Pac-Man Emulator

An emulator for the Zilog Z80 CPU and hardware specific to the 1980 arcade game: Pac-Man. It will also run Ms. Pac-Man, as well as any other homebrew ROMs that target the Pac-Man hardware.

This is based on the Intel 8080 CPU core from my Space Invaders emulator.

It emulates the graphics and sound, supports save states, has an interactive debugger, has reverse-stepping functionality, and includes 8000+ unit test cases.

Above: my emulator running Pac-Man, Ms. Pac-Man, and a homebrew Matrix-effect ROM.

Implementation

I wrote the emulator and disassembler in C# targeting the cross-platform .NET Core runtime. It runs as a desktop app on Windows/Linux/macOS as well as a UWP app on the Xbox One game console.

I used SDL2 for the GUI and audio via the SDL2# wrapper. The Xbox One version uses the same code wrapped in a UWP app wrapper, which is based on this starter project.

The keyboard mapping is hardcoded as:

It also supports using an Xbox-style game controller (both on desktop as well as when running on the Xbox One):

Compiling / Running

The project layout is as follows:

Windows/Linux/macOS Desktop App

1. Clone this repository
2. Install .NET Core 3.1
3. Install SDL2
   • macOS: brew install SDL2 && brew link sdl2
   • On M1/arm64 macOS, may also need to do: sudo ln -s /opt/homebrew/lib/libSDL2.dylib /usr/local/lib/
   • On Windows, download binaries and place SDL2.dll in the emulator.cli directory
4. cd emulator.cli
5. dotnet restore
6. dotnet run -- followed by the commands to pass to the CLI program

Currently there is only one command, run:

$ dotnet run -- run --help

Usage: pacemu run [arguments] [options]

Arguments:
  [ROM path]  The path to a directory containing the ROM set to load.

Options:
  -?|-h|--help          Show help information
  -rs|--rom-set         The name of an alternative ROM set and/or PCB configuration to use; pacman or mspacman; defaults to pacman
  -dw|--dip-switches    The path to a JSON file containing DIP switch settings; defaults to dip-switches.json in CWD.
  -l|--load-state       Loads an emulator save state from the given path.
  -sc|--skip-checksums  Allow running a ROM with invalid checksums.
  -wr|--writable-rom    Allow memory writes to the ROM address space.
  -d|--debug            Run in debug mode; enables internal statistics and logs useful when debugging.
  -b|--break            Used with debug, will break at the given address and allow single stepping opcode execution (e.g. --break 0x0248)
  -rvs|--reverse-step    Used with debug, allows for single stepping in reverse to rewind opcode execution.
  -a|--annotations      Used with debug, a path to a text file containing memory address annotations for interactive debugging (line format: 0x1234 .... ; Annotation)

For example, to run Pac-Man: dotnet run -- run ../roms/pacman

Or, to run Ms. Pac-Man: dotnet run -- run ../roms/mspacman --rom-set mspacman

Game settings (such as number of lives and difficulty) can be adjusted by editing the dip-switches.json file.

Homebrew ROMs that target the Pac-Man hardware can be run by leaving the --rom-set set to the default pacman value. You may also need to use a few extra switches depending on the ROM. e.g. dotnet run -- run ../roms/homebrew --skip-checksums --writable-rom

Xbox One

1. Install Visual Studio 2019 with the following components
   • Universal Windows Platform development
   • Windows 10 SDK 10.0.18362.0
2. Setup Xbox One in developer mode
   • Use DevMode activation app
   • Settings > System > Console Info > then quickly press LB, RB, LT, RT to enable
3. Clone this repository
4. Open emulator.uwp/pac-man-emulator-uwp.sln in Visual Studio
5. Set project configuration to Debug x64
6. Set signing certificate
   • Project Properties > Application > Package Manifest > Packaging
7. Setup for remote deploy to Xbox
   • Project Properties > Debug > Start Options > Authentication Mode: Universal (Unencrypted Protocol)
   • Enter IP address of Xbox
8. Place ROMs in emulator.uwp/roms
   • See readme for more details
9. Click the green play button labeled Remote Machine to deploy and start running/debugging

Interactive Debugger

If the emulator is launched with the --debug option, the debugger will be enabled. You can press the pause/break or F3 key which will stop execution and print the interactive debugger in the console.

From there you can use F1 and F2 to save and load the emulator state.

To single step over an opcode use F10, or F5 to continue until the next breakpoint.

Breakpoints can be set via the --break option at startup, or in the debugger by pressing F4.

If the emulator was started with the --annotations option, F11 can be used to toggle between the disassembler's generated psuedocode or the provided annotation file. This is used to show comments for each disassembled opcode inline in the debugger, which makes tracking down issues and/or understanding the game code easier. You can find an excellent annotated disassembly here.

F12 is used to print the last 50 opcodes, so you can see execution history.

Finally, if --reverse-step was specified at startup, F9 can be used to single step backwards over opcodes, effectively allowing you to rewind CPU state one instruction at a time. I found this to be very helpful when tracking down bugs in the CPU core.

Unit Tests

While building the emulator I found it essential to write unit tests for each opcode and along the way. This made it much easier to track down bugs late in development.

Each opcode test contains Z80 assembly code which is assembled using zasm. This assembled binary is then executed on the emulated CPU and then has assertions ran against the CPU state to verify opcode behavior.

Additionally, there is an integration test which uses a CPU test program written for the Zilog Z80 CPU. This test program executes instructions and then performs checksums against memory that were taken from real hardware. The assembled program along with its disassembly can can be found in the z80.tests/ZEX directory.

Zilog Z80 CPU tests (8000+ test cases):

1. cd z80.tests
2. dotnet restore
3. dotnet test

Finally, there are test cases for the Pac-Man specific hardware such as the video hardware which renders a screen of tiles and sprites in several different palettes and then compares them against the reference images:

Emulator tests:

1. cd emulator.tests
2. dotnet restore
3. dotnet test

Resources

I found the following resources useful in building this emulator:

• Zilog Z80 CPU
  • O.G. Programming Manual
  • Modern Programming Manual
  • z80.info
  • Z80 vs I8080 Differences
  • Opcode Table
  • Opcode List
  • Flag Behavior
  • Instruction Set
  • The Amazing Zilog Z80, Part 2
  • Interrupts
  • Interrupts Documentation
• Z80 Instruction Exerciser
  • Using Zexall/Zexdoc
  • Source 1
  • Source 2
• Pac-Man Hardware
  • Chris Lomont's Pac-Man Emulation Guide
  • Color PROM Info
  • Hardware Details
  • Homebrew ROMs
• Misc
  • 8-Bit Workshop
  • Let there be sound