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Intel 8080-based Space Invaders arcade machine implemented on an FPGA, written in CLaSH
Haskell Verilog
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README.md

Space Invaders arcade machine

This is a CLaSH implementation of the 1978 Space Invaders arcade machine by Taito.

Click to play YouTube video

Hardware architecture

  • CPU: Intel 8080.
  • Video: 256x224 monochrome at 60 FPS, backed by a 7168x8 bit framebuffer. IRQs triggered on lines 96 and 224. The screen was rotated 90 degrees to be in portrait orientation.
  • Sound: TI SN76477 connected to the CPU via two 8-bit ports
  • Inputs: coin deposit detector, two sets of buttons for starting the game, and moving and firing, for two-player mode.

CLaSH implementation

  • CPU: Implemented based on abstract descriptions of Intel 8080 ISA, with no reference to real implementation. Not cycle-accurate, but aims to pass various functional tests.
  • Video: Standard 640x480@60Hz VGA, with 25.175MHz pixel clock. Logical "pixels" are scaled 2x2, but the layout is not rotated to keep interrupt timings.
  • Framebuffer: The video system has priority over the CPU: memory reads from the framebuffer area will block the CPU until the video system is done reading.
  • Sound: Unimplemented.
  • Inputs: PS/2 keyboard. C to deposit a coin, Enter to start 1P game, and Left / Right / RCtrl to move and fire with P1.

Limitations

This implementation doesn't aim to be cycle-accurate; it wouldn't matter for much anyway, since the CPU runs at the VGA output pixel clock of 25 MHz, whereas the real cabinet ran at 2 MHz. Luckily, Space Invaders does all animations keyed to two 60 Hz timers triggered by the video system.

Sound is not implemented at all.

The back-panel DIP switches are not yet mapped to anything.

Building

Included are rudimentary Shake rules to build for various hobbyist FPGA dev boards:

  • The Xilinx Spartan-6 based Papilio Pro or the Spartan-3 based Papilio One, both with the Arcade mega-wing. These use the Xilinx ISE toolchain.

  • [WIP] The Xilinx Artrix-7 based Nexys A7 50T. This uses the Xilinx Vivado toolchain.

  • [WIP] The Altera Cyclone-V based DE10-Nano, using the Altera Quartus toolchain. Note that there is no input support for the DE10-Nano yet.

Make a file called build.mk with content similar to the following:

TARGET = papilio-pro-arcade
CLASH = stack exec --
ISE = ~/prog/docker/xilinx-ubuntu-12.04/run
VIVADO = ~/prog/docker/xilinx-2019.1-ubuntu-18.04/run

The CLASH, ISE and VIVADO fields are to optionally wrap invocations of the CLaSH compiler and the Xilinx ISE / Vivado toolchain in case you want to run them via Stack, Docker, Nix, etc.

Then you can build for the Papilio Pro by running the included mk script.

Useful links

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