
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beq _right
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jmp _tail
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_up:
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inc luminosity
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jmp _tail
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_down:
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dec luminosity
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jmp _tail
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_left:
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dec colour
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jmp _tail
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_right:
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inc colour
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;jmp _tail
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_tail:
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lda colour
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and #$0f
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sta colour
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lda luminosity
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and #$0f
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sta luminosity
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lda colour
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clc
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rol
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rol
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rol
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rol
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ora luminosity
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sta COLUP0
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lda #$06
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sta joystick_delay
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rts
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.scend
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;
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; Player (sprite) data.
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;
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; Because we loop over these bytes with the Y register counting *down*,
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; this image is stored "upside-down".
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;
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image_data:
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.byte %01111110
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.byte %10000001
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.byte %10011001
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.byte %10100101
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.byte %10000001
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.byte %10100101
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.byte %10000001
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.byte %01111110
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;
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; Standard postlude for Atari 2600 cartridge code.
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; Give BRK and boot vectors that point to the start of the code.
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;
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.advance $FFFC
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.word start
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.word start
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