IBM PC Emulation Module (PCx86)


PCx86 is the IBM PC emulation module powering all our IBM PC Machines.

This module divides PC functionality into variety of logical and visual components. In general, each JavaScript file is responsible for a single component or set of related components (eg, chipset.js). Most components represent familiar PC devices, such as video cards, disk drive controllers, etc.

Component is an overloaded term, since Component is also the name of the shared base class in component.js used by most machine components. A few low-level components (eg, the Memory and State components, the Card class of the Video component, the Color and Rectangle classes of the Panel component, etc) do not extend Component, so don’t assume that every PCx86 object has access to component.js methods.

Examples of non-device components include visual components like panel.js and debugger.js, and sub-components like x86ops.js and x86func.js, which separate the CPU functionality of x86.js into more manageable pieces.

These components should always be loaded or compiled in the order listed by the pcx86.scripts property in machines.json, which includes all the necessary shared components as well.

At the time of this writing, the recommended order is:

Some of the components can be reordered or even omitted (eg, debugger.js or embed.js), but you should observe the following:

  • component.js must be listed before any component that extends Component
  • panel.js should be loaded early to initialize the Control Panel (if any) as soon as possible
  • computer.js should be the last device component, as it supervises and notifies all the other device components

To minimize ordering requirements, the init() handlers and constructors of all components should avoid referencing other components. Device components should define an initBus() notification handler, which the Computer component will call after it has created/initialized the Bus component.

Major Features

[List of major existing features goes here]

Experimental Features

BackTrack Support

One experimental PCx86 feature is known as BackTrack Support, or simply BackTracks. When BackTracks are enabled, every memory location (at the byte level) and every general-purpose byte register may have an optional link back to its source. These links are called BackTrack indexes.

All the code that a virtual machine initially executes enters the machine either via ROM or disk sectors, and as that code executes, the machine is loading data into registers from memory locations and/or I/O ports and writing the results to other memory locations and/or I/O ports. BackTracks keep track of that data flow, allowing us to examine the history of any piece of data at any time, down to the byte level; while this feature could be extended to the bit level, it would make the feature dramatically more expensive, both in terms of size and speed.

A BackTrack index is encoded as a 32-bit value with three parts:

  • Bits 0-8: 9-bit BackTrack object offset (0-511)
  • Bits 9-15: 7-bit type and access info
  • Bits 16-30: 15-bit BackTrack object number (1-32767, 0 reserved for dynamic data)

This represents a total of 31 bits, with bit 31 reserved.

For example, look at one of the last things a ROM does during boot: loading a disk sector into RAM. It will be up to the drive controller (or DMA controller, if used) to create a BackTrack object representing the sector that was read, adding that object to the global BackTrack object array, and then associating the corresponding BackTrack index with the first byte of RAM where the sector was loaded. Subsequent bytes of RAM containing the rest of the sector will refer to the same BackTrack object, using BackTrack indexes containing offsets 1-511.

WARNING: BackTrack support is controlled by a global define (BACKTRACK in defines.js) which is false in the compiled version of PCx86, because it imposes a huge performance penalty. The only way to use the feature is with a machine explicitly configured to use “uncompiled” source and with the PCjs Debugger enabled (since, without the debugger, the feature is more or less useless).

Machines using “uncompiled” source also enable additional checks controlled by the global define DEBUG, which is another reason those machines are much slower. And DEBUG must be true for BACKTRACK to be enabled. As an aside, you can selectively disable either of those settings at run-time, by adding debug=false or backtrack=false parameters to an “uncompiled” machine’s URL; make sure there’s also a ? separating the original URL from any parameters.


Microsoft Bus Mouse

See this Microsoft Bus Mouse implementation, written by Michal Necasek for Oracle’s VirtualBox.

It references two Microsoft KnowledgeBase (KB) Articles of note:

Over the years, Microsoft has sporadically published and then deleted those and many other KnowledgeBase articles, for reasons known only to itself. See the blog post Corporations Are Crappy Archivists for further discussion.