A

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.

ACPI

Advanced Configuration and Power Interface. A specification that defines a new interface to the system board that enables the operating system to implement operating-system directed power management and system configuration. Following the ACPI allows system manufacturers to build systems consistent with the OnNow design initiative for instantly available PCs.

ACPI hardware

Computer hardware with the features necessary to support operating system power management and with the interfaces to those features described using the Description Tables as specified in Advanced Configuration and Power Interface Specification.

ACPI Name Space

A hierarchical tree structure in operating-system–controlled memory that contains named objects. These objects can be data objects, control method objects, bus and device package objects, and so on. All the information in the ACPI Name Space comes from the Differentiated System Description Table.

adapter

See device.

adaptive compression

Data compression software that continually analyzes and compensates its algorithm, depending on the type and content of the data and the storage medium.

add-on devices

Devices that are traditionally added to the base PC system to increase functionality, such as audio, networking, graphics, SCSI controller, and so on. Add-on devices fall into two categories: devices built onto the system board and devices on expansion cards added to the system through a system board connector such as PCI.

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.

AEC

Acoustic echo cancellation.

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.

algorithm

In compression software, refers to a specific formula used to compress or decompress video or other data.

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 section 16 of the Advanced Configuration and Power Interface Specification.

analog

A method of signal representation by an infinitely smooth universe of numeric values. Measurements that are characterized as analog include readings of voltage and current. Analog devices are characterized by dials and sliding mechanisms. Compare with digital.

analog video

A video signal that represents an infinite number of smooth gradations between given video levels. Compare with digital video.

anamorphic

Unequally scaled in vertical and horizontal dimensions.

ANSI

American National Standards Institute. A standards-setting, non-governmental organization that develops and publishes standards for voluntary use in the United States.

anti-aliasing

A form of interpolation used in graphics display technology when combining images; pixels along the transitions between images are averaged to provide a smooth transition.

API

Application programming interface. A set of routines that an applications program uses to request and carry out lower-level services performed by a computer operating system.

APM

Advanced Power Management. 1. A software interface (defined by Microsoft and Intel) between hardware-specific power management software (such as that located in a system BIOS) and an operating system power management driver. 2. An abridgment of the APM BIOS Interface Specification title.

arbitrator

1. A software module in Windows that handles the allocation of hardware resources among devices. 2. Under Windows 95, the Plug and Play device driver responsible for allocating a specific resource among all drivers that require the resource. For example, VDMAD is a DMA-channel arbitrator, and VPICD has services for allocating IRQ lines. Windows 95 provides arbitrators for standard I/O, memory, hardware interrupt, and DMA-channel resources.

architecture

A general term referring to the structure of all or part of a computer system. Also covers the design of system software, such as the operating system, as well as referring to the combination of hardware and basic software that links machines on a computer network.

artifact

An unintended, unwanted visual aberration in a video image.

ASCII

American Standard Code for Information Interchange. The most popular coding method used by small computers for converting letters, numbers, punctuation, and control codes into digital form.

ASIC

Application-specific integrated circuit.

ASL

ACPI Source Language. The programming-language equivalent for AML. ASL is compiled into AML images. The ASL statements are defined in section 15 of the Advanced Configuration and Power Interface Specification.

aspect ratio

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

assigned configuration

Drivers examine this portion of the device node to determine which resources have been allocated for the device. For Plug and Play cards, the assigned resources for a device can change dynamically or from one system start-up to the next.

asymmetrical 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 symmetrical compression.

asynchronous

An operation that proceeds independent of any timing mechanism, such as a clock. Compare with synchronous.

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 deals with pulse code modulation or similar digitized data or analog signals. A WDM audio minidriver provides support for audio devices under the WDM digital audio architecture.

audio mixing

The method of combining multiple streams of audio data into a single stream through some method such as addition and clipping.