Geartowns

Geartowns is a very accurate, cross-platform Fujitsu FM Towns emulator written in C++ that runs on Windows, macOS, Linux, BSD and RetroArch, with an embedded MCP server for AI debugging and development.

This is an open source project with its ongoing development made possible thanks to the support by these awesome backers. If you find it useful, please consider sponsoring.

Don't hesitate to report bugs or ask for new features by opening an issue.

Downloads

Notes:

• Windows: May need Visual C++ Redistributable and OpenGL Compatibility Pack
• Homebrew: If Homebrew asks you to trust the third-party tap, run brew trust --tap drhelius/geardome
• Linux: May need libsdl3

Features

• Very accurate 80386 core that passes test386 and SingleStepTests/80386.
• CD-ROM drive emulation with cue/bin, iso and chd images, m3u disc playlists and disc swapping, with optional compressed images (zip).
• Physical CD-ROM discs on Windows, macOS and Linux.
• Floppy disk emulation with two drives supporting d77, d88, hdm, xdf, img and HxC mfm images, m3u playlists, write protection and optional persistence of disk changes.
• SCSI hard disks on IDs 0-6 with raw images (hdd, h0-h6, and hdm over 8 MB) and chd images, the built-in interface of the 1F and later models and the Model 1/2 SCSI card, blank image creation and Towns OS installation.
• Save states with 5 slots and rewind support.
• Run-ahead support to reduce input latency.
• Video recording to AVI and screenshots.
• VGM recorder for the YM3438 and RF5C68 sound chips.
• Supported platforms (standalone): Windows, Linux, BSD and macOS.
• Supported platforms (libretro): Windows, Linux, macOS, Raspberry Pi, Android, iOS, tvOS, webOS, PlayStation 3, Nintendo GameCube, Nintendo WiiU, Nintendo Switch, Emscripten, Classic Mini systems (NES, SNES, C64, ...), OpenDingux, RetroFW and QNX.
• Full debugger with just-in-time disassembler, execute, data, I/O and interrupt breakpoints, code navigation, call stack, debug symbols, automatic labels, memory editor, trace logger, profiler, i386 registers, descriptor tables and paging inspectors, and viewers for CRTC, palettes, framebuffers, sprites, YM3438, RF5C68, CD-ROM, floppy controller and disks, SCSI controller and disks, interrupts, timers, DMA, RTC, keyboard and game ports.
• MCP server for AI-assisted debugging with Claude Code, Codex and similar, exposing tools for execution control, memory inspection, hardware status, rewind and more.
• Windows, Linux and macOS Portable Mode.
• Programmable Shader Chain.
• Media loading from the command line by adding the CD-ROM, floppy or hard disk image path as an argument.
• Media loading using drag & drop.
• Support for modern game controllers through gamecontrollerdb.txt file located in the same directory as the application binary.

