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To contribute to the emulators package do the following:

  1. Checkout emulators repository

    git clone

  2. Install emscripten sdk, and confgure environment to use it.

  3. Install cmake and gulp 4.

  4. Now you can build everything with gulp command

Native part of emulators is plain cmake project, you can open it in your favorite editor. Project have the following targets:

  1. sokol - js-dos v7 native version: dosbox + UI based on sokol. This version is exactly the same as the web version. You should use this target to contribute in js-dos v7.
  2. direct - target is used to build the web-direct version of js-dos v7. You can compile it only with emscripten.
  3. worker - target is used to build the web-worker version of js-dos v7. You can compile it only with emscripten.
  4. dosbox - original version of dosbox (UI based on SDL). You can use it to compare behaviour between original dosbox and js-dos v7.
  5. libzip - shared codes that contain implementation of zip.
  6. jsdos - shared codes that contain implementation of dosbox.


The idea of new js-dos v7 is that all targets (native and web) have exactly same way to communicate between client (native UI, browser UI) and dosbox.


For simplicity, you can think that the server is a dosbox. In the future servers can be implemented with different emulators. Now we support only dosbox implementation (look at jsdos.cmake).


Client should run this function when it's ready to start dosbox. This function will start the emulator. Client should prepare file system for dosbox it expects that cwd contains .jsdos/dosbox.conf file.

So you need to extract js-dos bundle in some directory and start sokol binary in this directory, and it will act exactly in the same way as direct/worker dosbox.

server_add_key(keycode, pressed, timeMs)#

This function adds keycode to the queue. They will be processed when dosbox poll keyboard events.


Terminates execution of dosbox and free resources.


Direct, worker, and sokol implementations share the same code for server part. But they are completely different, because they implement UI and sound system for different platforms. In original dosbox this was made by SDL, it was hard-coupled with dosbox. js-dos clearly detaches the emulator from its ui. You can easily add new UI/sound system to dosbox.

For example, let's look on sokol UI implementation. You can use it to debug and develop new features for js-dos. Worker is a primary web implementation for js-dos v7. sokol implementation tries to work in similar way: we start dosbox emulator in main thread and client in new thread.

void runRuntime() {
std::thread client(client_run);

client_frame_set_size(width, height)#

When the server starts it will send the frame size of the dosbox window by invoking client_frame_set_size. You should allocate rgba buffer to store frame content. This function will be called each time when dosbox window size is changed.

extern int frameWidth = 0;
extern int frameHeight = 0;
extern uint32_t *frameRgba = 0;
void client_frame_set_size(int width, int height) {
std::lock_guard<std::mutex> g(mutex);
if (frameRgba) {
delete[] frameRgba;
frameWidth = width;
frameHeight = height;
frameRgba = new uint32_t[width * height];

client_frame_update_lines(lines, count, rgba)#

This method will be called each time if contents of dosbox window are changed. dosbox implementation will send only changed lines. You need to update your frame buffer correctly.

Dirty region format (lines argument):

  • line number [0, height)
  • count of changed lines
  • offset in passed buffer (rgba argument)
void client_frame_update_lines(uint32_t *lines, uint32_t count, void *rgba) {
std::lock_guard<std::mutex> g(mutex);
if (!frameRgba) {
for (uint32_t i = 0; i < count; ++i) {
uint32_t start = lines[i * 3];
uint32_t count = lines[i * 3 + 1];
uint32_t offset = lines[i * 3 + 2];
memcpy(&frameRgba[start * frameWidth], (char *)rgba + offset,
sizeof(uint32_t) * count * frameWidth);

Implementing client_frame_set_size and client_frame_update_lines is enough to render dosbox window:

// ...
appDescription.frame_cb = []() { sokolFrame(); };
// ...
void sokolFrame() {
std::lock_guard<std::mutex> g(mutex);
// ...
sg_image_content imageContent = {};
imageContent.subimage[0][0] = {};
imageContent.subimage[0][0].ptr = frameRgba;
imageContent.subimage[0][0].size =
(state->width) * (state->height) * (int)sizeof(uint32_t);
sg_update_image(state->bind.fs_images[0], &imageContent);
sg_begin_default_pass(&state->pass, state->width, state->height);
sg_draw(0, 4, 1);
renderedFrame = frameCount;


Called when the dosbox needs to initialize the sound system.

void client_sound_init(int freq) {
saudio_desc audioDescription = {};
audioDescription.sample_rate = static_cast<int>(freq);
audioDescription.num_channels = 1;

client_sound_push(samples, num_samples)#

This method is called each time when new sound samples should be pushed to audio device. With sokol implementation is very simple:

void client_sound_push(const float *samples, int num_samples) {
saudio_push(samples, num_samples);

client_stdout(data, amount)#

This method will be called each time when dosbox prints something to its console.

Communicate to server#

Each time when key is pressed we should send event to dosbox:

// ...
appDescription.event_cb = [](const sapp_event *event) {
switch (event->type) {
// ...
void keyEvent(const sapp_event *event) {

When user closes sokol window we need to stop server:

appDescription.cleanup_cb = []() { server_exit(); };

That is. Check complete source of sokol implementation.


If the gulp command is finished successfully then you can run emulators tests. To do this, run a static web server to host the dist directory. For example, with http-server:

hs dist

and open test page in browser:


all tests should pass.

Running native js-dos v7#

As said above, you need to compile a sokol target with your favorite C++ toolkit. It will generate sokol executable. Next, you need to download some js-dos bundle for example digger.

js-dos bundle is a plain zip archive, you need to extract it in some folder. After that you should run sokol executable from that folder (cwd must be the root of the extracted bundle).

Using Docker#

You can use docker image to develop emulators core. The image has already configured everything to build emulators core and start emulators tests.

Build image#

docker build -t emulators .

Test image#

docker run -p 8080:8080 -ti emulators

Open http://localhost:8080 in browser, all tests should pass


Run inside the project directory:

docker run -v `pwd`/src:/app/src -v `pwd`/test:/app/test -v `pwd`/dist:/app/dist -ti emulators bash
source /emsdk/
gulp OR ./node_modules/.bin/tsc --watch

Use your code editor to edit the content of src and test. In the docker VM you can run gulp to build everything into dist OR use ./node_modules/.bin/tsc --watch if you need only compile time checks.

Contributing on github#

To contribute your code please create PR on github, and check if all tests passed.