= The VirtualBox architecture =

[wiki:"Virtualization"] is, by nature, extraordinarily complex, especially so on x86 hardware. Understanding the !VirtualBox source code therefore requires, at least for some components, a great deal of understanding about the details of the x86 architecture as well as great knowledge about the implementations of the host and guest platforms involved.

There are several ways how to approach how !VirtualBox works. This document shall describe them in order of increasing complexity.

== The !VirtualBox modules: a bird's eye view ==

When you start the !VirtualBox graphical user interface (GUI), at least one extra process gets started along the way -- the !VirtualBox service, normally dubbed `VBoxSVC`. The !VirtualBox GUI is only one of several "frontends" to the !VirtualBox "core", which is encapsulated in several libraries (among them `VBoxVMM.dll`, or `VBoxVMM.so`).

Once you start a virtual machine (VM) from the GUI, you have three processes running:

 1. `VirtualBox`, the GUI for the main window;
 1. `VBoxSVC`, a daemon running in the background that keeps track of all the processes involved (this was automatically started by the first GUI process);
 1. another `VirtualBox` process that was started with the `-startvm` parameter, which means that its GUI process acts as a shell for a VM.

(On Linux, there's another process called `VBoxXPCOMIPCD` which is necessary for our XPCOM implementation to work. We will ignore this for now; see [wiki:"COM-XPCOM interoperability"] for details.)