Specifications

4
Appendix A Glossary
access light The LED on the front of a drive or
other device (or on the front panel of the system)
that indicates the computer is reading or writing
data on the device.
access mechanism See actuator.
access time The time that elapses from the
instant information is requested to the point that
delivery is completed. It’s usually described in
nanoseconds (ns) for memory chips and in milli-
seconds (ms) for disk drives. Most manufacturers
rate average access time on a hard disk as the time
required for a seek across one third of the total
number of cylinders plus one-half the time for a
single revolution of the disk platters (latency).
accumulator A register (temporary storage) in
which the result of an operation is formed.
acoustic coupler A device used to connect a
computer modem to a phone line by connecting to
the handset of a standard AT&T-style phone. The
audible sounds to and from the modem are trans-
mitted to the handset through the coupler while
the handset is resting in the coupler. Although
often thought of as obsolete, an acoustic coupler
can be used to ensure the availability of a modem
connection when traveling and access to an RJ-11
jack is unavailable.
ACPI (Advanced Configuration and Power
Interface) A standard developed by Intel,
Microsoft, and Toshiba that is designed to imple-
ment power management functions in the operat-
ing system. ACPI is a replacement for APM. See
also APM.
ACR Short for Advanced Communication Riser,
this is an alternative to CNR advocated by the ACR
Special Interest Group (www.acrsig.org). ACR, like
CNR, is designed to allow motherboard designers to
add low-cost network capabilities to motherboards
but uses the same PCI connector used by PCI
expansion cards.
Acrobat Refers to the Adobe Acrobat program
for creating and reading cross-platform documents
created in Adobe’s Portable Document Format
(PDF) file format. Many computer and component
manuals are available online in Acrobat format.
The Acrobat Reader can be downloaded free from
Adobe’s website.
active heatsink A heatsink that includes a fan.
It’s commonly used to cool processors and North
Bridge/Memory Controller Hub chips.
active high Designates a digital signal that must
go to a high value to be true. Synonymous with
positive.
active low Designates a digital signal that must
go to a low value to be true. Synonymous with
negative.
active matrix A type of LCD screen that con-
tains at least one transistor for every pixel on the
screen. Color active matrix screens use three tran-
sistors for each pixel—one each for the red, green,
and blue dots. The transistors are arranged on a
grid of conductive material, with each connected to
a horizontal and a vertical member. See also TFT.
active partition Any partition marked as
bootable in the partition table. See also boot
manager.
actuator The device that moves a disk drive’s
read/write heads across the platter surfaces. Also
known as an access mechanism.
adapter The device that serves as an interface
between the system unit and the devices attached
to it. It’s often synonymous with a circuit board,
circuit card, or card, but it also can refer to a con-
nector or cable adapter that changes one type of
connector to another.
adapter description files (ADF) Refers to the
setup and configuration files and drivers necessary
to install an adapter card, such as a network
adapter card. Primarily used with Micro Channel
Architecture (MCA) bus cards.
add-in board See expansion card.
address Refers to where a particular piece of data
or other information is found in the computer. Also
can refer to the location of a set of instructions.
address bus One or more electrical conductors
used to carry the binary-coded address from the
microprocessor throughout the rest of the system.
ADSL (asymmetric digital subscriber line)
A high-speed transmission technology originally
developed by Bellcore and now standardized by
ANSI as T1.413. ADSL uses existing UTP copper
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