ICDL Module 1

In the early days of computer technology, it was easy to categorise computers. Today, even the basic desktop machines are extremely powerful by the standards of a few years ago and rival the early mainframes in computing power.

Cost comparisons

The cost of a computer depends on a range of factors including the components, labour and demand. As a result, any price that is quoted will be out of date within weeks. Further, as demand increases for one type of computer and decreases for another, their relative prices will change.

The following table attempts to give a graphical representation of the relative prices of different types of computer:

The performance of a computer is determined by a number of factors, all of which work together. Often a single item that is functioning poorly will cause a bottleneck resulting in poor performance.

CPU: The model of the CPU and its speed are the first factors that determine computer performance. Generally, the CPU so far outperforms the other components, that poor performance is usually due to other factors. One important factor in the performance of the CPU is the amount of on-board cache memory. If the CPU has sufficient cache memory it can queue future instructions and data in cache. Since access to cache memory is far faster than that to RAM, the overall processing performance is improved. On-board cache memory helps especially where the CPU is involved in processing of graphics.

RAM: If a computer does not have sufficient RAM, it has to make use of the hard disk to store intermediate data that it would normally store in RAM. This is referred to as virtual memory. Since hard disk access is much slower than access to RAM, this will slow down the computer. The more RAM a computer has, the less need there will be to make use of virtual memory.

Number of applications: Modern computers are designed to run more than one application at a time and to allow applications to be working on multiple sets of data at the same time. For example, a user may be working on four documents at once. However, the more open applications and documents there are, the more this will place a burden on the processing power of the computer. For best performance, only open the applications and documents you need. Close others.

Graphics cards: The graphics card is the unit that converts the signals from the CPU into a form that can be displayed on the monitor. A good graphics card can take over many of the tasks of the CPU in generating the output. This leaves the CPU free to do other processing tasks. The quality of the graphics card is a key factor in the performance of a computer, yet is one which is often overlooked.

There are three main components to the CPU: the arithmetic-logic unit (ALU), control unit and on-board cache memory.

Control unit: The control unit is responsible for loading and interpreting the individual instructions that comprise the computer program. These instructions are in a language called machine code. Machine code is a pattern of ones and zeros. The control unit also has the task of getting the data needed by the instructions and returning the results of the processing after the instruction has been executed.

Arithmetic-logic unit: The ALU is responsible for carrying out arithmetic operations such as addition and subtraction as well as logical decisions such as whether one number is bigger than another. All programs consist of complex sets of arithmetic and logical operations. Another way of thinking of a logical operation is as a decision making operation.

On-board cache memory: Because the CPU can perform its operations much faster than data can be transferred from RAM, many CPUs have on-board cache memory. This is memory that the control unit can access very quickly and use for intermediate storage. Further, data and instructions can be loaded into cache before they are actually needed. When they are needed, the transfer is much faster than it would have been if RAM had been used.

Speed of the CPU: The CPU operates as a result of electronic pulses sent to it by another device on the motherboard called the clock. The speed of a CPU is measured by the maximum number of pulses it is able to handle. This is measured in MHz (megahertz) or millions of pulses per second or GHz (gigahertz) thousands of millions of pulses per second. A good personal computer will use a CPU with a clock speed of over 2 GHz. This means it receives 2 000 000 000 million pulses every second from the CPU.

Previously CPUs could only do one operation per pulse. With improvements in technology, they have been able to improve on this. For example, they can do one operation at the start of the pulse and one at the end of the pulse.

Because we use very large numbers of bytes for storage, abbreviations are used for large numbers. These are based on powers of two and are set out in the following table.

The capacity of hard drives is measured in bytes. A modern hard drive has a capacity of 40 Gb or more.

When files are stored on disk, the amount of space they occupy is measured in bytes. The following screen shows a partial listing of files in a directory. Notice the fourth column which contains the size of the file in bytes. (Don't be concerned with the detail of this screen – it is shown purely for illustrative purposes.)

Figure 2.1. Directory Listing

When we work with files and directories, we often need to know how large the files are or how much space a directory and its files occupies.

The most familiar input device is the keyboard. Users type the text directly into the computer.

There are a number of layouts of the keyboard. The most important are the language variations. For example, the US and UK keyboards are quite similar but are very different from the French keyboard.

Figure 2.2. Keyboard

The mouse is a point and click device. As you move the mouse across a surface, it senses this movement either mechanically or optically. This is translated into the movement of a pointer on the screen. Functions are represented as icons on the screen. When you click on these using a mouse button, the function is executed.

