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
ISE = ~/prog/docker/xilinx-14.7-ubuntu-12.04/run
VIVADO = ~/prog/docker/xilinx-2019.1-ubuntu-18.04/run
QUARTUS = ~/prog/quartus/wrapper.sh --

The ISE, VIVADO, and QUARTUS fields are to optionally wrap invocations of the Xilinx ISE / Vivado and Intel Quartus toolchains in case you want to run them via Docker, Nix, shell scripts that set some environment variables, 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