Main memory and the CPU form the core of a computer. In this section,

we investigate how this core, which we will refer to as the computer,

communicates with peripheral devices such as mass storage systems,

printers, keyboards, mice, monitors, digital cameras, and even other

computers.

I. The Role of Controllers

Communication between a computer and other devices is normally

handled through an intermediary apparatus known as a controller. In the

case of a personal computer, a controller may consist of circuitry

permanently mounted on the computer's motherboard or, for flexibility', it

may take the form of a circuit board that plugs into a slot on the

motherboard. In either case, the controller connects via cables to peripheral

devices within the computer case or perhaps to a connector, called a port,

on the back of the computer where external devices can be attached.these

controllers are sometimes small computers themselves, each with its,own

memory circuitry and simple CPU that performs a program directing the activities of the controller

a controller translates messages and date back and forth between forms conpatible with the internal characteristics of the computer and those of the peripheral device to which it is attached. originally,each controller was designed for a particular type of device;thus,purchasing a new peripheral device often required the purchase of a new controller as well.

Recently, steps have been taken within the personal computer arena to

develop standards, such as the universal serial bus (USB) and FireWire,2

by which a single controller is able to handle a variety of devices. For

example, a single USB controller can be used as the interface between a

computer and any collection of USB-compatible devices. The list of devices

on the market today that can communicate with a USB controller includes

mice, printers, scanners, mass storage devices, and digital cameras.

Each controller communicates with the computer itself by means of

connections to the same bus that connects the computer's CPU and main

memory (Figure 7B-I). From this position it is able to monitor the signals

being.sent between the CPU and main memory as well as to inject its own.

signals onto the bus.

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With this arrangement, the CPU is able to communicate with the

controllers attached to the bus in the same manner that it communicates

with main memory. To send a bit pattern to a controller, the bit pattern is

first constructed in one of the CPU's general-purpose registers. Then an

instruction similar to a STORE instruction is executed by the CPU to

"store" the bit. pattern in the controller. Likewise, to receive a bit pattern

from a controller,an instruction similar to a LOAD instruction is used.