Figure 2.3. Mouse

A touchpad is a device that senses pressure to guide the pointer on the computer screen. It is generally a small square area below the keyboard. As the user moves his/her finger across the touchpad, the pointer moves on the screen. Next to the pad are two buttons used for clicking in exactly the same way as those on a mouse.

A trackball acts as a type of overturned mouse. The ball is on the top side of the object. By rolling the ball you can move the pointer across the screen. Some keyboards have an in-built trackball.

The trackball has been superseded by the touchpad.

Figure 2.4. Trackball

A light pen is a device which is sensitive to variations in patterns on a surface. Light pens act like a miniature scanner and can read text as they are dragged across the printed page. This can be transferred directly to the current open document.

Figure 2.5. Lightpen

A bar code is a pattern of vertical lines in which the spacing and thickness can be used to represent data. A bar code reader is a device that can read and interpret bar codes and input the data into the computer.

Figure 2.6. Bar code reader

A joystick is a device that is familiar for use in games to move objects on the screen. However, it is also used to control the movements in computerised industrial machines such as lathes. It consists of a small vertical lever which can move in any direction. These movements are translated to the computer which in turn uses them to control the movements of machinery.

Figure 2.7. Joystick

Instead of film, a digital camera uses a light sensitive screen at the back of the camera. A small computer inside the camera converts the pattern on the screen into a standard graphics file which can then be transferred to a computer.

A microphone translates speech into an electronic signal. Modern speech recognition software is able to translate this into either commands or data. This enables the user to use a microphone as an alternative to the keyboard.

A scanner is similar to a photocopier, except that instead of producing a paper copy of the document you place on it, you get an electronic copy which appears on your computer screen.

Text recognition software can be used with a scanner. This software is able to recognise the individual letters in the image. Instead of creating a single image of the document, the software inserts actual text into your application which can then be edited.

Figure 2.8. Scanner

Virtually all computers use some type of screen as their primary output device. There are two categories of screen: cathode ray tube and LCD.

CRT screens: The cathode ray tube (CRT) type screen is usually called a monitor and makes use of the same technology as a television screen. A beam of electronics is fired from an electronic gun at the back of the tube. This strikes the front of the tube which is covered in a phosphorescent material which glows when struck by electrons. Between the electron gun and the screen the beam is modulated by a signal to produce the image you see on the screen.

With CRT type screens, an important measure is the refresh rate. Roughly speaking, this is the number of times the image is refreshed every second. A low refresh rate makes the image appear to flicker. You need a refresh rate of at least 72 Hz (72 times a second) to avoid the appearance of flicker.

Solid state screens: Solid state screens, also known as LCD or Liquid Crystal Displays, make use of tiny transistors to emit light and create an image. Originally, LCD screens were confined to laptops, but they are increasingly used with desktops. They are usually called flat screens when used as separate units with desktops.

Resolution: An important characteristic of all screens is their resolution. Each point of light on the screen is called a pixel. The resolution of a screen is the maximum number of pixels that the screen can display. This is given as the number of pixels across (horizontal resolution) by the number of pixels down (vertical resolution). For example, 800 x 600. The greater the resolution the better. Modern screens can display 1024 X 768 or better.

Printers produce a hard copy of the output on paper. There are three main types of printer: Dot matrix, Inkjet and Laser. The following table compares the three types.

A plotter consists of a device that can move paper both backwards and forwards. On the top of the device one or more pens are able to move horizontally across the paper. The combined movement of the pens horizontally across the paper and the vertical movement of the paper allows complex continuous diagrams to be drawn.

Some plotters allow different colour pens to be used to create diagrams in multiple colours.

In other types of plotters, the paper lies on a flat bed. The mechanics of the plotter are so designed that the pens can move both across and down the paper to create the diagram.

Figure 2.9. Plotter

Plotters are usually used in conjunction with CAD (Computer Assisted Design) programs. These are used in everything from the design of ships and machines to buildings.

Modern computers using the appropriate software can turn text in a document into audible speech. This is known as speech synthesis. Other types of software allow music and other sounds to be created and played back.

The line between the computer and a home entertainment system is becoming blurred. Computers are able to play music directly from a CD or play a film from a DVD. You can even fit your computer with a radio or TV card to add these functions.

In all cases, the sound is transmitted through a speaker in the same way it is in a sound system or radio.

A diskette comprises a plastic flexible disk enclosed inside a tough plastic cover. At one end is a window. When the diskette is placed inside a diskette drive, the window is pushed to the side. The read-record head inside the drive makes contact with the magnetic disk.

