Standard Grade C o m p uting Studies. Syst e m s. Faculty of Business Education and ICT

Transcription

1 Standard Grade C o m p uting Studies C o m p u t e r Syst e m s Faculty of Business Education and ICT

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3 1. Foundation What is a computer? A computer is an information-processing machine. That means the it takes in raw information, makes some changes to it, and then puts the information back out in a new more useful form. This is called input, process, output (or I.P.O. for short). Information that is processed and stored by a computer is called data. This means that when we know the structure and meaning of the data it is called information. Everything that goes on in the world can be thought of as input, process, and output. For example, brushing you teeth! Input Process Output Toothpaste and Cleaning Clean teeth + fresh Brush Teeth breath Figure 1 - I.P.O. Input, Process, Output What makes a computer work? A computer must have a processor (a brain) and memory (so that it can remember the programs and data that it is using). It must also have an input device (a way of putting information into the computer) and an output device (so that the computer can show us what it has done). Finally a computer needs backing storage. Backing storage is where the computer puts all its programs and data when it is not using them. Data that needs to be saved for another day is written on to backing storage. When this data is needed again it is read from backing storage. How does all this fit together? Here is a diagram of a basic computer system. Central Processing Unit Input Devices Main Memory Processor Output Devices Data Flow Backing Storage Figure 2 - Simple block diagram of a computer system with arrows showing how data flows between parts of the system.

4 Main Memory and processor together make up the Central Processing Unit or CPU. The CPU is the brain of the computer because, like our brain, it contains both processor and memory. The part that thinks is the processor and the part that remembers is the memory. The CPU controls what happens to all the devices connected to it and carries out all the instructions held in programs. An input device allows the user to communicate with the CPU and the CPU uses an output device to show the user what it is doing. Programs and data are loaded from backing storage, when needed, and are stored in memory. What is memory? Computer memory is very much like our own. When the computer is working all the programs and data that it is using are stored in memory. The computer has two different types of memory. Read Only Memory or ROM The computer can only read this kind of memory. The computer cannot put any data into ROM. The computer uses ROM as a library where it can find important programs and data. These programs and data are built into the ROM chip when it is made. Random Access Memory or RAM This kind of memory can be read from and written to. The computer uses RAM to store programs and data that it has loaded from backing storage or received from an input device. RAM stores data for as long as the computer is switched on. When the computer is switched off, all the data in RAM is lost. This is why we use backing storage to keep programs and data when they are not being used. Types of Computers There are a number of different computer types. These computers are designed to do a range of different jobs. Microprocessor A microprocessor is a specialised electronic circuit or chip which can perform certain operations. Microprocessors can be found in many household appliances as well as in all computers. For instance the microprocessor in your Washing Machine would be used to control the machine by executing the wash programs (e.g. when to spin, add powder etc.).

5 Figure 3 - A microprocessor chip All computers are made up of different types electronic chips and some of these chips are microprocessors. Desktop A desktop computer is personal computer that can fit on a user's desk and perform business-computing tasks. Also, especially if linked to a network of other computers, it may be referred to as a workstation Desktops are the most common type of computer. Figure 5 - Laptop Computer Figure 4 - Desktop Computer Laptop/Notebook A laptop computer (also known as notebook computer) is a small mobile personal computer, usually weighing around from 1 to 3 kilograms (2 to 7 pounds). A laptop computer is small enough to sit on your lap. The laptop computer's small size allows you to take it almost anywhere and access the Internet. Laptops usually run on batteries and have a clam-shell constructions which allows them to be opened for work or closed. Palmtop Personal digital assistants (PDAs or palmtops) are handheld devices that were originally designed as personal organisers, but became much more versatile over the years. A basic PDA usually includes a clock, date book, address book, task list, memo pad and a simple calculator. One major advantage of using PDAs is their ability to synchronize data with desktop and laptop computers. Figure 6 - A Palmtop being used

6 Hardware Devices Now you will look at some of the different devices that we plug into a computer to allow the computer to do a useful job. There are three different types of device: input, output and backing storage. Input devices let you give information and instructions to the computer. Output devices let the computer show or give you the answers to your instructions. Backing storage devices let you to save all the programs and data used by the computer. You can also load programs and data from backing storage when they are needed. Input devices Figure 7 - Typical computer keyboard Keyboard The keyboard is made up of lots of keys that, when they are pressed, they cause a certain signal to be sent to the computer. This signal tells the computer which key on the keyboard has been pressed. A keyboard is the most common form of input device. Mouse Figure 8 Two-button mouse This is a simple input device that is used along with a keyboard. The mouse moves an arrow or pointer around the screen. As the mouse moves around the top of the desk, the arrow on the screen moves also. You use the mouse to point to things on the screen and then select them using one of the buttons on the top of the mouse. A mouse is often used with graphical data such as drawings or graphics on screen.

