This topic defines key terms used throughout this guide.


advanced programmable interrupt controller (APIC)

A controller that routes interrupts in a multiprocessor system, which can be either a VM or a physical computer.

child partition

Any partition that is created by the parent (or root) partition.


See logical processor.

In this guide, core is sometimes used interchangeably with virtual processor, especially in graphics. This usage will be corrected in a future edition of this guide.

device virtualization

A software technology that lets a hardware resource be abstracted and shared among multiple consumers.

emulated device

A virtualized device that mimics an actual physical hardware device so that guests can use the typical drivers for that hardware device. Emulated devices are less efficent than synthetic devices, but emulated devices provide support for “unenlightened” operating systems that do not have integration components installed.


An optimization to a guest operating system to make it aware of VM environments and tune its behavior for VMs. Enlightenments help to reduce the cost of certain operating system functions such as memory management. Enlightenments are accessed through the hypercall interface. Enlightened I/O can utilize the VMBus directly, bypassing any device emulation layer. An operating system that takes advantage of all possible enlightenments is said to be “fully enlightened.”

guest operating system

The operating system (OS) software running in a child partition. Guests can be a full-featured operating system or a small, special-purpose kernel.

Hypercall Interface

An application programming interface (API) that partitions use to access the hypervisor.


A layer of software that sits just above the hardware and below one or more operating systems. Its primary job is to provide isolated execution environments called partitions. The hypervisor controls and arbitrates access to the underlying hardware.


An asynchronous signal from hardware indicating the need for attention or a synchronous event in software indicating the need for a change in execution.


Input Output Memory Management Unit, remaps physical memory addresses to the addresses that are used by the child partitions

integration components (IC)

A set of services and drivers that improve performance and integration between the physical and virtual machines. Integration components enable guest operating systems to use synthetic devices, significantly reducing the overhead needed to access devices. See also enlightenment.

integration services

See integration components.

logical processor

A CPU that handles one thread of execution (instruction stream). A logical processor can be a core or a hyper-thread. There can be one or more logical processors per core (more than one if hyper-threading is enabled) and one or more cores per processor socket.

logical unit number (LUN)

A number used to identify a disk on a given disk controller or within a SAN.

parent partition

See root partition.


A virtual machine (VM) created by the hypervisor software. Each partition has its own set of hardware resources (CPU, memory, and devices). Partitions can own or share hardware resources.

pass-through disk access

A representation of an entire physical disk as a virtual disk within the guest. The data and commands are “passed through” to the physical disk (through the root partition’s native storage stack) with no intervening processing by the virtual stack.

root partition

A partition that is created first and owns all the resources that the hypervisor does not own including most devices and system memory. It hosts the virtualization stack and creates and manages the child partitions. The root partition is also known as the parent partition.

storage area network (SAN)

SANs are networks of storage devices. A SAN connects (typically) multiple servers and storage devices on a single high-speed fiber optic network.

synthetic device

A virtualized device with no physical hardware analog so that guests do not need a driver (virtualization service client) to that synthetic device. Drivers for synthetic devices are included with the integration components (enlightenments) for the guest operating system. The synthetic device drivers use the VMBus to communicate with the virtualized device software in the root partition.

virtual hard disk (VHD)

A virtual hard disk is a file stored on the physical computer’s native disk system. From within a virtual machine, the VHD appears as though it were a physical hard disk. VHDs are based on the Virtual Hard Disk Image Format Specification. For more information about the Virtual Hard Disk Format Specification see

virtual machine (VM)

A virtual computer that was created by software emulation and has the same characteristics as a real computer.

virtual machine management service (VMMS)

The VMMS is part of the Virtualization Windows Management Instrumentation (WMI) Provider interface. Management tools connect to this service during runtime to gather data about active partitions.

virtual machine worker process (VMWP)

Each VM has a worker process that runs in the parent partition. VMWPs run code for saving state, accessing emulated devices, and controlling the VM.

virtual processor

A virtual abstraction of a processor that is scheduled to run on a logical processor. A VM can have one or more virtual processors.

virtualization service client (VSC)

A software module that a guest loads to consume a resource or service. For I/O devices, the virtualization service client can be a device driver that the operating system kernel loads.

virtualization service provider (VSP)

A provider, exposed by the virtualization stack, that provides resources or services such as I/O to a child partition.

virtualization stack

A collection of software components in the root partition that work together to support VMs. The virtualization stack works with and sits above the hypervisor. It also provides management capabilities.


A channel-based communication mechanism used for inter-partition communication and device enumeration on systems with multiple active virtualized partitions.