Next to the speed with which a computer can perform its basic functions~

adding, subtracting, multiplying, dividing, and comparing one value with an-

other--no measure of such a machine s potential is more revealing than its

capacity for storing, and recalling on cue, information and the instructions for

handling it. Without the ability to squirrel away data and programs, a computer

would not deserve the name. Like the simplest electronic calculator, it could

handle only two numbers and one operation at a time, an unacceptable ]imi-

tation for any but the simplest of data-processing tasks.

Computers store information and programs by means that are, in essence,

either electronic or mechanical. Electronic methods, generally called memory,

are highly valued for their ability to keep pace with the computer s central

processing unit, or CPU, which typically shuttles bits of information and program

instructions in and out of memory several million times a second. Because these

electronic circuits are expensive and because the most common varieties lose

their contents when power to the computer is interrupted for even a split second,

the role of memory is that of a temporary niche for data and instructions that must

flow quickly in and out of the CPU.

The memories of computers that were constructed during the early part of the

1940s rarely exceeded a handful or two of bytes, a unit of measurement equal

to eight bits, or binary digits. (In their formative years, computers often handled

bits--the basic on-off currency of their circuits~one at a time. It is common

nowadays for a computer to work with groups of bits~known as words--of two,

four, and even eight bytes, a feature that by itself can increase the speed of a

computer many times over.)

Memory grew slowly, taking until the mid-1960s to pass the megabyte (one-

million-byte) mark, but the advent in the 1960s of integrated-circuit chips--

microscopic assemblages of transistors and other electronic components--

brought with it a tremendous increase in the memory capacity of computers.

Today, even the most unpretentious desktop computer may contain more than

500,000 bytes of memory and have the potential for controlling millions of bytes.

Some supercomputers, such as the Cray 2 and the ETA1~, have access to more

than two gigabytes of memory---or two billion bytes.