7 Trackball A trackball is really like an upside down mouse. Instead of moving the mouse around the desktop, you move a little ball that is inside a casing. You move the ball using the palm of your hand or your thumb. When the ball is moved, the pointer moves across the screen. Like a mouse, a trackball is often used with graphical data such as drawings or graphics on screen. Many laptop computers used a trackball because it takes up less space than a mouse. Graphics Tablet This device is often used along with specialised software such as Computer Aided Design systems. The tablet has a section which lists all the commands available and has a flat panel on which a digitiser can be moved. The digitiser is similar to a mouse in that it has buttons which can be pressed to make selections. However there is no rolling ball. The tablet detects the position and actions of the digitiser and passes this information to the CPU. For example, a plan of a house needs to be input into a computer. It would cost a lot of time and money to enter the plan from scratch into the computer. Instead, the paper version of the plan can be placed on the flat bed of the graphics tablet and the digitiser can be moved to all the important points on the plan (corners, walls etc). In this way, the whole plan can be transferred to the computer in a very short space of time. Touch sensitive screen This is a type of display screen that has a touch-sensitive panel covering the screen. Instead of using a pointing device such as a mouse or light pen, you can use your finger to point directly to things on the screen. Although touch screens provide a natural way for users to act with computers, they are unsatisfactory for most applications because the finger is really to big for this task. It is impossible to point accurately to small areas of the screen. Also most users find touch screens tiring to the arms after long use. Joystick A joystick is a computer device consisting of a handheld stick that moves about one end and transmits position to a computer. They are often used for computer games or to control devices for disabled people such as wheelchairs. Trackpad A touchpad is an input device commonly used in laptop computers. It is used to move the cursor, using the movement of the user's finger. A trackpad is a substitute for a computer mouse. Trackpads vary in size but are rarely made larger than 50 cm².

8 Output Devices VDU The Visual Display Unit is another common form of output device. This is the television like screen that we look at to see what we are doing on the computer. VDUs are also called monitors. Printers A printer is an output device that produces a printed copy of what you have been doing on the computer. The printed copy is usually on paper. There are three main types of printers. Inkjet Printer Inkjet printers produce printouts by firing tiny drops of ink at the paper. Each drop makes up part of the letter or picture. Inkjet printers are more expensive than dotmatrix but are cheaper than laser. They produce good quality printouts and do not make much noise. They can print out between two and five pages in a minute. Many ink jet printers can print out in colour. Laser Printer Laser printers make printouts in the same way that a photocopier makes a copy. They produce very high quality printouts and work very quietly but are expensive to buy. They can produce between eight and twelve pages in a minute. Resolution Most ink-jet printers produce poor quality printouts because they have a very low resolution. This means that only a few large dots are used to make a character. The more dots used to make up a picture, the smaller the dots are and the higher the quality of the printout. Ink-jet printers are good quality because they use a resolution of between 300 to 600 dots per inch. Laser printers are very good quality because they use a resolution of between 600 and 2400 dots per inch. Dot Matrix Printer 72 dpi Ink-jet Printer 300 dpi Summary of Printers A summary of the different features of printers is shown below. Laser Printer 600 dpi Type of Printer Speed Cost Noise Quality Ink jet Medium Mid Quiet Good Laser Fast Expensive Very Quiet Very Good

9 Other devices used for output Plotter A plotter is a device used for creating a hard copy of the output from a computer system. Plotters use special pens to draw the output as a series of lines. A plotter consists of a movable set of pens, which move across the paper, plotting (or drawing), as it moves. Plotters are most commonly used for producing detailed drawings such as electronic designs and building plans that often have to be printed on very large sheets of paper. Backing Storage Devices and Media Backing storage is a way of permanently storing programs and data. We need backing storage because when we switch off the computer all the data in RAM, Random Access Memory, is lost. Magnetic tape, hard disc and floppy disc are all examples of backing storage media. A backing storage device is what we use to read or write the media. This means that to read or write a floppy disc we must put it into the floppy disc drive. When we write data to a backing store, we often say that the data is being saved. When we read data from a backing store, we say that the data is being loaded from the backing store. Magnetic storage Magnetic disc Magnetic discs are made out of metal or plastic which is coated in magnetic material. There are two types of magnetic disc. Floppy disc A floppy disc is made out of flexible plastic and is coated with magnetic material. The plastic disc is inside a protective case. Floppy discs store programs and data when the computer is not using them. A floppy disc is an example of a backing storage medium. Three and a half inch (3½ ) is the most common size of floppy disc, but there are some other sizes too. Floppy discs are very useful because you can copy information from one computer to another using them. A floppy disc must be inserted into a floppy disc drive so that it can be read and written. All magnetic disc drives, allow the computer to read information from or write information to the disc in any order. The computer can jump from file to file on the disc just like choosing the tracks on a music compact disc. Hard disc This is a solid disc (made of metal or very hard plastic) which is permanently fixed inside the computer or close by it in its own box. A hard disc is not portable like a floppy disc. Figure 9 - Hard disc

