rdp-studio / em-dosbox

DOSBox ported to Emscripten

About

DOSBox is an open source DOS emulator designed for running old games. Emscripten compiles C/C++ code to JavaScript. 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 emterpreter sync. A few programs can still run into problems due to paging exceptions.

This is a highly experimental version that implements most of the incoming branch of DOSBox, there are still sections to port (namely a rewrite of the SDL____.cpp code).

Other issues

• This version uses the latest "incoming" SVN branch of DOSBox hosted at Sourceforge, updated to 20/09/2020, the SDL UI code has not yet been merged, the application should run "fine" with the old codebase.
• 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.
• Compiling in Windows is supported via Windows Subsystem for Linux. The build process requires a Unix-like environment due to use of GNU Autotools. See Emscripten issue 2208.
• Currently the runtime expects a canvas to be present with the id of "canvas" because a querySelector exists as #canvas. This is automatically generated but it's something to be aware of.
• The same origin policy prevents access to data files when running via a file:// URL in some browsers. Use a web server such as python -m SimpleHTTPServer 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.
• 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

The use of the legacy asmjs backend of emscripten is not recommended and not [currently] tested, consider another fork. This version specifically targets web assembly. To build you will need the LATEST version of Emscripten (as of 17/09/2020).

First we have to build GNU compatible configure scripts, this requires Autotools, automake, autoconf etc

./autogen.sh

We then need to run the configure script targetting web assembly

emconfigure ./configure --enable-wasm --disable-opengl --host=none-none-none

We can then build the application

emmake make

All relevant files are located in src/

Running basic DOS shell

Enter the src/ directory

cd src

And run any compliant static HTTP Server, for example:

python2 -m SimpleHTTPServer

With your browser, set url to http://localhost:8000/dosbox.html to be presented with a basic DOS shell:

Running DOS Programs

To run DOS programs, you need to provide a suitable web page, 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 2 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:

Some browsers have a same origin policy that prevents access to the required data files while using file:// URLs. To get around this you can use Python's built in Really Simple HTTP Server and point the browser to http://localhost:8000.

python -m SimpleHTTPServer 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.