An audio standard for delivering 5.1 audio developed by Dolby Laboratories. This system compresses six channels of digital audio into 384 Kbps versus 4 Mbps uncompressed.

accelerated graphics port (AGP)

A technology that provides a dedicated, high-speed port through which large blocks of 3-D texture data can be moved between the computer's graphics controller and system memory.


In Microsoft DirectX Video Acceleration, the functional unit that executes simple but high-rate operations such as Inverse Discrete Cosine Transform(IDCT), motion-compensated prediction (MCP), and display format conversion.

access control entry (ACE)

An individual entry in an access control list (ACL). An access control entry (ACE) contains a SID and describes the access rights to a system resource by a specific user or group of users. Each object has a set of all ACEs, which is used to determine whether an access request to the object is granted.

See also security descriptor.

access control list (ACL)

An ordered list of access control entries (ACEs).

access point (AP)

A base station that controls an 802.11 LAN Basic Service Set (BSS). Access points are required for 802.11 infrastructure configurations.

access right

A permission granted to a process to manipulate a specified object in a particular way (by calling a system service). Different system object types support different access rights, which are stored in an object's access control list (ACL).

See access control list (ACL).

access violation

An attempt to execute a memory operation that is not allowed by the underlying page protection. Applies to memory operations; does not apply to checking user-mode access rights to objects by the security manager.

See also probe and structured exception handling.


See access control entry (ACE) and Advanced Computing Environment (ACE).


See acknowledge control packet (ACK).

acknowledge control packet (ACK)

A control packet used by a destination station to acknowledge the receipt of a packet of data.


See access control list (ACL) or asynchronous connectionless link (ACL).


See Advanced Configuration and Power Interface (ACPI).

ACPI Machine Language (AML)

Pseudocode for a virtual machine supported by an ACPI-compatible operating system and in which ACPI control methods are written. The AML encoding definition is provided in the Advanced Configuration and Power Interface Specification.

ACPI Source Language (ASL)

The programming-language equivalent for AML. ASL is compiled into AML images. The ASL statements are defined in the Advanced Configuration and Power Interface Specification.

Active Server Page (ASP)

A server-side scripting environment used to create Web pages and build Web applications. ASP pages are files that contain HTML tags, text, and script commands. ASP pages can call Microsoft ActiveX components to perform tasks such as connecting to a database, performing a business calculation, or creating a printer Web page.

ad hoc mode

Enables direct communication between stations. Ad hoc mode does not require an infrastructure. Stations operating in ad hoc mode operate in an independent basic service set (IBSS) without an Extended Service Set (ESS).

adapter object

An object type available only in kernel mode, defined by the I/O manager and supported by the underlying hardware abstraction layer (HAL) component. An adapter object represents a hardware bus adapter or DMA controller channel. Adapter objects "connect" different kinds of devices on the bus or DMA controller, each device (or kind of device) with its own driver.

For more information, see Adapter Objects and DMA.

adaptive differential pulse code modulation (ADPCM)

An encoding format for storing audio information in a digital format.

Adobe Font Metrics (AFM)

Files that contain font metrics and pair kerning information for Type 1 fonts. Also called PostScript fonts.


See adaptive differential pulse code modulation (ADPCM).


See asymmetric digital subscriber line (ADSL).

Advanced Computing Environment (ACE)

A consortium of hardware and software companies formed to promote an open computing environment.

Advanced Configuration and Power Interface (ACPI)

An abstract interface that defines a power management and configuration mechanism for hardware and operating systems. Part of the industry-wide OnNow Initiative, ACPI is defined in the industry-sponsored ACPI Specification.

advanced programmable interrupt controller (APIC)

A controller that routes interrupts in a multiprocessor system (virtual machine or standalone computer).

advanced RISC computing

A reduced instruction set computer (RISC)-based computer architecture standard, associated with the ACE consortium. Advanced RISC computing (ARC) machines include both RISC- and CISC-based platforms, and supply the following components to the operating system loader:

  • On ACPI-compliant platforms, a set of machine firmware that supports bootstrap loading and execution as an "abstracted" set of ARC routines and ARC devices.
  • On non-ACPI-compliant platforms, the NT-based operating system provides a hardware recognizer () that detects the necessary devices to bootstrap the system. queries the ROM BIOS and builds a hardware database that emulates the RISC-based ARC firmware.
  • On both kinds of platforms, a HAL that interfaces between the hardware and the system. See also HAL.

Device drivers for certain types of devices create an alias between the names of their device objects and the corresponding ARC device name by calling IoAssignArcName.

  1. A programmer-defined attribute of a process and/or thread on a multiprocessor platform:
    • For a process, affinity is the programmer-determined set of processors on which its threads are permitted to run.
    • For a thread, affinity by default is identical to that of the process to which it belongs; otherwise, its affinity must be a proper subset of the default process affinity.
  2. The set of processors on which a particular interrupt is enabled in a given machine.