10 A hard disc works in the same way as a floppy disc. You can store more data on a hard disc because the surface is hard. Although a hard disc and a floppy disc may be the same size to look at, you can store more data on the hard disc. Hard discs can also spin at higher speeds than floppy discs. This means that data can be read from a hard disc faster than it can be read from a floppy disc. Hard discs are permanently fixed inside their drives. Magnetic Tape Reading and writing to and from magnetic tape is much slower than to and from a magnetic disc. But magnetic tape is very cheap and, because of this, large magnetic tapes are used to store lots of data. Storing data on magnetic tape is much cheaper than storing data on magnetic disc. Magnetic tapes are read and written using a magnetic tape drive. Another name for a magnetic tape drive is tape streamer. Figure 10 - Magnetic tapes in a computing room Non-Magnetic Storage Optical Disc Optical disks allow us to store very large amounts of data in small spaces. We shall look at two types of optical discs. CD ROM This works in just the same way as music compact discs. Data is stored in a digital form on the surface of the disc in a series of pits in the surface of the disc. eg. These pits can be read by shining laser light onto the surface of the disc. The disc rotates at a very high speed and the laser beam is reflected off the discs surface. Data cannot be written to CD ROM (in the same way that we cannot record on a music CD). Figure 11 - CD-ROM Disc CD ROMs are manufactured by pressing a glass disc into plastic. This glass disc has the data to be stored written on to it. When the glass disc is pressed into the plastic the binary data is transferred to the plastic disc (the CD). DVD-ROM These DVDs are read-only disks that also have enough storage capacity for a fulllength feature film. They are accessed using a special DVD drive attached to a personal computer. Most of these drives are backward-compatible with CD-ROMs and can play DVD video disks. DVD-ROMs are manufacture using a process similar to that used for CD-ROMs.

11 How does the computer store things in memory? The computer works on electricity. Electricity inside a computer can be ON or OFF. It has what we call two states. The ON state has a value of 1 and the OFF state has a value of 0. All information used by a computer system must be stored as a pattern of ones and zeros. Bits, Bytes and binary units of storage Each of these ones or zeros is called a binary digit or bit for short. Eight of these bits make up what is called a byte. About one thousand of these bytes makes a kilobyte. Again roughly one thousand kilobytes makes a megabyte and roughly one thousand megabytes makes a gigabyte. Roughly 1000 gigabytes makes a terabyte. Each storage location in memory can store a certain number of these ones and zeros. Binary Unit Value Bit 1 or 0 Byte 8 bits Kilobyte (K) 1024 bytes Megabyte (MB) 1024 kilobytes Gigabyte (GB) 1024 megabytes Terabyte (TB) 1024 gigabytes These terms are used to measure the size of anything that is used to store data in a computer system such as a hard disc, CD-ROM, memory etc. For example, most modern computers have between 512 Mb and 1 Gb of RAM. Main memory size This size of a computers memory is referred to as main memory size. So a typical home computer main memory which could stored 1 Gb of data while the computer is running. Remember, main memory is where the computer does most of it s data storage while it is running programs. The operating system (the program which makes the computer work), programs (such as word processors etc.) and data (such as word processing file) are all loaded into main memory when the computer is being used.

12 The Operating System The operating system software (OS) is the most important part of a computer s system software. Without the operating system software the computer would be useless. All computers need an operating system to tell them how to work. The operating system software is the very first program or set of programs loaded when the computer is switched on. The operating system software controls and monitors the operation of the computer system and how the hardware connected to the CPU is used. Filing System The system that an operating system uses to organise and keep track of files. Directory/Folder The filing system can access the contents of floppy and hard discs and other backing stores. Data is stored on most modern backing stores in areas called directories or folders. Each folder can contain more folders (called sub folders) and/or files. Each file (or sub folder) inside a folder must have a unique name. Answer these questions. 1. What happens to your data when the power is disconnected from a desktop computer? 2. Mr Robert Sleigh runs a snowboard shop and has just bought a laptop computer. He keeps on losing the data he has created because the battery for his computer keeps running down. What would you suggest he does, so that he does not lose his data? 3. Write down the following backing storage media along side its own backing storage device. Media Floppy disc CD-ROM Hard disc Device Hard disc drive CD-ROM drive Floppy disc drive 4. Describe, in your own words, why we need backing storage. 5. Explain clearly the meaning of the terms: bit, byte, kilobyte and megabyte. 6. What is main memory?

