Gearsystem

Gearsystem is a very accurate, cross-platform Sega Master System / Game Gear / SG-1000 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

Supported Machines

• Sega Mark III
• Sega Master System
• Sega Game Gear
• Sega Game 1000 (SG-1000)
• Othello Multivision

Features

• Very accurate Z80, VDP, PSG and FM emulation.
• Supported cartridges: ROM, ROM + RAM, SEGA, Codemasters, Korean, MSX + Nemesis, Janggun, SG-1000, and many Korean multi-carts.
• Automatic region detection: NTSC-JAP, NTSC-USA, PAL-EUR.
• Support for YM2413 (OPLL) FM sound chip.
• Gear-to-Gear cable support for two local desktop processes, including serial and parallel communication.
• Mark III Joy-Joy cable support for two local desktop processes.
• Light Phaser and Paddle Control support.
• Internal database for ROM detection.
• Battery powered RAM save support.
• Save states with preview and rewind support.
• Run-ahead support to reduce input latency.
• Compressed ROM support (ZIP).
• Game Genie and Pro Action Replay cheat support, with RetroArch .cht import.
• VGM recorder.
• Supported platforms (standalone): Windows, Linux, BSD and macOS.
• Supported platforms (libretro): Windows, Linux, macOS, Raspberry Pi, Android, iOS, tvOS, webOS, PlayStation Vita, PlayStation 3, Nintendo 3DS, Nintendo GameCube, Nintendo Wii, Nintendo WiiU, Nintendo Switch, Emscripten, Classic Mini systems (NES, SNES, C64, ...), OpenDingux, RetroFW and QNX.
• Full debugger with just-in-time disassembler, CPU breakpoints, memory access breakpoints, code navigation (goto address, JP JR and CALL double clicking), debug symbols, automatic labels, memory editor, trace logger, IO inspector and VRAM viewer including tiles, sprites, backgrounds and palettes.
• 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.
• ROM loading from the command line by adding the ROM path as an argument.
• ROM 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

• BIOS: Gearsystem can run with or without a BIOS. You can optionally load a BIOS and enable it. It's possible to run the title included in the BIOS by setting Media Slot to None in the Emulator menu. This emulates a machine without any media plugged in.
• Overscan: For a precise representation of the original image, select Overscan Top+Bottom and Aspect Ratio Standard (4:3 DAR) in the Video menu. Game Gear will ignore any overscan settings.
• Mouse Cursor: Automatically hides when hovering over the main output window or when Main Menu is disabled.
• 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.
• Gear-to-Gear and Mark III Link Cable: Open Link Cable in two desktop instances, select the same session, and connect both, or start each process with --link-session N. Select Auto, Gear-to-Gear, or Mark III under Protocol; the session is independent of this selection. Auto uses Gear-to-Gear for native Game Gear software and Mark III Joy-Joy for F-16 Fighting Falcon (Japan). For Mark III, Return, 1, 2, and Space are mapped to the SK-1100 keyboard. Reset the console if Mark III hardware is attached after startup. Independent instances should use separate application copies or --portable data directories so configuration, saves, and single-instance handling do not conflict.

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 ROM while another instance is running will send the ROM to the running instance instead of starting a new one.
• Debug Symbols: The emulator automatically tries to load a symbol file when loading a ROM (.sym, .noi). For example, for path_to_rom_file.sms it tries to load path_to_rom_file.sym. You can also load a symbol file using the GUI or the CLI. It supports sjasmplus/Pasmo (EQU), SDCC/NoICE (.noi), wla-dx and vasm/generic file formats.

Command Line Usage

gearsystem [options] [rom_file] [symbol_file]

Arguments:
  [rom_file]                  ROM file: accepts ROMs (.sms, .gg, .sg, .mv) or ZIP (.zip)
  [symbol_file]               Optional symbol file for debugging

Options:
  -f, --fullscreen            Start in fullscreen mode
  -w, --windowed              Start in windowed mode with menu visible
      --load-state SLOT       Load save state slot 1-5, or a state file, once the media is loaded
      --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 or Link Cable)
      --link-session N        Connect to link cable session 1-255
      --link-stall-us N       Override barrier stall threshold (0=default)
      --portable              Store configuration and user data beside the application
  -v, --version               Display version information
  -h, --help                  Display this help message

MCP Server

Gearsystem 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, and more. STDIO and HTTP transports are supported, with STDIO preferred.

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

Agent Skills

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

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

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

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 Gearsystem Visual Studio solution platforms/windows/Gearsystem.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 Gearsystem:

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

Accuracy Tests

Zexall Z80 instruction exerciser (from SMS Power!)

SMS VDP Test (from SMS Power!)

Screenshots

Contributors

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

License

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