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DOSBox ported to Emscripten

About

DOSBox is an open source DOS emulator designed for running games. Emscripten compiles code to JavaScript and WebAssembly. This is a version of DOSBox which can be compiled with Emscripten to run in a web browser. It allows running old DOS games and other DOS programs in a web browser.

DOSBox is distributed under the GNU General Public License. See the COPYING file for more information.

Status

Em-DOSBox runs most games successfully in web browsers. Although DOSBox has not been fully re-structured for running as an Emscripten main loop, most functionality is available thanks to Emscripten Asyncify. A few programs can still run into problems due to paging exceptions.

Other issues

  • Game save files are written into the Emscripten file system, which is by default an in-memory file system. Saved games will be lost when you close the web page. It is possible to interface other JavaScript code with the Emscripten file system to address this issue.
  • Compiling in Windows is not supported. The build process requires a Unix-like environment due to use of GNU Autotools. See Emscripten issue 2208.
  • Emscripten issue 1909 used to make large switch statements highly inefficient. It seems fixed now, but V8 JavaScript Engine issue 2275 prevents large switch statements from being optimized. Because of this, the simple, normal and prefetch cores are automatically transformed. Case statements for x86 instructions become functions, and an array of function pointers is used instead of the switch statements. The --enable-funarray configure option controls this and defaults to yes.
  • Most web browsers prevent access to other files when using file:// URLs. Use a web server such as python -m http.server instead.
  • In Firefox, ensure that dom.max_script_run_time is set to a reasonable value that will allow you to regain control in case of a hang.
  • Firefox may use huge amounts of memory when starting asm.js builds which have not been minified.
  • The FPU code uses doubles and does not provide full 80 bit precision. DOSBox can only give full precision when running on an x86 CPU.

Compiling

This code retains the ability to build a normal version of DOSBox, for running as an ordinary computer application. These instructions are focused on how to build using Emscripten, for running in a web browser.

Use the latest stable version of Emscripten. For more information see the the Emscripten installation instructions. Em-DOSBox depends on bug fixes and new features found in recent versions of Emscripten. Some Linux distributions have packages with old versions, which should not be used.

First, create ./configure by running ./autogen.sh. Then configure with emconfigure ./configure and build with make. DOSBox will be contained in src/dosbox.js, and if WebAssembly is enabled (the default), src/dosbox.wasm. Also, src/dosbox.html will be created, which is a simple web page that provides an interface to DOSBox. You can customize it or use DOSBox in your own web page. If you built with Asyncify enabled (the default), you can open src/dosbox.html via a web server, and run DOSBox. Without Asyncify DOSBox won't work like that, and you need to add files to run. See below for information on "Running DOS Programs".

This branch supports SDL 2 and uses it by default. Emscripten will automatically fetch SDL 2 from Emscripten Ports and build it. Use of make -j to speed up compilation by running multiple Emscripten processes in parallel may break this. Once SDL 2 has been built by Emscripten, you can use make -j. To use a different pre-built copy of Emscripten SDL 2, specify a path as in emconfigure ./configure --with-sdl2=/path/to/SDL-emscripten.

To use SDL 1, give a --with-sdl2=no or --without-sdl2 argument to ./configure. There will be more problems, like some keys not working and the mapper not displaying. Emscripten's SDL 2 is better.

Emscripten Asyncify is used by default. This enables more DOSBox features to work, and only causes a very small performance penalty.

If building with Emscripten versions between 1.29.4 and 1.39.17, you could try using Emterpreter Sync instead of Asyncify. This is depreciated, worse than Asyncify and not recommended. Such builds create a memory initialization file src/dosbox.html.mem needs to be in the same folder as dosbox.js. The memory initialization file is large. Serve it in compressed format to save bandwidth.

Running DOS Programs

When compiled using Asyncify, you can interactively load files. In DOSBox, first use mount c: / and c: commands to provide a space for new files. Then use the wget command to retrieve files from the same web server. You can use relative URLs, simply providing file names to load files from the same directory as the HTML page.

If not using Asyncify, or generally, for convenience, you can create web pages which run particular games. Such a page needs to load files into the Emscripten file system and give command line arguments to DOSBox. The simplest method is by using the included packager tools.

The normal packager tool is src/packager.py, which runs the Emscripten packager. It requires dosbox.html, which is created when building Em-DOSBox. If you do not have Emscripten installed, you need to use src/repackager.py instead. Any packager or repackager HTML output file can be used as a template for the repackager. Name it template.html and put it in the same directory as repackager.py.

The following instructions assume use of the normal packager. If using repackager, replace packager.py with repackager.py. You need Python 3 to run either packager.

If you have a single DOS executable such as Gwbasic.exe, place it in the same src directory as packager.py and package it using:

./packager.py gwbasic Gwbasic.exe

This creates gwbasic.html and gwbasic.data. Placing those in the same directory as dosbox.js and viewing gwbasic.html will run the program in a web browser:

Most browsers prevent access to the required data files while using file:// URLs. To get around this you can use Python's built in HTTP server and point the browser to http://localhost:8000.

python -m http.server 8000

If you need to package a collection of DOS files. Place all the files in a single directory and package that directory with the executable specified. For example, if Major Stryker's files are in the subdirectory src/major_stryker and it's launched using STRYKER.EXE you would package it using:

./packager.py stryker major_stryker STRYKER.EXE

Again, place the created stryker.html and stryker.data files in the same directory as dosbox.js and view stryker.html to run the game in browser.

You can also include a DOSBox configuration file that will be acknowledged by the emulator to modify any speed, audio or graphic settings. Simply include a dosbox.conf text file in the package directory before you run ./packager.py.

To attempt to run Major Stryker in CGA graphics mode, you would create the configuration file src/major_stryker/dosbox.conf and include this body of text:

[dosbox]
machine=cga

Then package it using:

./packager.py stryker-cga major_stryker STRYKER.EXE

Credits

Most of the credit belongs to the DOSBox crew. They created DOSBox and made it compatible with a wide variety of DOS games. Ismail Khatib got DOSBox to compile with Emscripten, but didn't get it to work. Boris Gjenero started with that and got it to work. Then, Boris re-implemented Ismail's changes a cleaner way, fixed issues and improved performance to make many games usable in web browsers. Meanwhile, Alon Zakai quickly fixed Emscripten bugs which were encountered and added helpful Emscripten features.

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