13 2. General More on Main Memory The two types of memory (RAM and ROM) can be added to our diagram of a computer system. Central Processing Unit Input Devices RAM ROM Output Devices Processor Data Flow Backing Storage Figure 12 - Block diagram of a computer system with RAM and ROM How is memory organised? Both RAM and ROM are made up of small spaces called storage locations. Each storage location has its own special number called an address. Each storage location can store a certain amount of data. Addresses in Memory Each location in memory will have an address. This is a number which identifies the location in memory. This is like the number on the outside of a house and is different from any number stored inside the location e.g. The lowest memory location is zero but the highest depends on the size of memory that the machine has. It is possible to fetch data from these locations in any order. This type of access is called RANDOM ACCESS or DIRECT ACCESS. Data held in RAM is lost when the power is switched off. Both data and instructions are held in Main Memory. More on Types of Computer A large, expensive, powerful central computer. Called a mainframe because early computers occupied a number of metal frames, the main one of which contained the processor and memory.

14 Figure 13 - Main frame Computer Room with operators These machines can and do run successfully for years without interruption, with repairs taking place whilst they continue to run. Mainframes often support thousands of simultaneous users who gain access through "dumb" terminals or terminal emulation. More on Hardware Devices More on Input Devices Figure 14 - A Hand-held Scanner and Flatbed Scanner Scanner A scanner is used to take information stored on paper and read it into a computer system. Scanners can be used to convert photographs, paintings and typed text into a form that can be stored in a computer. There are two types of scanners. Hand-held scanners read documents by rolling the scanner across them. Flatbed scanners read documents placed face down on them like a photocopier. Digital Still Camera A digital still camera looks and behaves like a regular camera, except instead of using film, it stores the image it sees in memory as a file for later transfer to a computer. Many digital cameras offer additional storage besides their own internal memory; a few sport some sort of disk but the majority utilize some sort of flash card which can be removed and inserted in a special reader connected to a computer. Many digital cameras currently lack the resolution and colour palette of real cameras, but are usually much more convenient for computer applications. Still cameras do not recording moving images.

15 Digital Video Camera A digital video camera takes stores video on digital tape, disc or memory card. The video is stored in a digital format. The video can then be easily transferred to a computer for editing. Figure 15 - Digital Video Camera Web Cam a digital camera designed to take digital photographs and/or video and transmit them over the internet Figure 15 - Web cam Microphone A device that converts sound waves into audio signals. These audio signals can then be converted into digital values and stored in the computer. Microphones are often used with webcams for video conferencing. More on Output Devices Figure 16- Liquid Crystal Display Diagram Liquid Crystal Display (LCD) The type of display found on digital watches, lap-top computers and some flat-screen monitors. Their major advantage is low power consumption, leading to long battery life for portable devices.

16 TFT Abbreviation of thin film transistor, a type of LCD flat-panel display screen where each pixel is controlled by its own transistor. This technology provides the best resolution of all the flat-panel techniques. Also sometimes called active-matrix LCDs. Loudspeakers Loudspeakers are audio output devices which take the output from a computers soundcard and covert it back into audio waves. Loudspeakers can be anything from small headphones to surroundsound cinema style speakers. Figure 17 - Surround Sound Loudspeakers More on Backing Storage CD-R recordable compact disk. Disk can be written to by a user with the proper kind of CD drive. Cannot be erased to be written on again. One time write only! CD-RW similar in virtually all respects to a CD-R, except that a CD-RW disc can be written and erased many times. This makes them best suited to many backup tasks. CD-RW drives can also create CD-R discs. DVD-R A write-once, recordable format. DVD-R drives can write DVD-R discs, which can be written only once. DVD-RW DVD-RW (recordable/rewritable) drives can write both DVD-R discs and DVD-RW rewritable discs, which can be written to and erased many times. USB Flash Drive Universal Serial Bus is an interface which allows various devices to be attached to a computer. A flash drive is a device which contains memory chips which can be used to store data permanently. These devices are small and portable. They have capacities up to 2Gb (the most common being 512 Mb (0.5 Gb).

17 Comparison of printers in terms of: You may be asked in an exam to compare printing devices based on the following factors. Speed (ppm) Pages per minute is used as a means of comparing printers. General the more pages a printer can print in a minute the better for the user (the user has less time to wait for the printouts). Capital Costs The initial cost of the printer and it s installation are capital costs. Printers can range in price from a few tens of pounds to thousands or tens of thousands of pounds. Some printers require network cabling, special power supplies, additional computers etc. All of this adds to the capital cost of the printer. Running costs The running costs are the costs to operate the printer. These include the cost of: Paper and other printing media Power Ink or toner for the printer Service contacts or support for hardware failure Resolution Is an indication of the sharpness of images on a printout. It is based on the number and density of the dots used. The more dots used in an image, the more detail can be seen and the higher the image's resolution. Resolution for printers is measured in dots per inch (dpi). The higher the dpi the better quality the image. Comparison of backing storage You may be asked in an exam to compare backing storage devices based on the following factors. Speed (data transfer) Backing storage devices can be compared in terms of the time it takes to transfer data from the disc to the computer. Speeds are most commonly measured in Mbits (megabits i.e. thousands of bits per second) A typical floppy disc drive transfers data at 1 Mbits/sec A typical 32x CD-ROM drive transfers data at 4.8 Mbits/sec A typical 16x DVD-ROM drive transfers data at 16.7 Mbits/sec A typical Hard disc drive transfers data at 150 Mbits/sec These are all typical statistics and given as a rough guide Cost Floppy drives are cheaper than CD-ROM drives which are cheap than DVD-ROM which are cheaper than Hard-discs. All devices can be compared on a cost basis.

