Specifications

Glossary
7
processing equipment. The standard ASCII charac-
ter set consists of 128 decimal numbers ranging
from 0 to 127, which are assigned to letters, num-
bers, punctuation marks, and the most common
special characters. In 1981, IBM introduced the
extended ASCII character set with the IBM PC,
extending the code to 8 bits and adding characters
from 128 to 255 to represent additional special
mathematical, graphics, and foreign characters.
ASCII character A 1-byte character from the
ASCII character set, including alphabetic and
numeric characters, punctuation symbols, and
various graphics characters.
ASME (American Society of Mechanical
Engineers; www.asme.org) ASME Inter-
national has nearly 600 codes and standards in
print, and its many committees involve more than
3,000 individuals, mostly engineers but not neces-
sarily members of the society. The standards are
used in more than 90 countries throughout the
world.
aspect ratio The measurement of a film or tele-
vision viewing area in terms of relative height and
width. The aspect ratio of most modern motion
pictures varies from 5:3 to as large as 16:9, which
creates a problem when a wide-format motion pic-
ture is transferred to the more square-shaped televi-
sion screen or monitor, with its aspect ratio of 4:3.
See also letterbox.
assemble The act of translating a program
expressed in an assembler language into a com-
puter machine language.
assembler language A computer-oriented lan-
guage whose instructions are usually in one-to-one
correspondence with machine language instructions.
asymmetrical modulation A duplex transmis-
sion technique that splits the communications
channel into one high-speed channel and one
slower channel. During a call under asymmetrical
modulation, the modem with the greater amount
of data to transmit is allocated the high-speed
channel. The modem with less data is allocated the
slow, or back, channel. The modems dynamically
reverse the channels during a call if the volume of
data transfer changes.
asynchronous communication Data trans-
mission in which the length of time between trans-
mitted characters can vary. Timing depends on the
actual time for the transfer to take place, as
opposed to synchronous communication, which is
timed rigidly by an external clock signal. Because
the receiving modem must be signaled about when
the data bits of a character begin and end, start and
stop bits are added to each character. See also syn-
chronous communication.
asynchronous memory Memory that runs
using a timing or clock rate different from (usually
slower than) the motherboard speed.
AT clock Refers to the Motorola 146818 real-
time clock (RTC) and CMOS RAM chip, which first
debuted in the IBM AT and whose function has
been present in all PC-compatible systems since.
Keeps track of the time of day and makes this data
available to the operating system or other software.
ATA (AT Attachment Interface) An IDE disk
interface standard introduced in March 1989 that
defines a compatible register set, a 40-pin connec-
tor, and its associated signals. ATA standards are
developed and published by Technical Committee
T13 (www.t13.org). ATA has evolved over time,
resulting in a number of standards, the latest of
which is in development as ATA8. See also IDE,
ATAPI and SATA.
ATAPI (AT Attachment Packet Interface) A
specification that defines device-side characteristics
for an IDE-connected peripheral, such as a CD-
ROM or tape drive. ATAPI is essentially an adapta-
tion of the SCSI command set to the IDE interface.
ATA-4 and newer ATA standards include ATAPI.
Athlon An AMD sixth-generation processor fam-
ily roughly comparable to the Intel Pentium III and
Pentium 4. Later models (beginning with the
Thunderbird core) include on-die L2 cache running
at full core speed. It includes MMX and AMD
3DNow! instructions for multimedia performance.
Originally available in a Slot-A cartridge package,
all Athlons are now available only in the Socket-A
(462-pin) package. The Mobile Athlon XP, which
replaced the Athlon 4, is designed for mobile appli-
cations, the Athlon MP is designed for work-
station/server multiprocessor configurations, and
Appendix A
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