Software that runs on a client computer for use by administrative software running on a server. Agents are typically used to support administrative actions, such as detecting system information or running services.


Alpha-Index 4-4. A video surface subtype used by decoders that implement Microsoft DirectX VA. AI44 has a bits-per-pixel value of 8. The character "A" represents 4 bits of transparency information (also known as per-pixel alpha). The character "I" is a 4-bit index into a 16-entry YUV palette. An AI44 surface allows for 16 different colors at 16 different transparency values, or 256 different pixel representations. In AI44, the alpha is stored in the hi-nibble.


A Boolean value that provides a way to break into a thread's execution at a point where the thread either:

  • Is in an alertable wait state (as specified when the wait service was called).
  • Polls the alerted flag.
alpha blending

A technique by which the color in a source bitmap is combined with that in a destination bitmap to produce a new destination bitmap.

GDI performs per-pixel alpha blending, according to the formula: Blend = Alpha * Source + (1 - Alpha) * Destination. This formula is used to compute the alpha blend at each pixel, for each of the red, green, and blue color channels.


See ACPI Machine Language (AML).

analog simultaneous voice and data (ASVD)

A signaling method that mixes voice and data.


Unequally scaled in vertical and horizontal dimensions.


The display of a series of graphic images, simulating motion. Animation can be frame-based or cast-based.

antenna diversity

Using two or more receiving antennas on a wireless device to reduce signal distortion.


A content delivery method in which only one client, typically the client nearest to the server, receives and responds to a network data packet. Otherwise similar to multicast, in that more than one client can be configured with the same address. Anycast is specific to IPv6 and is defined in RFC 2373.


See asynchronous procedure call (APC).


See advanced programmable interrupt controller (APIC).


See advanced RISC computing.


See ACPI Source Language (ASL).

aspect ratio

The relationship between the width and height of an image. When an image is displayed on different screens, the aspect ratio must be kept the same to avoid either vertical or horizontal stretching.

associated IRP

One of a set of IRP, created by the highest-level driver in a chain of layered drivers, that specifies some part of an I/O request to be sent on to a lower-level driver. When all associated IRPs in the set have been processed, the I/O manager completes the original request.


The exchange of information about the station and BSS capabilities. The process of association occurs after authentication is completed. A station that has completed the process of association can receive and transmit data frames.


See analog simultaneous voice and data (ASVD).

asymmetric compression

A system that requires more processing capability to compress than to decompress an image. It is typically used for the mass distribution of programs on media such as CD-ROM, where significant expense can be incurred for the production and compression of the program, but the playback system must be low cost.

Compare with symmetric compression.

asymmetric digital subscriber line (ADSL)

A method for moving data over regular phone lines. An ADSL circuit is much faster than a regular phone connection, and the wires coming into the subscriber's premises are the same (copper) wires used for regular phone service.

asynchronous connectionless link (ACL)

A packet-switching data link between two Bluetooth devices.

asynchronous I/O

A model for I/O in which the operations carried out to satisfy I/O requests do not necessarily occur in sequence. The application that originally made the request can continue executing (rather than waiting for its I/O to complete), the I/O manager or a higher-level driver can reorder I/O requests as they are received, and a lowest-level driver can start an I/O operation on a device before it has completed the preceding request, particularly in a multiprocessor machine.

asynchronous procedure call (APC)

A kernel-defined control object which represents a procedure that is called asynchronously within a particular thread context. For more information, see Asynchronous Procedure Calls.

asynchronous rendering

A display driver supports asynchronous rendering if it can hand off a batch of one or more DDI drawing operations to the graphics coprocessor, returning immediately to GDI. In order to perform graphics operations of its own, GDI must synchronize itself with the bitmap or surface. To do this, GDI calls a display driver-implemented synchronization routine called either DrvSynchronize (for device-managed surfaces) or DrvSynchronizeSurface (for arbitrary surfaces).

See also synchronization routine.

asynchronous transfer mode (ATM)

A transmission protocol that segments user traffic into small, fixed-size units called cells, which are transmitted to their destination, where they are reassembled into the original traffic. During transmission, cells from different users may be intermixed asynchronously to maximize utilization of network resources.


A protocol used to control modems. Command set A telecommunication (AT) device control protocol.


See AT attachment (ATA).


AT Attachment Packet Interface. A hardware and software specification that documents the interface between a host computer and CD-ROM drives using the ATA bus.

AT attachment (ATA)

An integrated bus usually used between host processors and disk drives. Used interchangeably with IDE.


See asynchronous transfer mode (ATM).

audio class

The class of filters that deal with Pulse Code Modulation (PCM) or similar digitized data or analog signals.


The interchange of information between the access point and the station, where each side validates a given password. Authentication is required before a station can join a BSS through an access point that the station has located.


The protocol used to communicate with Audio/Video Control (AV/C) devices. Examples of AV/C devices include Mini-Digital Video Camcorders and time-shifting digital recorders.



Send comments about this topic to Microsoft