18 Capacity Refers to how much data a medium can store. Floppy disc is 1.44 Megabytes Most common USB Flash Drive size is 512 Megabytes CD-ROM is 673 Megabytes DVD-ROM is a maximum of 17 Gigabytes The most common hard disc size if currently 120 Gigabytes Representing data inside the computer The computer can only store data in binary (ones and zeros). This means that all the numbers, text and graphics that we use must be stored as ones and zeros inside the computer. Numbers We use the decimal system (0,1,2,3,4,5,6,7,8,9) when we count things, but the computer uses the binary system (0,1). We count like this The computer counts like this Figure 18 - Decimal and Binary numbers from zero to sixteen The decimal system uses these headings Tens of Thousands Thousands Hundreds Tens Units 10 x 10 x 10 x x 10 x x So the number 2379 is 2 thousands = 2 x 10 x 10 x 10 = hundreds = 3 x 10 x 10 = tens = 7 x 10 = 70 9 units = 9 = 9 = 2379 The headings for the binary system look like this Sixty Four Thirty Two Sixteen Eight Four Two Unit 2 x 2 x 2 x 2 x 2 2 x 2 x 2 x 2 x 2 2 x 2 x 2 x 2 2 x 2 x 2 2 x 2 2 1

19 So the number is 1 thirty-two = 1 x 2 x 2 x 2 x 2 x 2 = 32 0 sixteen 1 eight = 1 x 2 x 2 x 2 = 8 1 four = 1 x 2 x 2 = 4 0 two 1 unit = 1 = 1 = 45 So the binary number for 45 is This shows you that any decimal number that the user inputs to the computer can be stored and processed by the computer as a binary number. Before the computer displays its results, it converts any binary numbers back to decimal so that the user can understand them. Text All the characters that the user enters at the keyboard need to be stored in the computer using ones and zeros. To do this, each character is given its own special number. This number is called its code. We can then store this code in the computer using binary ones and zeros. There are several different ways of giving characters codes, but the most popular one is called ASCII. Pronounced ask-ee, ASCII stands for American Standard Code for Information Interchange. What happens is that each letter, number or symbol is given a number code from 0 to 127. For example, the ASCII code for uppercase M is 77 and for m is 109. Most computers use ASCII codes to represent text, which makes it possible to transfer data from one computer to another. Here is a table of ASCII codes.

20 Code Character Code Character Code Character Code Character 0 33! 66 B 99 c C 100 d 2 35 # 68 D 101 e 3 36 $ 69 E 102 f 4 37 % 70 F 103 g 5 38 & 71 G 104 h 6 39 ' 72 H 105 i 7 Bell 40 ( 73 I 106 j 8 Backspace 41 ) 74 J 107 k 9 42 * 75 K 108 l 10 New line L 109 m 11 44, 77 M 110 n 12 New page N 111 o 13 Return O 112 p 14 Shift (in) 47 / 80 P 113 q 15 Shift (out) Q 114 r R 115 s S 116 t T 117 u U 118 v V 119 w W 120 x X 121 y Y 122 z Z 123 { : 91 [ ; 92 \ 125 } 27 Escape 60 < 93 ] 126 ~ = 94 ^ 127 _ > 95 _ 30 63? 96 ` a 32 Space 65 A 98 b Figure 19 - ASCII Codes ASCII uses one byte to store each character. Remember that eight bits make a byte. The letter M has an ASCII code of 77. When 77 is converted to binary it is So in the computer s memory or on the backing storage, M would be represented as the byte

21 Graphics Computer need some way of storing graphical images such as drawings from graphics packages and images from computer games. Pictures that appear on computer screens are generally made up of a grid of dots. The dots are called pixels, which is short for picture elements. Pixels are set ON for black and OFF for white. Remember ON has a value of 1 and OFF has a value of 0. Here is an example of a graphic that has been enlarged so that you can see each pixel. Original image 2 x size Figure 20 - Pixels in a graphic 4 x size Each pixel is stored in memory as either a 1 or a 0. A graphics is made up of a grid of pixels and stored in memory as a bit pattern of ones and zeros Low resolution Figure 20 Low resolution graphic with its binary representation. The resolution (or the detail) of the graphics depends on how many pixels make up the grid. The more pixels in the grid, the higher the resolution and the greater the amount of detail that can be included in the graphic. Also, as the resolution gets higher, the pixels have to get smaller.