Figure 2.10. Diskette

Diskettes are slow and have a low capacity (1,44 Mb). Since they are cheap, they still tend to be commonly used for storing small amounts of data. Another advantage is that they can be used over and over again. Unfortunately, many are not very good quality and data can become corrupted and unusable. Because of this they are not suited to backup purposes. The disks can become corrupt through many causes including disk, heat, magnetism or moisture.

A zip disk is a removable magnetic disk which fits into a special zip drive. The surface is coated with a special scratch resistant material which makes a zip disk a very robust storage device. It comes in a number of capacities: 100, 250 and 750 Mb. The speed of a zip drive is faster than all but the very fastest of CD drives. Its robustness and speed make it an excellent backup device. However, its capacity is much less than that of hard drives and tape drives which limits its use for very large amounts of data.

A data cartridge is a tape very similar to that found in a tape recorder, only much higher quality. These are used in a device called a tape streamer to record data. Data cartridges, especially if good quality, are reliable and cheap devices for creating backups of large quantities of data. They are, however, rather slow. Data cartridges are sequential devices which means that to access an item of data on them, all the preceding data reads to be read first. They are tending to become obsolete as newer faster and more reliable technologies are available. A single tape can stored many Gb of data.

A CD ROM uses optical technology. When data is written, small pits are burned into the surface using a highly focussed laser beam. These are read by another laser beam.

There are two types of CD ROM used for storage. The CD-W disks can only be written to once. Once data has been written to part of the surface, this part can no longer be used. CD-RW disks are designed so that one set of data can overwrite another. This allows the disks to be re-used many times.

CD ROM provides a reliable and storage medium for backing up and storing data. The speed is greater than that of a diskette but slower than that of a hard drive. Writing to a CD ROM is a much slower process than reading it. The capacity of a CD ROM is 640 Mb. It is sometimes possible to store about 700 Mb on a disk.

CD ROM technology is improving all the time with continued improvements in quality and speed.

The Digital Versatile Disk is a development of the storage technology of the CD ROM. Using newer storage methods and higher quality media, a DVD can store about 4 Gb of data. This is enough to store a full length film.

A hard disk drive can be internal or external. An internal drive is housed inside the main unit and is connected directly to the motherboard of the computer. An external drive is housed inside a special caddy which connects to the computer through one of its ports. Most now use the USB or firewire ports to achieve maximum performance.

An external hard drive is a good backup medium and allows large quantities of data to be stored. Since the same drive can be connected at different times to different computers, these drives provide a useful way of transferring data between computers that are not connected through a network. As they are electromechanical devices, they are subject to mechanical failure if not handled with care. The small 2½“ drives used in laptops make excellent external hard drives since they are constructed to be moved around.

Modern hard drives have capacities from 40 Mb to 120 Mb. They are also relatively cheap in terms of the storage capacity they offer.

Because they contain moving parts, they do eventually fail. When a hard drive fails is quite unpredictable. Any suspicious noise coming from a hard drive should be viewed with great caution and the data it contains should backed up immediately. The expected life span of a hard drive is measured as the mean time between failures. This is a very rough average of the working life. Figures of 250 000 hours are often quoted but these should be viewed with caution.

Numerous systems have been developed to protect data on hard disks. One of these is mirroring where the data is stored simultaneous on two disks. The one disk becomes the mirror image of the other. If one fails, the data is still one the other. In this case, the first disk is replaced, the system creates a mirror image of the first disk automatically and the system continues.

There are a number of measures of performance of a hard disk. One is the speed at which the platters turn. Typically this is somewhere between 4800 and 7200 rpm. There are faster, more expensive disks. Another is the access time. This is the time it takes the disk to access an item of data. A good figure here would be around 10 ms. An ms or millisecond is one thousandth of a second. Disks are also sometimes compared in terms of their data transfer rates. This is a measure of how many bytes can be read or written per second.

A new type of external memory is the flash disk or memory stick. This is a solid state device (no moving parts) that connects to the computer via the USB port. It provides a very fast and reliable method of storing data externally.

They are at the moment fairly expensive, especially the larger capacity devices. They tend to be limited to a maximum of about 2 Gb. This figure can be expected to increase quite dramatically over time.

As in the case of computers, the cost of memory is continually changing. The price varies from country to country as well as according to international demand. In order to compare the cost of memory, a common measure is to calculate the cost per Mb. The following table compares the cost per Mb of the different media.

This table tells you that it is about 100 times as expensive to store a Mb of data on a floppy disk as it is on a hard drive.

Use this table with caution. Treat the values as very approximate relative values. In other words use them as comparative values, not as monetary values.

A new diskette or hard disk is not able to record data immediately. The disk first needs to be prepared by a process known as formatting. This marks out concentric circles called tracks. Each track is divided into a number of sectors. The tracks and sectors are marked out using magnetic markers.

