The Simplest Possible Tutorial... Understanding How Computers Work

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The Simplest Possible Tutorial... Understanding How Computers Work

This tutorial was developed to help Chris Bell and Robert Wright to come to terms with, and be more confident in handling, their personal computers. However I believe that there are others that can benefit from a gentle introduction to what may seem to be a technical subject.

Any 'new' technical words that are introduced in the body text, will be highlighted at their first use, clicking on the highlighted word will take you directly to the entry in the glossary for the word concerned. Entries in the menu at top left of this page will take you to a sub page describing the topic.

I have heard people say that they are frightened of doing damage by pressing the 'wrong' buttons. There are a few occasions when pressing a button at an inappropriate time will cause you to lose some data or disrupt a program application, but these are few and far between. If we take the right steps in the first place then those events that do occur will be of minor consequence as the situation will be a recoverable one.

First we must establish what a computer is...

	A computer is simply a machine ( Hardware) that is capable of receiving and processing data in accordance with a series of instructions it receives, these instructions may be generated externally from the user via the keyboard, mouse or may be generated internally from 'applications' stored as programmed instructions (software). When the data has been processed, the results of the processing are usually sent to an output device, which can be a screen (Monitor), printer, plotter or another computer.

A computer is simply a machine ( Hardware) that is capable of receiving and processing data in accordance with a series of instructions it receives, these instructions may be generated externally from the user via the keyboard, mouse or may be generated internally from 'applications' stored as programmed instructions (software). When the data has been processed, the results of the processing are usually sent to an output device, which can be a screen (Monitor), printer, plotter or another computer.

Most computers can be classified into one of Five types...

	Mainframe Computers These are huge computers often housed in a dedicated room or complete building. They are capable of executing a wide variety of applications at great speed as well as supporting a very wide range of equipment (peripheral Devices) and large number of users via many individual terminals. Often costing several millions of pounds or dollars each, this type of computer is only used by the larger companies, universities and government departments.

Mainframe Computers

These are huge computers often housed in a dedicated room or complete building. They are capable of executing a wide variety of applications at great speed as well as supporting a very wide range of equipment (peripheral Devices) and large number of users via many individual terminals. Often costing several millions of pounds or dollars each, this type of computer is only used by the larger companies, universities and government departments.

	Mini Computers A mini computer is a smaller version of the mainframe computer. These are the next largest in terms of physical size and cost. They are floor standing machines which commonly take up the same floor space as a filing cabinet. This type costs several tens of thousands of pounds or dollars each and have largely been replaced by micro computers that have very high specifications.

Mini Computers

A mini computer is a smaller version of the mainframe computer. These are the next largest in terms of physical size and cost. They are floor standing machines which commonly take up the same floor space as a filing cabinet. This type costs several tens of thousands of pounds or dollars each and have largely been replaced by micro computers that have very high specifications.

	Micro Computers (PCs) A micro computer is small and less expensive, sizes vary, but generally they will fit on to an office desk and are thus known as 'desk top computers'. This type of computer is the most numerous and they are employed by large, medium and small companies as well as individuals in their homes. This type is what most people mean when they use the word 'computer'. They are constructed of standardised modules or cards and can be easily worked on, altered or upgraded. Also fitting into this category are 'laptop' computers, 'notebook' and 'sub notebook' computers, which are typified by briefcase styling and 'all in one' construction. Laptop and notebook types are not quite so easily worked on for alteration or upgrading, indeed they can be difficult to even disassemble to get at the internal parts. Some types of micro computer have been 'ruggedised' for service in an industrial or military environment.

Micro Computers (PCs)

A micro computer is small and less expensive, sizes vary, but generally they will fit on to an office desk and are thus known as 'desk top computers'. This type of computer is the most numerous and they are employed by large, medium and small companies as well as individuals in their homes. This type is what most people mean when they use the word 'computer'. They are constructed of standardised modules or cards and can be easily worked on, altered or upgraded. Also fitting into this category are 'laptop' computers, 'notebook' and 'sub notebook' computers, which are typified by briefcase styling and 'all in one' construction. Laptop and notebook types are not quite so easily worked on for alteration or upgrading, indeed they can be difficult to even disassemble to get at the internal parts.
Some types of micro computer have been 'ruggedised' for service in an industrial or military environment.

	Palm Top Computer (PDAs) Palm Top Computers (also known as 'Personal Digital Assistants') are so small that they will fit on the palm of your hand or slide into a jacket pocket. Generally these as slower, with less storage and have a lower specification than desktop computers, but they can be coupled to other computers to download or upload data. They are extremely portable and some incorporate mobile phone technology and digital imaging giving a high degree of functionality for their relatively small size.

