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Computer Systems

Hardware is the name that is given to any part of a computer that you can actually touch.

An individual piece of hardware is called a device.

The basic hardware of any computer consists of a central processing unit (CPU) along with input, output and backing storage devices.

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Backing Storage Devices

I nput Devices

Central Processing Unit (CPU)

Output Devices

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The central processing unit (CPU)

This is the part of the computer where the searching and sorting of data, calculating and decision-making goes on.

The CPU contains the Main Memory, the Control Unit and the Arithmetic and Logic Unit (ALU).

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CENTRAL PROCESSING UNIT

PROCESSOR

Control Unit

ALU

Input and

Output Devices

control flow data flow instruction flow

Main

Memory

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The Control Unit fetches instructions from the main memory, decodes them and causes them to be executed by the ALU.

The ALU performs calculations and makes decisions using these instructions.

The Main Memory contains two types of memory chip called ROM and RAM which hold program instructions and data.

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Random Access Memory (RAM)

RAM is the computer’s ‘working memory’.

RAM temporarily stores programs and data that are being used at a given time.

The contents of RAM can be changed and are lost when the computer is turned off.

Memory which is wiped clean when the computer is turned off is called volatile memory.

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Read Only Memory (ROM)

The main use of ROM memory chips in a computer is to store the program that runs when the computer is turned on which loads the operating system (e.g. Windows 2000) from disk.

The contents of ROM can’t be changed and aren’t lost when the computer is switched off.

Memory which isn't wiped clean when the computer is turned off is called non-volatile memory.

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PROM and EPROM

PROM and EPROM are both special types of programmable read only memory.

PROM stands for Programmable Read Only Memory. This type of memory can be programmed once but can’t be changed again afterwards.

EPROM stands for Erasable Programmable Read Only Memory. This type of memory can be programmed and then changed whenever necessary.

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The size of a computer’s memory is normally measured in kilobytes (Kb), megabytes (Mb) or gigabytes (Gb). The table below shows some of the main units of size that we use to measure computer memory.Measurement Size (bytes) Symbol

kilobyte 1,024 Kb

megabyte 1,048,576 Mb

gigabyte 1,073,741,824 Gb

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Mainframe computers

Mainframe computers cost millions of pounds to buy and install. They can process massive amounts of data extremely quickly, which is stored on hundreds of disk drives. A mainframe can have hundreds of terminals and users connected to it at the same time. The most powerful mainframes are called supercomputers.

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Microcomputers

Microcomputers are sometimes called PCs, or personal computers.

A typical PC system consists of:

A base, or tower, unit that contains a CPU and backing storage devices, normally a floppy disk drive, a hard disk drive and CD-ROM drive, and often a Zip drive;

Input devices, normally a mouse and keyboard.

Output devices, typically a screen, printer and speakers.

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MinicomputersMinicomputers cost tens of thousands of pounds to buy and install.

They are much more powerful than microcomputers but not as powerful as a mainframes.

Medium-sized businesses use minicomputers to control the company network and act as a central store for all the company’s data.