Tips

Basic Usage

• Firmware: Geartowns requires the FM Towns system ROMs to run. Use Machine → Firmware... to choose the folder that holds them, with one subfolder or MAME zip per set. See BIOS_README.md for the sets each model needs, where to put them and their checksums.
• CD-ROM Images: Geartowns supports chd, zipped and unzipped cue/bin and iso images, and m3u playlists for multi-disc games. CUE audio tracks can use raw BIN, compatible WAV (44100 Hz, 16-bit, stereo) or Ogg Vorbis files. Change discs from the CD-ROM menu.
• Floppy Images: Insert floppy disks from the Floppies menu, or load them from the command line. Supported images are d77, d88, hdm, xdf, img and mfm, also inside zip files, and m3u playlists.
• Hard Disks: Attach raw images of 512-byte sectors to SCSI IDs 0-6 from the Hard Disks menu, with drag & drop or from the command line (h0-h6 pick that ID, hdd the first free one, and so does an hdm over 8 MB, smaller ones being floppies). A MAME hard disk chd is read-only: its writes go to a raw working copy in the Save Files folder, made at the first write and used from then on. In libretro, load the image as content, alone or in an m3u playlist with the CD; changes go to a working copy in the save directory and the original is never written. On a running machine changes apply at the next reset, and writes are saved a few seconds after the machine stops writing. Save states, rewind and run-ahead hold no disk data: loading one undoes the disk writes made since (the last 32 MB of them per disk), and warns when it can't. SCSI is built into the 1F/2F and later, fitted as a card on the Model 1/2 while a disk is attached, and absent on the Marty. With Formatted checked, New Blank Disk... makes a disk partitioned and formatted like Towns OS does, an empty drive per 127 MB (up to ten), so games booted from their CD can install to it; it holds no system and doesn't boot. To install Towns OS, create a disk with Formatted unchecked, run HD Install from the Tools folder of TownsMENU V2.1, then eject the CD and reset. A bootable CD boots before the disk unless you pick the disk in Reset and Boot From, which holds the system ROM's boot keys (H and the ID) through the reset; the Model 1/2 and Marty ROMs don't read them. Towns OS V2.1 needs 4 MB to start from a disk, which Main RAM on Auto gives while one is attached. Towns OS only uses the partitions that have a drive letter in the model's CMOS. With Assign Drive Letters on, the default, each reset gives the partitions of an attached disk that have none the free letters from D:, so a disk installed on another model or emulator boots; letters already set are kept. With it off, assign them in Towns OS: boot the Towns OS V2.1 CD, open Settings → Drive Configuration, click the disk's SCSI ID, Add Drive, Execute, then eject the CD and reset.
• Machine: The FM Towns (Model 1/2), 1F/2F, 10F/20F, FM Towns II CX, UX, UG, HG and HR, and the FM Towns Marty are emulated. Machine → Model, CPU Speed and Main RAM default to Auto, which picks the machine, CPU speed and RAM the game database names for the game, the FM Towns II HG for 1.44 MB floppy disks, or the FM Towns (Model 1/2) for other content.
• Portable Mode: Run with --portable, or create an empty file named portable.ini in the same directory as the application binary. On macOS, place the file next to the .app bundle.

Debugging Features

• Docking Windows: In debug mode, you can dock windows together by pressing SHIFT and dragging a window onto another.
• Multi-viewport: In Windows or macOS, you can enable "multi-viewport" in the debug menu. You must restart the emulator for the change to take effect. Once enabled, you can drag debugger windows outside the main window.
• Single Instance: You can enable "Single Instance" in the Emulator menu. When enabled, opening a file while another instance is running will send it to the running instance instead of starting a new one.
• Debug Symbols: The emulator automatically tries to load a symbol file when loading a CD-ROM or floppy image. For example, for path_to_image.cue it tries to load path_to_image.sym. You can also load symbol files using the GUI or the CLI. Each line of a symbol file defines one symbol as address name, name = address, name EQU address or address type name (GNU nm output, for example nm -n program.elf > program.sym), where the address can be linear or selector:offset. Comments start with ; or #. GNU ld and NASM map files can be loaded too.

Command Line Usage

geartowns [options] [media_file] [symbol_file]

Arguments:
  media_file                CD-ROM (.cue .chd .iso), floppy (.d77 .d88 .hdm
                            .xdf .img .mfm), .m3u playlist, .zip, or hard disk
                            (.hdd .h0-.h6, hard disk .chd, .hdm over 8 MB)
  symbol_file               Symbol file for debugging

Media:
      --cd FILE             CD-ROM image or .m3u playlist
      --fd1 FILE            Floppy image in drive 1
      --fd2 FILE            Floppy image in drive 2
      --load-state SLOT     Load save state slot 1-5, or a state file, once the
                            media is loaded

Machine (for this run only, the saved settings stay):
      --model MODEL         auto, model1_2, 1f_2f, 10f_20f, cx, ux, ug, hg, hr,
                            marty
      --cpu-mhz MHZ         auto, 16, 20, 25, 33, 40, 50, 66, 100
      --ram MB              auto, 1, 2, 4, 6, 8, 10, 12, 16, 20, 24, 26, 28
      --fpu FPU             auto, none, 80387 (a 487SX on the HR)

Options:
  -f, --fullscreen          Start in fullscreen mode
  -w, --windowed            Start in windowed mode with menu visible
      --mcp-stdio           Auto-start MCP server with stdio transport
      --mcp-http            Auto-start MCP server with HTTP transport
      --mcp-router          Enable compact MCP tool routing
      --mcp-http-address A  HTTP bind address (default: 127.0.0.1)
      --mcp-http-port N     HTTP port for MCP server (default: 7777)
      --headless            Run without GUI (requires MCP)
      --portable            Store configuration and user data beside the
                            application
  -v, --version             Display version information
  -h, --help                Display this help message

MCP Server

Geartowns includes a Model Context Protocol (MCP) server that enables AI-assisted debugging through AI agents like Claude Code, Codex and similar. The server provides tools for execution control, memory inspection, breakpoints, disassembly, hardware status, rewind and more. STDIO and HTTP transports are supported, with STDIO preferred.

