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1.2 / STRUCTURE AND FUNCTION 13

sources or destinations of data.When data are received from or delivered to a device
that is directly connected to the computer, the process is known as input–output
(I/O), and the device is referred to as a peripheral. When data are moved over longer
distances, to or from a remote device, the process is known as data communications.
Finally, there must be control of these three functions. Ultimately, this control
is exercised by the individual(s) who provides the computer with instructions.Within
the computer, a control unit manages the computer’s resources and orchestrates the
performance of its functional parts in response to those instructions.
At this general level of discussion, the number of possible operations that can
be performed is few. Figure 1.2 depicts the four possible types of operations. The
computer can function as a data movement device (Figure 1.2a), simply transferring
data from one peripheral or communications line to another. It can also function as
a data storage device (Figure 1.2b), with data transferred from the external environ-
ment to computer storage (read) and vice versa (write). The final two diagrams
show operations involving data processing, on data either in storage (Figure 1.2c) or
en route between storage and the external environment (Figure 1.2d).
The preceding discussion may seem absurdly generalized. It is certainly possi-
ble, even at a top level of computer structure, to differentiate a variety of functions,
but, to quote [SIEW82],
There is remarkably little shaping of computer structure to fit the
function to be performed.At the root of this lies the general-purpose
nature of computers, in which all the functional specialization occurs
at the time of programming and not at the time of design.

Structure
Figure 1.3 is the simplest possible depiction of a computer. The computer interacts
in some fashion with its external environment. In general, all of its linkages to the
external environment can be classified as peripheral devices or communication
lines.We will have something to say about both types of linkages.

Peripherals Communication lines

COMPUTER
• Storage
• Processing

Figure 1.3 The Computer