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AC-3

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 3D texture data can be moved between the computer's graphics controller and system memory.

accelerator

In DirectX® Video Acceleration, the functional unit that executes simple but high-rate operations such as IDCT, MCP, display format conversion.

access control entry

An individual entry in an access control list (ACL). An access control entry (ACE) contains an 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

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 also 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.

ACE

See access control entry and advanced computing environment.

ACK

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

ACL

See access control list.

ACPI

Advanced Configuration and Power Interface. 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.

Active Server Page (ASP page)

A server-side scripting environment used to create Web pages and build Web applications. Active Server Pages (ASP pages) are files that contain HTML tags, text, and script commands. ASP pages can call 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 BSS (IBSS) without an ESS.

adapter object

An object type available only in kernel mode, defined by the I/O Manager and supported by the underlying 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.

Adobe Font Metrics (AFM)

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

ADPCM

Adaptive Differential Pulse Code Modulation. An encoding format for storing audio information in a digital format.

ADSL

Asymmetric Digital Subscriber Line. 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.

advanced computing environment

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

advanced RISC computing

A 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 (Ntdetect) that detects the necessary devices to bootstrap the system. Ntdetect 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.

affinity

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.
The set of processors on which a particular interrupt is enabled in a given machine.

agent

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.

AI44

Alpha-Index 4-4. A video surface subtype used by decoders that implement 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.

alert

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.

AML

ACPI Machine Language. 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.

analog simultaneous voice and data (ASVD)

A signaling method that mixes voice and data.

anamorphic

Unequally scaled in vertical and horizontal dimensions.

animation

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

APC

See asynchronous procedure call.

ARC

See Advanced RISC Computing.

ASL

ACPI Source Language. 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.

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 IRPs, 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.

association

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.

ASVD

See analog simultaneous voice and data.

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

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

A kernel-defined control object which represents a procedure that is called asynchronously. Asynchronous procedure calls (APCs) are thread-context dependent; that is, they are queued to a particular thread for execution. There are three different types of APCs in the kernel:

User APC—used by certain asynchronous system services to allow user-mode applications or protected subsystems to synchronize thread execution with the completion of an operation or with an event such as a timer’s expiration. User APCs are, by default, disabled; that is, they are queued to the user-mode thread, but are not executed, except at well-defined points in the program. Specifically, they can be executed only when an application or protected subsystem has either called a wait service and has enabled alerts to occur, or called the test-alert service.
Kernel APC—normal kernel-mode APCs. They are much like a normal user APC except that they are executable by default. That is, they are enabled except when the thread is already executing a Kernel APC. (Note that a special Kernel APC always preempts a normal Kernel APC.)
Special Kernel APC—an APC that cannot be blocked, except by running at a raised IRQL. Special Kernel APCs are executed at APC_LEVEL IRQL (see IDT), in kernel mode. These types of APCs are used by the system to force a thread to execute a procedure in the thread’s context. For example, when the I/O Manager needs to get back into the context of the original requester of the I/O operation so that it can copy buffers, and so forth. In order to do this, the I/O Manager must be able to access the virtual address space of the thread/process, and the most efficient way to complete the operation is to be in the calling thread’s context.

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 synchronization routine.

AT

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

See also, TIA-602 and V.25ter.

ATA

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

ATAPI

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.

ATM

Asynchronous transfer mode. 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.

audio class

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

authentication

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

AV/C

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.