Computer Terms

A/D converter
(Analog to Digital Converter) Device that converts
continuously varying analog signals from
instruments that monitor such conditions as
movement, temperature, sound, etc., into binary
code for the computer. It may be contained on a
single chip or can be one circuit within a chip.
See modem and codec. Contrast with D/A converter.


A/UX
Apple\'s version of UNIX for the Macintosh. It is
based on AT&T\'s UNIX System V with Berkeley
extensions.


A20
PC address line that points beyond one megabyte.
For downward compatibility with 8086/8088s, this
line is gated in 286s and up and is controlled by
circuitry on the motherboard. When disabled, it
keeps the machine in Real Mode (under 1MB). When
enabled, the CPU can address beyond 1MB (Protected
Mode).

The HMA (High Memory Area) can also be accessed
in Real Mode when this line is turned on. There
are 32 address lines on a 386DX (A0-A31), 24 on a
386SX and 286 (A0-A23).


A:
Designation for the first floppy disk drive in a
PC.


ABC
(Atanasoff-Berry Computer) First electronic
digital computer. Completed in 1942 by Iowa State
Professor John Atanasoff and graduate student
Clifford Berry, it embodied the input, memory and
arithmetic unit of future computers.

John Mauchly, cobuilder of the ENIAC, visited
Atanasoff in 1940 and corresponded with him.
Although Eckert and Mauchly are considered the
creators of the first electronic digital computer,
Atanasoff and Berry are acknowledged contributors.

In 1990, nearly 50 years after his invention,
87-year-old Atanasoff was awarded the National
Medal of Technology.


abend
(ABnormal END) Also called a crash or bomb, occurs
when the computer is presented with instructions or
data it cannot recognize or the program is reaching
beyond its protective boundary. It is the result
of erroneous software logic or hardware failure.

When the abend occurs, if the program is running
in a personal computer under a single-task (one
program at a time) operating system, such as DOS,
the computer locks up and has to be rebooted.
Multitasking operating systems with memory
protection halt the offending program allowing
remaining programs to continue.

If you consider what goes on inside a computer,
you might wonder why it doesn\'t crash more often.
A mainframe\'s memory can easily contain over 300
million storage cells (bits). Within every second,
millions of these cells change their state from
uncharged to charged to uncharged. If only one
cell fails, the computer can abend.


ABI
(Application Binary Interface) Specification for a
particular hardware platform and operating system.
It details the machine language of the CPU family
as well as the calls between the application and
the operating system.


abort
(1) To exit a function or application without
saving any data that has been changed.

(2) To stop a transmission.