Palm Top Computer (PDAs)

Palm Top Computers (also known as 'Personal Digital Assistants') are so small that they will fit on the palm of your hand or slide into a jacket pocket. Generally these as slower, with less storage and have a lower specification than desktop computers, but they can be coupled to other computers to download or upload data. They are extremely portable and some incorporate mobile phone technology and digital imaging giving a high degree of functionality for their relatively small size.

	Embedded Computers Are microprocessors that are built into various industrial and consumer products. They are usually dedicated to specific tasks and most of the data that they use will come from sensors or transducers rather than a keyboard or mouse.

Embedded Computers

Are microprocessors that are built into various industrial and consumer products. They are usually dedicated to specific tasks and most of the data that they use will come from sensors or transducers rather than a keyboard or mouse.

Throughout the rest of these tutorial pages we will use the word 'computer' to mean a micro computer.

Computers have several depths of complexity. The first of these is the physical units that a machine is made up from. It is possible to work with a computer for many years without understanding what goes on inside these boxes, but if you do gain an understanding of the internals and the way they work, you will make better use of the resources at your command.

In the previous paragraph, I used the word 'command' and this is truly the correct word to use... You are in control of the machine and not the other way around.

Standalone Computer versus Networked Computers

	A standalone computer system is a computer (CPU and peripherals) that the operator has sole use of and which are not connected to any other computer systems, although they may be connected to the internet (which is a massive network) the computer itself is 'in charge'. There are NO shared facilities or features with this type of computer system. The operating system, applications software and user data files are all stored on the computers hard disk or on floppy disks which are inserted into the computer's drive(s).

A standalone computer system is a computer (CPU and peripherals) that the operator has sole use of and which are not connected to any other computer systems, although they may be connected to the internet (which is a massive network) the computer itself is 'in charge'. There are NO shared facilities or features with this type of computer system. The operating system, applications software and user data files are all stored on the computers hard disk or on floppy disks which are inserted into the computer's drive(s).

Advantages of Standalone Computers

	If the PC fails... That PC is the only one affected, since the PCs are completely separate, if it breaks down it will not affect any other PCs. Portability... A standalone PC can easily and quickly be un-plugged from the power and telephone sockets and moved to a new location. Can be dedicated to single or a limited number of tasks... The relatively low cost of a PC system make it economical to be used for one or a small number of jobs. Dedicated CPU... The Central Processor in the PC is working for only one user. This utilises all the processor's speed. One PC costs less than setting up a network for a single user. The break even point is typically around four users.

If the PC fails... That PC is the only one affected, since the PCs are completely separate, if it breaks down it will not affect any other PCs.

Portability... A standalone PC can easily and quickly be un-plugged from the power and telephone sockets and moved to a new location.

Can be dedicated to single or a limited number of tasks... The relatively low cost of a PC system make it economical to be used for one or a small number of jobs.

Dedicated CPU... The Central Processor in the PC is working for only one user. This utilises all the processor's speed.

One PC costs less than setting up a network for a single user. The break even point is typically around four users.

The main hardware components of a typical standalone computer system consists of:-

The basic layout of a stand alone desktop computer

The diagram above shows the CPU or Systems unit as a horizontal form that the monitor will stand on top of, there is an alternative style of CPU case that is vertically arranged and is known as a 'tower unit'. Whatever style you have, it will contain similar components within it's case and perform the same functions. All the individual chunks have cables that are all plugged into the Systems unit or Tower, in addition there will be a power cable that leads to the Systems unit from the domestic electricity supply point. The Monitor will also have a mains supply cable and this may connect direct to the mains supply or it may connect to an outlet socket on the back of the Tower unit or CPU box.

Networked Computer Systems

	A network of computers is formed by linking two or more computers together via a communications system to a central computer called the 'network file server' often just called 'the server'. The reason for networking computers is to share computing power and peripherals, and to exchange application software or user data files and information, both within and outside the organisation.

Networks may be either local, wide or 'peer to peer'.

The simplest of these is the peer to peer, which as it's title suggests has workstations that are of equal status. Generally this setup only has two workstations, each with their own operating systems, but able to share each other's data and peripherals. If more than two linked workstations are required then it is usual to go to a local area network setup, but it is possible to link many individual computers to work in combination on massive problems by this method.


Local area networks (LANs) are networks where the workstations are connected to a network file server, and are situated on the same site. A site being defined as one or more rooms within the same building or a group of buildings next to each other. LAN workstations are usually connected by a cable (hard wired) to the file server.


Wide area networks (WANs) are networks where the workstations can be situated on the same site or anywhere in the world, or even in space, and may be hard wired to the file server or connected via land lines, radio links or even satellites.

On a network it is usual to store the applications software, network communications software and user data files on the file server's hard disk(s). Each workstation then communicates with the file server through the networking software to load the application software that it needs and relevant data files. Hard copy output is produced from the network printer(s) which are shared between all the workstations since each workstation prints via the file server the output of any machine is placed in a print queue.