22 High resolution Figure 21 - High Resolution Graphic Often the resolution of monitors and printers is measured in dots per inch or dpi for short. Each dot is equal to a pixel. Operating Systems - Different modes of processing The operating system is a program which can process instructions in two different ways. Interactive Accepting input from a human. Interactive computer systems are programs that allow users to enter data or commands. Most popular programs, such as word processors and spreadsheet applications, are interactive. In interactive processing, the application responds to commands as quickly as it can once they have been entered. Sometimes there is a delay while the processor finishes another task. Real Time Real-time operating systems are systems that respond to input immediately. Automatic teller machines for banks are an example of real time processing. Real time systems are used for tasks such as navigation, in which the computer must react to a steady flow of new information without interruption. Types of File Software can be stored on backing storage as files. There are two types of files. Program files (such as the operating system, word processors etc). Data files (such as graphics images, documents etc).

23 High level languages A High Level Language is a tool which allows us to write computer programs without having to use the binary codes that the computer understands. High level languages make it easier for us to write programs, because we can use command words that mean something to us such as PRINT, END, IF, THEN. Examples of modern high level languages are Java, Visual Basic, Perl and Delphi. Figure 41 shows an example program written in the Perl high level language. Perl uses words like OPEN for opening a file, CLOSE for closing a file and PRINT for sending information to the VDU or to a file. These words are much easier for us to understand than the machine code program above. open (MAIL, " /usr/lib/sendmail -t"); print MAIL "To: $sendtoaddress\n"; print MAIL "From: $my address\n"; print MAIL "Subject: $subject\n"; print MAIL "Put your message here in the body.\n"; close (MAIL); Figure 22 - A sample PERL program an example of a High Level Language Common features of high level languages There are lots of different HLLs but they all still have several things in common. The commands that tell the computer what do to are written in English-like words and phrases. This makes it easy for us to write and understand the program. Programs written in a high level language must be translated into machine code before they can be run. A program called a translator does this. High level languages use standard arithmetic. So all the computer symbols for addition, subtraction, multiplication and division can be used in high level language programs. When we write the program, we only have to think about what the program has to do, not how the computer will actually do it. It s a bit like driving a car. To drive a car we just need to know how to drive. We don t need to know how the car engine works. High level language programs are portable. This means that we can easily change them so that they can run on different computer systems.

24 Answer these questions. 7. Copy and complete this block diagram of the computer system including the contents of the Central Processing Unit. OUTPUT RAM 8. a) What happens to ROM when power is disconnected from a computer? b) What happens to RAM when power is disconnected from a computer? 9. A program is needs to be loaded automatically every time the computer is started up (booted). Should the program be stored in RAM or in ROM? Explain your answer. 10. How many bytes are required to store this graphic? a) Yusuf compares two computer printers in a shop. One has a resolution of 300 dpi and the other has a resolution of 600 dpi. Which printer is better for printing graphics? b) Explain your answer. 11. Describe how black and white graphics are stored in a computer system. 12. Copy and convert these binary numbers into decimal. The first one is done for you = Describe a typical computer keyboard.

25 14. Describe how a mouse is used as part of a computer system. 15. a) What type of input device would be used by architects and designers for creating drawings and plans? b) Why is this device particularly useful for this type of task? 16. Jenny has painted a picture for the school magazine. The magazine staff would like her to give them the picture as a computer graphic. What input device should she use to convert the picture? 17. Shelley works in an office. Everyone in the office is fed up with the noise that the ink-jet printers make. Shelley suggests changing over to laser printers. Give two advantages and one disadvantage of changing to laser printers. 18. Shamiel works in a design studio where they create drawings for the latest computer chips. The designs often have to be printed out on very large sheets of paper. Which type of output device would you recommend? 19. Paul is a manager in a busy stock room. He needs to print lots of information at a very cheap price. Paul is not worried about the quality of the printouts as long as they are readable. Which type of printer would you recommend? 20. Printers use d.p.i. as a measurement of their resolution. What does d.p.i. stand for? 21. What does VDU stand for? 22. Corrie needs a printer that can print out in colour. Which type of printer: ink-jet or laser, is most likely to be able to print in colour? 23. Data can be stored and retrieved from the following media FLOPPY DISC HARD DISC MAGNETIC TAPE List them in order of speed and price, starting with the lowest. 24. State two reasons why you would recommend a hard disc rather than a floppy disc for storing files. 25. What do we mean by the capacity of a backing storage device?

