Computers: What is a computer?

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What is a computer?

A computer is a device which enables you to process information (1) automatically and (2) in accordance with instructions that you give it.

(1) means that it can carry out the capture-process-store­ communicate sequence described in Chapter 1 all by itself, or at least with the minimum of human intervention. It does this by means of hardware, i.e. information capture, pro­cessing, storage, and communicating devices, linked together electronically.

(2) means that it can be adapted to carry out almost any kind of information processing task, whether word process­ing or stock control. It does this by means of software, i.e. sets of instructions or programs which control these hard­ ware devices so that they carry out the required operations on the information.

In this chapter and the next we look at the various pieces of hardware. In subsequent chapters we turn our attention to the software.

Computer hardware

The hardware of a computer system consists of the devices listed below. They are all described in this chapter and the next.

• Input devices to capture the information, such as the keyboard, mouse, microphone, or scanner.

• The central processing unit (CPU), containing the arith­metic logic unit (ALU), which carries out the logical and arithmetic computations that lie at the heart of comput­ing (see below), and the control unit, which executes commands typed at the keyboard or stored in software.

• Storage is split into short-term memory, using micro­ chips inside the computer, which hold data and pro­ grams while processing is being carried out, and long­ term storage, normally using magnetic disks.

• Output devices for communicating the results of the processing, such as a printer, monitor, or loudspeaker.

• I should also mention the circuitry linking these various devices, consisting of 8, 16, or 32 strands of parallel wire, called buses, along which all the bits that make up each item of information travel in parallel.

We can represent the computer hardware by the usual sort of systems diagram, though it is helpful to represent storage by a separate box linked to the CPU (see Figure 2.1).

image

Microprocessors

I'll start by describing microprocessors (microchips), as these lie at the heart of computers and other information processing devices. The CPU inside a modern computer will consist of one or more of these chips, which contain the logic circuitry needed to perform computations. A chip is packaged in a holder containing the electrical connections needed to link it to other devices such as a keyboard (see Figure 1.1). Microchips which act as memory are described later in this chapter.

In essence, a microprocessor works in the same way as the rooms full of glass 'thermionic' valves which made up the CPUs of the earliest computers. Each valve was a switch which could be turned OFF by applying a strong negative electrical charge to it, or ON by applying a weak electrical charge. If it was ON a current could flow through it, if it was OFF no current could flow.

Instead of thermionic valves, which were bulky, unrelia­ble, and expensive, we now have an alternative type of electrically operated switch, the tiny transistor. A transistor is shown in diagrammatic form in Figure 2.2.

image

A transistor consists of three layers of doped silicon, i.e. silicon to which a small amount of another substance has been added. Doped silicon is a semiconductor. This means that under certain circumstances it will conduct electricity, otherwise it won't. Silicon can be positively doped, i.e. the added substance has fewer valence electrons than pure silicon, or negatively doped, i.e. the added substance has more valence electrons. In either case, an electric current can flow under the right conditions.

A strong current applied to the central positively doped layer in the transistor allows a current to flow between the two negatively doped layers, but a nil or weak current will prevent its flow. In this way the transistor acts like a switch.

A microprocessor is made up of many thousands of such transistors, linked to each other by microscopic circuitry etched into the surface of the chip.

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