Glossary of terms - PC Based Instrumentation and Control - page 445

Appendix A Glossary of
terms

Accelerated Graphics Port (AGP) A high speed interface for video cards.
AGP typically runs at 1 (66 MHz), 2 (133 MHz), or 4 (266 MHz).

Access time The time taken to retrieve data from a memory/storage device,
i.e. the elapsed time between the receipt of a read signal at the device and the
placement of valid data on the bus. Typical access times for semiconductor
memory devices are in the region 100-200 ns whilst average access times for
magnetic disks typically range from 10-50 ms.

Accumulator A register within the processor (or CPU) in which the result of
an operation is placed.

Acknowledge (ACK) A signal used in serial data communications which
indicates that data has been received without error.

Active high A term used to describe a signal which is asserted in the high

(logic 1) state.

Active low A term used to describe a signal which is asserted in the low
(logic 0) state.

Address A reference to the location of data in memory or within I/O space.
The processor (or other controlling device) places addresses (in binary coded
form) on the address bus (see also Address bus ).

Address bus The set of lines used to convey address information. The original
PC bus had twenty address lines (A0 to A19) and these were capable of address-
ing a linear address space with a little more than a million address locations.
One byte of data may be stored at each address.

Address decoder A hardware device (often a single integrated circuit) which
provides chip select or chip enable signals from address patterns which appear
on an address bus.

Address selection The process of selecting a specific address (or range of
addresses). In order to prevent conflicts, expansion cards must usually be con-
figured (by means of DIP switches or links) to unique addresses within the I/O
address map.

Amplifier A circuit or device which increases the power of an electrical signal.

Analogue The representation of information in the form of a continuously
variable quantity (e.g. voltage).