26 3. Credit THE PROCESSOR The Processor consists of two main parts. These are the ALU (Arithmetic and Logic Unit) and the Control Unit. Figure 23- Detail of the CPU Processors must be able to: Address Main Storage Fetch, Decode and Execute instructions Do Arithmetic Calculations Perform Logical Operations Control the order (sequence) of operations throughout the system (INPUT, OUTPUT, BACKING STORE and within the CPU) ALU The two main functions of the ALU are: 1. To carry out the arithmetic required by the system (ADD, SUBTRACT, DIVIDE and so on) 2. To perform certain 'logical operations'. e.g. To test if one data item is larger than another and then to perform some action depending on the result of this test. Control Unit This is the nerve centre of the computer. It controls the sequencing of all CPU operations both within the CPU and in connection with INPUT, OUTPUT and BACKING STORAGE devices. Later on, we will look at input, storage and output devices. Now we will go on to look at how information is stored inside a computer system.

27 Registers The registers are small storage locations inside the processor where data can be stored while the processor is executing instructions. Registers include the MDR (Memory Data Register), MAR (Memory Address Register), IR (Instruction Register), ACC (Accumulator) etc. Control Unit MDR ACC MAR IR X Y Registers ALU Figure 23 - Detail of the Processor Word Remember each location in memory can store a certain amount of data. This amount of data is often called a WORD. A word is the total number of bits that the processor can process in one operation. It is usually the same as the number of bits stored in each memory location. An X-Box is a 32-bit computer system because it can process 32 bits in one operation, hence it s word size is 32 bits. The old ZX Spectrum (a computer popular in the early 1980 s) had a word size of 8 bits. The next generation of Play Station (PS3) is 64-bit, a 64 bit word size. Making backups Imagine someone has given you some very important information written down on paper. If you lose the information on the paper you will get into trouble. So what do you do to make sure that nothing can happen to the information? You make a copy of it and keep it in a safe place. Original Backup Copy of original is made Backup is kept in a safe place Figure 24 - Backing up data If anything happens to the original you still have the copy. You can take the copy and use it to replace the original.

28 Backing up to floppy One or two small files Backup copy on floppy disc Backing up to magnetic tape High capacity hard disc Figure 24 - Backup media Backup copy of hard drive on magnetic tape. When we have important data on a computer system, we make a backup of it and keep this backup in a safe place. If you were backing up one or two small files, you would just copy the files on to a floppy disc. If you were backing up a large capacity hard disc, then you would copy the data on to magnetic tape because it is a cheap way of storing large amounts of data. We can copy these backup copies of the data back to where they came from if the original data is lost or damaged somehow. Common features of operating system software. All operating systems have certain standard functions. These standard functions can be found in all operating system software. These include: Managing Memory The operating system controls how blocks of the computer s memory are given to programs and data. Managing Input and Output The operating system checks input devices like the mouse and keyboard to see if data has been input to the computer system. The operating system controls the sending of data to output devices like the VDU and the printer. File Management The access to all types of backing storage devices is controlled by a special part of the operating system called the filing system. Whenever data is saved or loaded the filing system controls what happens. Whenever data is saved the operating takes care of how the data is written to the backing store. So, if you are using a piece of application software and wish to save your data to the backing store, you would use the SAVE command in the application software which

29 would then pass the data to be saved to the operating system which would then write the data to the computer hardware. Application software Operating System Backing store Figure 24 System software allows us to manage and use the computer hardware. The part of the operating system that manages backing storage is the filing system The filing system will prevent two programs trying to access the same files at the same time. More on this later. Hierarchical filing system A filing system in which directories have files and subdirectories beneath them. A hierarchical filing system is one that uses directories to organise files into a tree structure. Computer C:/ D:/ Drives Folders Files A hierarchical filing system Types of access Random and Sequential Magnetic discs use random access and magnetic tapes use sequential access. But what does this mean?

30 The easiest way to explain this is as follows. Think of a compact disc and a cassette tape. Each holds information about sound. If we want to listen to a certain track on the compact disc, say track number five, all we have to do is enter the track number and the player will jump straight to the start of that track. But if we want to listen to the same track on our cassette machine we have to fast forward past all the first four songs on the tape to get to track five. The compact disc is a random access medium and the cassette tape is a sequential access medium. Sequential Access To go from file A to file Z in a sequential-access system, you must pass through all intervening files. Sequential access is sometimes called serial access. Random Access Refers to the ability to access data at random. In a random-access system, you can jump directly to file Z. Disks are random access media, whereas tapes are sequential access media. Security In some operating systems, especially ones that run on computer networks, the access to the computer system is controlled by the operating systems through passwords.

