Space Invaders arcade machine

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

This code is part of the book Retrocomputing with Clash: Haskell for FPGA Hardware Design at https://unsafePerform.IO/retroclash/.

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:
  • Direct pushbuttons
  • 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.

• The Xilinx Artrix-7 based Nexys A7. This uses the Xilinx Vivado toolchain.

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

TARGET = papilio-pro-arcade
CLASH = stack exec --
ISE = ~/prog/docker/xilinx-14.7-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

• Wikipedia
• Computer Archeology's Space Invaders page
• Original arcade machine schematics
• Intel 8080 opcodes
• Intel 8080 opcodes
• Intel 8080 implementation in Clash
• Papilio Pro FPGA dev board
• Nexys A7 FPGA dev board