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.
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. - 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. - 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 SimpleHTTPServerinstead. - 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
Use the latest stable version of Emscripten (from the master branch). 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.
This will create src/dosbox.js which contains DOSBox and src/dosbox.html,
a web page for use as a template by the packager. These cannot be used as-is.
You need to provide DOSBox with files to run and command line arguments for
running them.
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.
Emscripten emterpreter sync is used by default. This enables more DOSBox
features to work, but requires an Emscripten version after 1.29.4 and may
cause a small performance penalty. To disable use of emterpreter sync,
add the --disable-sync argument to ./configure. When sync is used,
the Emscripten
memory initialization file
is enabled, which means 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.
WebAssembly
WebAssembly is a
binary format which is meant to be a more efficient replacement for asm.js.
To build to WebAssembly instead of asm.js, the -s WASM=1 option needs to
be added to the final emcc link command. There is no need to rebuild the
object files. You can do this using the --enable-wasm option when running
./configure. Since WebAssembly is still under very active development, use
the latest incoming Emscripten and browser development builds like
Firefox Nightly and
Chrome Canary.
Building with WebAssembly changes the dosbox.html file. Files packaged
using a dosbox.html without WebAssembly will not work with a WebAssembly
build.
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.