Communications between the server and the individual workstations needs to be specific to each workstation. If for any reason it is not, the network becomes inoperable. In order to maintain the integrity of the network, a human administrator is required. Known as the 'network manager', whose duty is to administer the day to day operation of the network, to solve any and all network problems and queries, to maintain the network configuration and set-up new hardware, new software, user passwords etc..

Advantages of Networked Computers

	Hardware Costs are usually lower when the number of workstations exceeds four. Sharing Facilities such as the software applications and data on the server's hard drive(s) and allowing many machines to use a small number of printers, saves cost and desk space. Transmission of data files and messages between users connected by a network is easy and rapid. High levels of security can be achieved since only authorised users have access to the data on the network. However, once a 'hacker' has gained access, the data becomes easily available to them and since networks can be designed to be accessed from a remote location via a modem, security may be lower than a standalone system which has no remote access. (future link to Quarantine machine) Software upgrades are easier on a network than on the same number of stand alone machines, since only one copy is installed and configured on the file servers hard disk instead of an individual copy on each standalone machine.

Hardware Costs are usually lower when the number of workstations exceeds four.

Sharing Facilities such as the software applications and data on the server's hard drive(s) and allowing many machines to use a small number of printers, saves cost and desk space.

Transmission of data files and messages between users connected by a network is easy and rapid.

High levels of security can be achieved since only authorised users have access to the data on the network. However, once a 'hacker' has gained access, the data becomes easily available to them and since networks can be designed to be accessed from a remote location via a modem, security may be lower than a standalone system which has no remote access. (future link to Quarantine machine)

Software upgrades are easier on a network than on the same number of stand alone machines, since only one copy is installed and configured on the file servers hard disk instead of an individual copy on each standalone machine.

Note... The advantages of networks tend to be disadvantages of standalone systems and vice versa.

Network Limits... There can be almost any number of users that are allowed access to a network. However, they may not all be able to use the network at the same time. There is a finite limit to the number of workstations that can be connected to any one server.

Passwords and User Names... All network users are supplied with some form of security. This restricts access to the network to those who are entitled to use it. Security is controlled by means of group or individual user names, and/or by passwords.

Logging On and Logging Off... When a user wishes to access the network they must 'Log On'. This involves typing in his/her user name or code followed by his/her password if they have one. Once typed in, the network software checks the details against a list of authorised users. If the typed data matches then, and only then, the user is allowed to proceed. If either the user identity or password is incorrect the user is prompted to log on again. When the user wishes to leave the network, say at the end of a work period, then he/she must 'Log Off' the network. This involves quitting the software application(s) in use, returning to the network menu screen and then selecting or typing exit, logoff, quit, logout or whatever the 'logging off' command is.

Note... Standalone systems can be set-up with passwords for authorised users, but this is not very sensible as they are inherently less secure than networks, owing to most standalone systems having at least one floppy disk drive which, with a suitable boot disk, the machine can be booted and run independently of any security system.

Input Devices

	These are peripheral devices connected to the CPU or systems unit in order to command or inform the computer of the user's desired course of action.

	Input devices include... Keyboard.. Used to select or enter commands, enter text, QWERTY keyboards are not particularly user friendly but they are likely to be replaced by voice/speech recognition at some as yet unspecified time in the future. Mouse... Used to move the cursor, select commands and features from the on-screen menus, also used for many actions in drawing diagrams. Digitising Tablets and the associated puck are used to move the cursor, select commands and features, enter co-ordinates, mainly used for computer aided drawing (CAD) or map making. Joysticks can be used to move the cursor, select commands and features, but they are not often used for general work as they are slower than a mouse. However they are commonly used in computer games.

Input devices include...

Keyboard.. Used to select or enter commands, enter text, QWERTY keyboards are not particularly user friendly but they are likely to be replaced by voice/speech recognition at some as yet unspecified time in the future.
Mouse... Used to move the cursor, select commands and features from the on-screen menus, also used for many actions in drawing diagrams.
Digitising Tablets and the associated puck are used to move the cursor, select commands and features, enter co-ordinates, mainly used for computer aided drawing (CAD) or map making.
Joysticks can be used to move the cursor, select commands and features, but they are not often used for general work as they are slower than a mouse. However they are commonly used in computer games.

Output Devices

	Output devices are peripheral devices which are used to present, to a human operator, the result of the programming instructions on the data provided.

Output devices include...

Printers... Are used to produce the hardcopy of text or drawings.
Plotters... Can be used to produce the hard copy of large drawings, architectural plans.
Screens, VDUs (Visual Display Unit) and Monitors... Used to display in graphic or textual form, the results of the CPU's processing of data in accordance with the instructions given.
Ports... Are connections by which the CPU can pass data to the outside world as raw data for use by other computers or electronic devices.

