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.
• Game Gear Gear-to-Gear cable support for two local desktop processes, including serial and parallel communication.
• 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.
• 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 GitHub Copilot, Claude, 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: Open Gear-to-Gear in two desktop instances, select the same session, and connect both, or start each process with --geartogear-session N. 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