For complete setup instructions and tool documentation, see MCP_README.md.

Agent Skills

Geartowns provides Agent Skills that teach AI assistants how to effectively use the emulator for specific tasks:

• geartowns-debugging — Game debugging, code tracing, breakpoint management, hardware inspection, and reverse engineering.
• geartowns-romhacking — Cheat creation, memory searching, data modification, text translation, and game patching.

Install with npx skills add drhelius/geartowns. See the skills README for details.

Hardware Accuracy

Geartowns is being developed based on the results of the FM Towns Hardware Tests.

Build Instructions

Windows

• Install Microsoft Visual Studio Community 2026 or later.
• Download the latest SDL3 VC development libraries from SDL3 Releases (the file named SDL3-devel-x.y.z-VC.zip).
• Extract the archive and rename the resulting folder (e.g. SDL3-x.y.z) to SDL3.
• Place the SDL3 folder inside platforms/windows/dependencies/ so that the include path is platforms/windows/dependencies/SDL3/include/SDL3/.
• Open the Geartowns Visual Studio solution platforms/windows/Geartowns.sln and build.

macOS

• Install Xcode and run xcode-select --install in the terminal for the compiler to be available on the command line.
• Run these commands to generate a Mac app bundle:

brew install sdl3
cd platforms/macos
make dist

Linux

• Ubuntu / Debian / Raspberry Pi (Raspbian):

If you are using Ubuntu 26.04, you can install SDL3 directly. Use the following commands to build:

sudo apt install build-essential pkg-config libsdl3-dev libgl-dev
cd platforms/linux
make

For Ubuntu 24.04, you need to build SDL3 from source first. Use the following commands to build both SDL3 and Geartowns:

sudo apt install build-essential cmake git curl jq pkg-config \
  libx11-dev libxext-dev libxrandr-dev libxcursor-dev libxfixes-dev \
  libxi-dev libxss-dev libxkbcommon-dev libwayland-dev libdecor-0-dev \
  libdrm-dev libgbm-dev libgl1-mesa-dev libegl1-mesa-dev libdbus-1-dev libudev-dev libxtst-dev
SDL3_TAG=$(curl -s https://api.github.com/repos/libsdl-org/SDL/releases/latest | jq -r '.tag_name')
git clone --depth 1 --branch "$SDL3_TAG" https://github.com/libsdl-org/SDL.git /tmp/SDL3
cmake -S /tmp/SDL3 -B /tmp/SDL3/build -DCMAKE_INSTALL_PREFIX=/usr -DSDL_TESTS=OFF -DSDL_EXAMPLES=OFF
cmake --build /tmp/SDL3/build -j$(nproc)
sudo cmake --install /tmp/SDL3/build
cd platforms/linux
make

• Fedora:

sudo dnf install @development-tools gcc-c++ SDL3-devel
cd platforms/linux
make

• Arch Linux:

sudo pacman -S base-devel sdl3
cd platforms/linux
make

BSD

• FreeBSD:

su root -c "pkg install -y git gmake pkgconf sdl3"
cd platforms/bsd
USE_CLANG=1 gmake

• NetBSD:

su root -c "pkgin install gmake pkgconf SDL3"
cd platforms/bsd
gmake

• OpenBSD

doas pkg_add gmake sdl3
cd platforms/bsd
LDFLAGS=-L/usr/X11R6/lib/ USE_CLANG=1 gmake

Libretro

• Ubuntu / Debian / Raspberry Pi (Raspbian):

sudo apt install build-essential
cd platforms/libretro
make

• Fedora:

sudo dnf install @development-tools gcc-c++
cd platforms/libretro
make

Contributors

Thank you to all the people who have already contributed to Geartowns!

License

Geartowns is licensed under the GNU General Public License v3.0 License, see LICENSE for more information.