As data is recorded on a disk, it fills up. To be able to re-use a disk, it may also be formatted. This releases the areas that contain data so that new data may be stored in its place. When a disk is reformatted, the old data is lost.

When data is stored on a disk, it is not always stored in a continuous pattern. Rather, the system stores data in the first free area it finds. When this has been filled, it looks for additional free space and continues storing the data. A file ends up being stored as a series of segments across the disk. This breaking up a a file into many segments is called fragmentation.

Fragmentation slows down the operation of the disk as the system needs to keep track of all the different segments. A disk may be re-organised to reduce fragmentation by a process called defragmentation.

There are different file systems available, but in all cases the disk needs to be prepared with the index area, tracks and sectors through formatting.

As mentioned previously, disk can be reformatted. In this process everything on the disk is erased and the disk is formatted as if it was a new disk.

The following diagram represents the tracks (green) and sectors (red triangular area) that are created when the disk is formatted and ready to store data.

Figure 2.11. Disk Sectors and Tracks

The development of computer programs is a highly developed and structure process involving a number of distinct stages as shown in the table below. The stages of development are known as the program development life cycle.

The following are often also included as part of the development cycle.

Networks on which all computers have equal status are called peer-to-peer networks.

On most networks, certain computers have special dedicated tasks. Since these machines provide services to other computers, the are called servers. The computers that make use of the services or servers are called clients or workstations. A network such as this is called a client-server network.

A server which is used for the central storage of files is called a file server. Using a file server, users can access their work from anywhere on the network. They can also make these files available to users on other computers. File servers also enable users to co-operate on a task by centralising the results of their work.

A computer attached to a printer which users can access is called a print server. Having a single printer rather than a printer attached to each computer obviously saves capital.

An increasingly important type of server is an applications server. In the case of applications servers, application packages are not installed on the workstations but on the applications server.

A communications server is a computer dedicated to connecting a network to the outside world. These are often called proxy servers.

As the case of print servers illustrates, one of the values of having a network is that it enables resources to be shared.

A LAN or Local Area Network is a group of computers within the same building, or within a group of buildings that are in close proximity, that are connected together.

A WAN or Wide Area Network is a group of widely dispersed computers that are connected together. These could be across the same town, or across a country or even across the world. Apart from distance, the other feature that distinguishes as WAN from a LAN is that the WAN would make use of a range of communication technologies such as telephone, microwave and satellite links.

A peer-to-peer network is where two or more computers are linked together in order to share information and hardware.

It is a major advantage to be able to share printers, plotters and scanners. When computers are networked together, there can be many pc's sharing a printer as opposed to each one having to have it's own printer which is much more costly.

It is also possible to share data files across the network by creating a share on the harddrive and allowing other people access to that information.

If you want to share applications you would need to investigate a client/server network solution and some of the advantages are listed below.

Proxy servers contain a repository of internet sites recently visited and cached for quicker access at a future date.

The internet is the collection of all computers across the world which can access each other in some way. The links between computers might include telephone, fibre optic cable, radio, microwave or satellite. Today tens of millions of computers are able to access each other. The Internet has no central organisation which controls its use. Because the Internet knows no borders, many governments have tried to control the flow of information across the Internet. However, communications technology is so varied and so widespread that this is a near impossible task.

An intranet is similar to the Internet in operation. However, it is limited to an organisation. Users may browse computers within an intranet using a browser but will usually not be able to access the wider Internet. In the same way, outsiders will not be able to access the intranet of an organisation. An intranet can be thought of as a private Internet.

An extranet is an extension of a organisation's intranet to include outside users. In an extranet, outside organisations or individuals are allowed access to certain parts of the intranet. This access is usually controlled by means of passwords and access rights. These restrict which users can access the extranet and what they can do once they have access. The purpose of the extranet is to facilitate business transactions with other organisations.

The PSTN or Public Switched Telephone Network refers to the originally telephone network. From a communications perspective it was slow and unreliable. Some of the exchanges on a PSTN may still make use of mechanical switches to route telephone calls. These add additional noise to the line. When lines are noisy, signals have to be resent repeatedly between the source and the destination. The PSTN makes use of analogue technology. Analogue technology uses continuously variable signals. An example of an analogue signal is ordinary speech. Newer digital technologies make use of pulses of fixed magnitude and duration.

In order to improve connections, it is possible to have an analogue leased line. This is a dedicated permanent telephone connection between two computers using the PSTN.

In order to connect a computer to a telephone network, you need a modem. This is an abbreviation for modulator-demodulator. The function of the modem is to convert the digital signals from the computer into an analogue form suitable for trans