was designed to be modular and flexible. When the graphical user interface (GUI) is opened and a VM is started, at least the following three processes are running:

• VBoxSVC, the service process which always runs in the background. This process is started automatically by the first client process and exits a short time after the last client exits. The first service can be , VBoxManage, VBoxHeadless, the web service amongst others. The service is responsible for bookkeeping, maintaining the state of all VMs, and for providing communication between components. This communication is implemented using COM/XPCOM.

  When we refer to clients here, we mean the local clients of a particular VBoxSVC server process, not clients in a network. employs its own client/server design to allow its processes to cooperate, but all these processes run under the same user account on the host operating system, and this is totally transparent to the user.

• The GUI process, VirtualBoxVM, a client application based on the cross-platform Qt library. When started without the --startvm option, this application acts as , displaying the VMs and their settings. It then communicates settings and state changes to VBoxSVC and also reflects changes effected through other means, such as the VBoxManage command.

• If the VirtualBoxVM client application is started with the --startvm argument, it loads the VMM library which includes the actual hypervisor and then runs a virtual machine and provides the input and output for the guest.

Any front end, or client, will communicate with the service process and can both control and reflect the current state. For example, either the VM selector or the VM window or VBoxManage can be used to pause the running VM, and other components will always reflect the changed state.

The GUI application, called , is only one of several available front ends, or clients. The complete list shipped with is as follows:

• VirtualBoxVM: The Qt front end implementing and running VMs.

• VBoxManage: A less user-friendly but more powerful alternative. See VBoxManage.

• VBoxHeadless: A VM front end which does not directly provide any video output and keyboard or mouse input, but enables redirection through the VirtualBox Remote Desktop Extension. See .

• vboxwebsrv: The web service process which enables control of an host remotely. This is described in detail in the Software Development Kit (SDK) reference. See .

• The Python shell: A Python alternative to VBoxManage. This is also described in the SDK reference.

Internally, consists of many more or less separate components. You may encounter these when analyzing internal error messages or log files. These include the following:

• IPRT: A portable runtime library which abstracts file access, threading, and string manipulation. Whenever accesses host operating features, it does so through this library for cross-platform portability.

• VMM (Virtual Machine Monitor): The heart of the hypervisor.

• EM (Execution Manager): Controls execution of guest code.

• TRPM (Trap Manager): Intercepts and processes guest traps and exceptions.

• HM (Hardware Acceleration Manager): Provides support for VT-x and AMD-V.

• GIM (Guest Interface Manager): Provides support for various paravirtualization interfaces to the guest.

• PDM (Pluggable Device Manager): An abstract interface between the VMM and emulated devices which separates device implementations from VMM internals and makes it easy to add new emulated devices. Through PDM, third-party developers can add new virtual devices to without having to change itself.

• PGM (Page Manager): A component that controls guest paging.

• TM (Time Manager): Handles timers and all aspects of time inside guests.

• CFGM (Configuration Manager): Provides a tree structure which holds configuration settings for the VM and all emulated devices.

• SSM (Saved State Manager): Saves and loads VM state.

• VUSB (Virtual USB): A USB layer which separates emulated USB controllers from the controllers on the host and from USB devices. This component also enables remote USB.

• DBGF (Debug Facility): A built-in VM debugger.

• emulates a number of devices to provide the hardware environment that various guests need. Most of these are standard devices found in many PC compatible machines and widely supported by guest operating systems. For network and storage devices in particular, there are several options for the emulated devices to access the underlying hardware. These devices are managed by PDM.

• Guest Additions for various guest operating systems. This is code that is installed from within a virtual machine. See Guest Additions.

• The "Main" component is special. It ties all the above bits together and is the only public API that provides. All the client processes listed above use only this API and never access the hypervisor components directly. As a result, third-party applications that use the Main API can rely on the fact that it is always well-tested and that all capabilities of are fully exposed. It is this API that is described in the SDK. See .