Internal Devices

	These are not normally visible without removing the outer covers of the systems unit or tower.

CPU (Central Processing Unit) 'Micro Processor' or just 'Processor'

The CPU is the brain of the computer. The CPU organises all the functions of the computer system and consists of three sections.
The control unit ... Co-ordinates all the computers functions by interpreting and carrying out the instruction sent to it by an input device or program.
ALU (Arithmetic Logic Unit)... this is the computational engine that makes the calculations, decisions and logical comparisons.
Cache Memory... This is an array of memory cells which can be easily and quickly accessed by the ALU to store data until it is needed for another part of a calculation.
The processor is totally solid state with no moving parts. They are usually fabricated on one thin plate of doped silicon with circuitry architecture that is incredibly small, resulting in short transmission paths and high data rates.

	Memory storage can be either 'chip' based or 'backing store' based. Chip based memory mainly consists of 'RAM memory' and 'ROM memory'.

RAM (Random Access Memory)

This type of memory is available only when the computer is switched on and is therefore a non-permanent or 'volatile' memory store. Copies of device drivers for screens, scanners, printers etc. are all placed here by the main processor during the start up sequence, as well as application software and currently active data files.
Note:- When a computer is switched off or re-booted, all the data stored in the RAM is lost.

	ROM (Read Only Memory)

This type of memory is a permanent or non-volatile memory store and is used to store data or applications which the computer needs in order to function. These may be... Character sets, internal programs such as the BIOS, certain device drivers that control the disks, cooling fans etc.

Note:- When the computer is switched off or re-booted, the data stored in read only memory is not lost. It is permanently burnt into the memory chips and cannot be erased or deleted.

There are other types of ROM that are electrically alterable or re-programmable, but the principle is the same... The information is retained when the power is switched off. They can only be re-programmed by a deliberate act and sometimes this requires removal from the circuit board concerned.

	Backing Store Memory

Backing store memory is usually some type of magnetic storage media i.e. magnetic cassette tape or magnetic disks (both hard and floppy), but other media forms can be used including punched paper tape, punched cards, plastic media and CD-ROMs (including DVDs).

Magnetic media has advantages over other media storage means including...

Ease of use and reuse.
Speed of access of data. Transfer speeds of several million bytes per second are commonplace.
Capacity of storage, sizes from approximately 360,000 characters for older styles of floppy disk up to many thousands of millions of characters on a hard disk.
Small physical size of the media for the amount of data held.

Operating System

The operating system is a suite of utility routines which form the link between the operator, the computer (C.P.U.) and the application software by providing the facilities of...

File, Disk and System Management (via the BIOS) comprising a range of commands and utilities that allow the computer to carry out it's file and disk management.
Software Application Support enabling the software application to carry out (via the BIOS)... Data file operation, character input and mouse position sensing.

Software Applications

Software applications are the programs that are required to satisfy a particular operational need. They include Word Processing, Database, Spreadsheet, Internet Browsers, Graphics and image manipulators, Games, CAD, CAM etc. These programs are usually stored on the computers hard disk, but may be stored on a floppy disk or other removable media. Software can be of many types to achieve the same object, some are simple, some are complicated, some are available freely others command high fees. Matching the precise requirement to the software is much a part of the selection process as evaluating value for money. It is possible to run quite a sophisticated system without spending a penny on software, I personally have never expected payment for any software that I have written, nor do I expect to have to pay for anything anyone else has written.

Output Devices

Hard copy output devices fall into one of two categories... Plotters and Printers.

Plotters are used mainly for producing line drawings that have been generated by CAD methods. They can be ink jet or pen type and flatbed or drum.

Printers are of various types...

Ink Jet... These work by spraying ink onto the paper through a number of very fine nozzles. A set amount of ink is sprayed through selected nozzles depending on the character being printed. The print head which contains the nozzles moves along the paper spraying ink as it goes. Both text and graphics can be printed in black or in colour.
Dot matrix... Although they are considered old fashioned, they are ideal for some label printing applications. The work rather like typewriter but use, an array (matrix) of pins to print dots on paper via an inked ribbon. They are inexpensive to run, but are noisy and slow in operation. They exist in both colour and black only versions, but are mainly only used for black these days.
Laser Printers... These are the latest technology printers and are capable of printing at high speeds. They are the most expensive table top printers when comparing like for like, but they do produce good quality. Laser printers can print both text and graphics in black or full colour. They work rather like a photocopier, using a laser beam to selectively discharge an electrostatically charged surface, this charge is transferred to the paper which picks up powdered ink particles (toner). The toner is bonded onto the paper by passing through heated rollers.

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Originated... July 2003, Revised... Early November 2003, Upgraded... 16 July 2006,