INPUT/OUTPUT
As we mentioned at the start of this chapter, a computer system has three
major components: the CPU, the memories (primary and secondary), and the I/O
(Input/Output) equipment such as printers, scanners, and modems. So far we
have looked at the CPU and the memories. Now it is time to examine the I/O
equipment and how it is connected to the rest of the system.
2.4.1 Buses
Physically, most personal computers and workstations have a structure similar
to the one shown in Fig. 2-1. The usual arrangement is a metal box with a large
printed circuit board at the bottom, called the motherboard (parentboard, for the
politically correct). The motherboard contains the CPU chip, some slots into
which DIMM modules can be clicked, and various support chips. It also contains
a bus etched along its length, and sockets into which the edge connectors of I/O
boards can be inserted. Sometimes there are two buses, a high-speed one (for
modern I/O boards) and a low-speed one (for older I/O boards).
SCSI controller
Sound card
Modem
Edge connector
Card cage
Figure 2-1. Physical structure of a personal computer.
The logical structure of a simple low-end personal computer is shown in
Fig. 2-2. This one has a single bus used to connect the CPU, memory, and I/O
devices; most systems have two or more buses. Each I/O device consists of two
parts: one containing most of the electronics, called the controller, and one containing
the I/O device itself, such as a disk drive. The controller is usually contained
on a board plugged into a free slot, except for those controllers that are not
optional (such as the keyboard), which are sometimes located on the motherboard.
Even though the display (monitor) is not an option, the video controller is sometimes
located on a plug-in board to allow the user to choose between boards with
or without graphics accelerators, extra memory, and so on. The controller connects
to its device by a cable attached to a connector on the back of the box