PCjs Documentation

PCjs is a full-featured IBM PC, PC XT and PC AT emulator written entirely in JavaScript. After you’ve read the Documentation, play with the Demos.

The simulation above features an Intel 8088 running at 4.77Mhz, with 64Kb of RAM and an IBM Monochrome Display Adapter. To create your own simulation, all you need is the PCjs script and a machine XML file, along with a couple of XSL and CSS support files (included in the ZIP file below).

Creating Machine XML Files

A PCjs machine XML file defines all a machine’s components. Components include:

Here’s a simple machine XML file that includes an 8088 CPU and 16Kb of RAM:

<machine id="ibm">
	<computer id="pc" name="IBM PC"/>
	<cpu id="cpu8088" model="8088"/>
	<ram id="ramLow" addr="0x00000" size="0x04000"/>

However, that machine isn’t usable, since it lacks a keyboard, screen, or any code (ROMs) to execute.

A simple machine definition that actually works might look like:

<machine id="ibm" class="pc" width="720px">
	<computer id="pc" name="IBM PC"/>
	<cpu id="cpu8088" model="8088"/>
	<ram id="ramLow" addr="0x00000" size="0x04000"/>
	<rom id="romBASIC" addr="0xf6000" size="0x8000" file="ibm-basic-1.00.json"/>
	<rom id="romBIOS" addr="0xfe000" size="0x2000" file="1981-04-24.json"/>
	<keyboard id="keyboard"/>
	<video id="videoMDA" model="mda" screenwidth="720" screenheight="350" charset="ibm-mda-cga.json">
		<name>Monochrome Display</name>
	<chipset id="chipset" model="5150" sw1="01000001" sw2="11110000"/>

Here is a demo of this machine’s XML file.

Machine definitions can also include visual elements. For example, we can include a “Run” button with the CPU component. Note that as soon as the machine is ready and the CPU starts running, the “Run” button will change to “Halt”.

<machine id="ibm" class="pc" width="720px">
	<computer id="pc" name="IBM PC"/>
	<cpu id="cpu8088" model="8088">
		<control type="button" class="input" binding="run">Run</control>

Next, we can add a Floppy Disk Controller (FDC) component. And since we want to be able to “load” and “unload” floppy disks at will, we’ll include some UI controls.

<machine id="ibm" class="pc" width="720px">
	<fdc id="fdcNEC" automount="{A: {name: 'PC-DOS 1.0', path: 'pcdos-1.00.json'}}">
		<control type="container">
		<control type="list" class="input" binding="listDrives"/>
			<control type="list" class="input" binding="listDisks">
				<disk path="">None</disk>
				<disk path="pcdos-1.00.json">PC-DOS 1.0</disk>
			<control type="button" class="input" binding="loadDrive">Load</control>

Here is a demo of the updated machine’s XML file.

Loading Machine XML Files

Inside a web page, add a <div> to contain the machine, load the pc.js script (pc-dbg.js if you need the PCjs Debugger), and then call window.embedPC():

<div id="sample2"/>
<script type="text/javascript" src="pc.js"/>
<script type="text/javascript">
	window.embedPC("sample2", "sample2.xml", "components.xsl");

In fact, this is exactly what we did in the previous demo.

window.embedPC() requires 3 parameters:

  • The id of the machine <div> (e.g., “sample2”);
  • The url of the machine XML file (e.g., “sample2.xml”);
  • The url of the components XSL file (e.g., “components.xsl”)

components.xsl (and the corresponding components.css) are included with the PCjs scripts in the samples download below.

Using the PCjs Debugger

To create a configuration that includes the PCjs Debugger, you need to:

  • Add Debugger and Control Panel components to the machine XML file;
  • Add debugger controls to the Control Panel, such as Run, Step, Reset, etc;
  • Change your web page to load pc-dbg.js instead of pc.js.

Take a look at the sample3a demo (with XML file) for an example.

The debugger gives you access to more capabilities than mere debugging. For example, you can use the load command to load diskette sectors into memory (“l <addr> <drive> …”) or dump an entire diskette as JSON (“l json <drive>”). You can also halt a machine (“h”) and dump its entire state as JSON (“d state”). You can save that state in a .json file, and then use that state to initialize a new machine (as long as it uses the same machine id).

In fact, the sample3b demo (with XML file) does just that, using JSON dumps created from sample3a after starting VisiCalc. See the state attribute on the Computer component for more information on state files.

Running PCjs On Your Own Server

All of the demos described above are available for download.

Creating PCjs-Compatible Disk Images

If you have (or find) an IMG disk image file on a server, the PCjs web server provides a DiskDump API via endpoint “/api/v1/dump” that creates PCjs-compatible disks in JSON:


For example, let’s say you found a disk image online:


To convert it to a PCjs-compatible JSON format, use the following command:


Save the resulting JSON file to a folder on your server, and then update your machine XML file(s) to use that file. If necessary, you can also reverse the process, converting a JSON disk image back into an IMG file:


Although PCjs will accept IMG disk image files, it must call the DiskDump API to convert the image every time it’s loaded, so it’s much faster and more efficient to use pre-converted JSON-encoded disk images.

Remember that PC and PC XT machines supported only 160Kb diskettes (on any version of PC-DOS), 320Kb diskettes (on PC-DOS 1.1 and higher), and 180Kb and 360Kb diskettes (on PC-DOS 2.0 and higher).

The 1.2Mb diskette format was introduced with the PC AT, and 720Kb and 1.44Mb diskette formats were supported later on 8Mhz PC AT and PS/2 models. So, when using any of these larger formats, be sure you’re also using a compatible machine configuration.

Learn more about PCjs disk images here.