JavaScript Sega Master System & Game Gear dynamic recompiling emulator.
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jsSMS is a dynamic recompiling emulator for Sega Master System & Game Gear ROMs written in JavaScript and designed to be fast on mobile browsers.


Originally, this is a JavaScript port of JavaGear by Chris White.

Original copyright: Copyright (C) 2002-2008 Chris White

The code then evolved from a basic interpreter to a complex recompiler able to run on mobile at full speed.

The recompilation code is located in src/compiler folder as well as technical insights.

Related talk

Video of the talk about JSSMS I gave at Reject.js 2013. There are also the slides in HTML.

Current state

  • Though state save and load functions are ported, there are not used and should be more JavaScript friendly (using JSON for example, see JSNES for an example of implementation).
  • Lightgun is not yet supported.
  • The recompiler is not compatible with Closure Compiler in advanced optimizations mode and will likely never be.

Many parts of the script are borrowed from JSNES, a NES emulator in JavaScript by Ben Firsh.

If you are looking for another SMS emulator in JavaScript, go to Miracle.


  • Support latest IE.
  • Experiment with web workers for recompiled code generation.
  • Experiment with generator for calling interrupts.
  • Experiment with asm.js.


jsSMS - A dynamic recompiling emulator for Sega Master System & Game Gear written in JavaScript

Copyright (C) 2018 Guillaume Cedric Marty (

Based on JavaGear Copyright (c) 2002-2008 Chris White

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see