| computer memory
physical device that is used to store such
information as data or programs (sequences of
instructions) on a temporary or permanent basis for use
in an electronic digital computer. The memory of
a typical digital computer retains information of
this sort in the form of the digits 0 and 1 of the
binary code. It contains numerous individual storage
cells, each of which is capable of holding one such
binary digit (or "bit") when placed in either
of two stable electronic, magnetic, or physical states
corresponding to 0 and 1. The main memories of digital
computers usually operate by means of transistor
circuits. In these electronic circuits, binary digits
are represented as states of electric charge--on or off,
closed or open, conducting or nonconducting, resistive
or nonresistive--that can be held, detected, and changed
for purposes of storing or manipulating the data
represented by the digits.
Most digital computer systems have two levels
of memory--the main memory and one or more auxiliary
storage units. Besides the main memory, other units of
the computer (e.g., the control unit,
arithmetic-logic unit [ALU], and input/output units)
also use transistor circuits to store electronic
signals.
The flow of electric current through the transistors
in memory units is controlled by semiconductor
materials. Semiconductor memories utilizing
very-large-scale integration (VLSI) circuitry are
extensively used in all digital computers because of
their low cost and compactness. Composed of one or more
silicon chips only about a quarter of an inch in size,
they contain several million microelectronic circuits,
each of which stores a binary digit. Semiconductor
memories provide great storage capacity but are
volatile--i.e., they lose their contents if the
power supply is cut off.
A special type of transistor circuit for temporary
storage of a binary digit is called a flip-flop. A
single flip-flop consists of four or a few more
transistors. Once a flip-flop stores a binary digit 0 or
1, it keeps that digit until it is rewritten to 1 or 0,
respectively. A set of flip-flops that temporarily
stores a program instruction (or two or three
instructions in the case of certain types of computers)
or a number (as in a computational result) is called a
register. Numerous flip-flops and registers are used not
only in the memory unit but in the ALU and control unit
as well.
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