V.25 is the ITU-T standard for Serial Asynchronous Automatic Dialing and Control, commonly known as the "AT" command set.


V.4 is an ITU-T standard for the general structure of signals over the public telephone network. The V.4 standard governs the data transmission of signals created from international alphabet number 5 code.

valid page

A virtual page that is currently in physical memory.

See also invalid page.

value entry

A named value with assigned data within a registry key.

See also registry and key object.


See vertical blanking interval (VBI).


See virtual block number (VBN).


In Windows, a 32-bit protected-mode cache driver.


See volume control block (VCB).


In Windows, a 32-bit protected-mode communications driver.


See video cassette recorder (VCR).


See virtual device driver (VDD).


See virtual DOS machine (VDM).

vertical blanking interval (VBI)

The length of time required to move a graphics display's electron beam from the bottom scan line back up to the top scan line. Also refers to digital data encoded on video lines that are not displayed during this interval. For example, closed-captioned data can be encoded on VBI line 21.


See video graphics array (VGA).

video cassette recorder (VCR)

An analog magnetic recording and playback machine. Usually used for recording and viewing full-motion video; also useful as a data backup device.

video codec

An algorithm for compression and decompression of video data. Can be implemented in hardware or software or both. Commonly used video codecs include AVI and MPEG.

Video Electronics Standards Association (VESA)

An organization that sets standards for video devices.

video graphics array (VGA)

A video adapter that supports 640x480-pixels color resolution. Video display standard for boot devices under Windows operating systems. Provides medium-resolution text and graphics.

video mixer-renderer (VMR)

A DirectShow sink filter that displays the contents of a video stream.

video port extensions (VPE)

The video port extensions to DirectX. Driver developers for devices with a hardware video port should implement these extensions. The hardware video port is a dedicated connection between video devices, typically between an MPEG device or NTSC decoder and the video card. This dedicated connection carries horizontal sync (Hsync) and vertical sync (Vsync) information with the video data. The hardware video port and overlay can use this sync information to flip automatically between multiple buffers, writing to one surface while the overlay displays another. This allows tear-free video without burdening the application.

video request packet (VRP)

A mechanism used to communicate device I/O control requests from a display driver to its corresponding adapter-specific miniport driver. For example, when a display driver calls EngDeviceIoControl, this function calls a system service causing the NT-based operating system I/O manager to set up an IRP and call the Microsoft-supplied video port driver with that IRP. The video port driver uses the IRP to set up a VRP in a VIDEO_REQUEST_PACKET structure. The video port driver then calls the corresponding video miniport driver's StartIo entry point with the VRP.

VideoPort routines

An interface to adapter-specific miniport drivers exported by the system-supplied video port driver. Video miniport drivers call these routines to obtain all system support they need to carry out I/O operations.


A whole or partial mapping of a section object in the virtual address space of a process. For more information, see Section Objects and Views.

virtual block number (VBN)

A VBN identifies a block (in other words, "sector") relative to the start of a file. For a file with N blocks of data, the corresponding VBNs are numbered 1 through N.

virtual device driver (VDD)

A device driver that operates in user mode and communicates with a corresponding device driver in kernel mode. A VDD supports only special-purpose hardware devices from an MS−DOS application. The VDD acts as a layer between MS-DOS applications and the hardware attached to the machine running an NT-based operating system.

virtual device driver (VxD)

A device driver that runs on non-NT-based Windows. Such device drivers run at the privileged ring 0 protected mode of the microprocessor. They can extend the services of the Windows kernel, supervise hardware operations, or perform both functions. Driver file names generally use the VxD format, where x is a string that represents the supported device or service.

virtual DOS machine (VDM)

A protected subsystem that emulates MS-DOS and 16-bit Windows.

See Windows NT virtual DOS machine and NTVDM.

virtual machine (VM)

Software that mimics the performance of a hardware device. For example, a software program that allows applications written for an Intel processor to be run on a Motorola chip interprets the Intel machine instructions, becoming a virtual Intel machine.

virtual memory

A view of memory that does not necessarily correspond to the underlying physical memory structure. For example, a given range of virtual addresses might be mapped to and backed by some number of discontiguous physical pages, even though the corresponding virtual pages can be accessed as a single, contiguous range.


See virtual machine (VM).


See volume map control block (VMCB).


See video mixer-renderer (VMR).


Volume is the general term that refers to all of the following entities that you can create and use on a computer running Windows:

  • Primary partition
  • Logical drive in an extended partition
  • Volume set
  • Stripe set
  • Mirror set
  • Stripe set with parity

A volume has a single drive letter assigned to it, and is formatted for use by a file system.

volume control block (VCB)

An internal file system structure in which a file system maintains state about a mounted volume.

volume file

A virtual file, maintained by certain file systems, whose contents map metadata structures of the on-disk file system. A volume file is a type of stream file.

volume map control block (VMCB)

An opaque structure that stores VBN-to-LBN mappings for an IFS's volume file.

volume parameter block (VPB)

A VPB is a structure, defined by the I/O manager, that maps a file system's volume device object to the device or partition upon which the volume is mounted. The file system's device object is actually used to represent the volume (VPB) mounted on the actual device (physical device object). Device objects for physical disks, tapes, CD ROMs, and RAM disks have associated VPBs.

See also mount.


See volume parameter block (VPB).


See video request packet (VRP).


See virtual device driver (VxD).



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