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7.1 / EXTERNAL DEVICES 219

Address lines

System
Data lines bus

Control lines

I/O module

Links to
peripheral
devices

Figure 7.1 Generic Model of an I/O Module

Thus, an I/O module is required. This module has two major functions
(Figure 7.1):
• Interface to the processor and memory via the system bus or central switch
• Interface to one or more peripheral devices by tailored data links

We begin this chapter with a brief discussion of external devices, followed by an
overview of the structure and function of an I/O module.Then we look at the various
ways in which the I/O function can be performed in cooperation with the processor and
memory: the internal I/O interface. Finally, we examine the external I/O interface,
between the I/O module and the outside world.

7.1 EXTERNAL DEVICES

I/O operations are accomplished through a wide assortment of external devices that
provide a means of exchanging data between the external environment and the
computer. An external device attaches to the computer by a link to an I/O module
(Figure 7.1). The link is used to exchange control, status, and data between the I/O
module and the external device. An external device connected to an I/O module is
often referred to as a peripheral device or, simply, a peripheral.