31 User interface The operating system provides user interface. This is the means of communicating with the computer system. Features such as icons, commands etc. are all provided by the operating system. Icon Menu Window Toolbar (added by a utility program) Desktop Pointer Figure 24 - Operating System Desktop Error Reporting When an error occurs the operating system provides information to the user about what has happened and, sometimes, how it can be fixed Special Functions of Operating Systems Interactive systems with background job capability Some operating systems allow a background process to occur at the same time as a foreground interactive process. The foreground process is the one that accepts input from the keyboard, mouse, or other input device. Background processes cannot accept interactive input from a user, but they can access data stored on a disk and write data to the video display unit. For example, some word processors print files in the background, enabling you to continue editing while files are being printed.

32 Programming Languages You remember from earlier that computers can only store binary; that is, programs and data written in binary form, ones and zeros. This means that as well as storing numbers, text and graphics, the computer must also store programs as binary. The name for a computer program written in binary is machine code. Machine code programs look like figure Figure 25 - Machine Code Program There are many different makes and models of processor chip and all of these have their own machine code language that is specific to them. This means that a machine code program written for an Apple Macintosh computer will not work on a Pentium PC, because the machine codes used by the two processors are different. Machine code is very difficult for people to use and understand. But it is the only language that the processor can understand. To make programming easier we need to write the program in a language that we understand. Then the computer converts it into machine code for processing. The computer translates the program from our language into machine code for the CPU to run it. High level languages We use high level languages (HLLs) because programming in low level languages like machine code is VERY difficult. Translators As mentioned earlier computers can only understand instructions and data which are written in binary form. Programs written in high level languages need to be altered in some way so that they can be understood by the computer. This is done by a piece of systems software called a translator. There are 2 types of translator.

33 Compiler This program converts high level language code into machine code. The compiler takes all the source code (the high level language program before anything is done to it) as one block and translates it into machine code. This translated program can be run at any time without the source code or the compiler because the translated program is stored as machine code and this machine code can be understood by the processor. Figure 26 - Compilation If a change is made to the source code then the program must be compiled again so that the change is incorporated in the finished compiled program. The finished compiled program is called object code. Object code is created every time the source code is compiled. Figure 27 - An interpreter translates and executes each program instruction one at a time The compiler gets its name from the way it works, looking at all of the source code and collecting, reorganising and converting the instructions into machine code. Compilers require some time to create an executable program. However, programs produced by compilers run much faster than the same programs executed by an interpreter. The program produced by the compiler is executable. It does not need the compiler or the source code. Many compilers are available for the same language. For example, there is a FORTRAN compiler for PCs and another for Apple Macintosh computers. This makes the program source code PORTABLE (able to run on a number of different computer processors).

34 Interpreter An interpreter translates and executes each source code statement in turn as the program is run. In simple terms the interpreter does the following: 1. Fetch instruction 2. Translate instruction into machine language equivalent(s) 3. Execute machine language instructions. Programs translated using an interpreter can only be run in conjunction with the interpreter. If the interpreter is not present then the source code instructions cannot be translated into their machine code equivalents. An interpreter translates high-level instructions into a machine code line by line. Each HLL statement is converted, in turn, into machine code and then executed. An interpreter can immediately execute high-level programs (unlike a compiler). For this reason, interpreters are sometimes used during the development of a program, when a programmer wants to add small sections at a time and test them quickly. Also interpreters are often used in education because they allow students to program interactively. The advantage of an interpreter over a compiler is that it does not need to go through the compilation stage during which machine code instructions are generated. Many interpreters are available for the same language. For example, there is a BASIC interpreter for PCs and another for Apple Macintosh computers. This makes the program source code PORTABLE. Relative Advantages Of Compilers and Interpreters. 1. Compilers produce executable programs. eg. They produce programs that can be run on a computer system without the need for the source code or the compiler/assembler to be present in the system. 2. Compiled programs run much faster than interpreted programs. This is because interpreted programs read and translate each instruction in turn and this takes time. But compiled programs already have all their instructions converted to machine code. 3. Interpreted programs take less time to develop (write) because if a change is made to the source code then this change is included in the next program run. But with compilers any changes to the source code are not included in the executable code until the source code is re-compiled or re-assembled. Each time a change is made a new version of the executable program must be made.

35 Specialised Hardware Devices Input / Output Devices Virtual Reality Virtual Reality describes a system that enables one or more users to look, move, and react in a computer-simulated world. Various types of interface devices allow users to see, touch, pick up, and even manipulate virtual objects. Currently there are two general types of virtual reality experience: In Desktop VR the user views the Virtual World using a normal 2 dimensional computer monitor. Interaction is normally controlled with the mouse or the keyboard. Such Desktop VR systems are used for visualising information such as the plan of a house or to play computer games. Immersive virtual reality is by far the most exciting development of virtual reality. It involves the user 'physically' entering the virtual environment. To see the virtual world, users wear a special headset that presents a computerscreen display to each eye. The headset also contains a position tracker to measure the location of the user's head and the direction in which they are looking. With this information, a computer workstation can recalculate images of the virtual world a slightly different view for each eye to match the direction in which a user is looking, and display these images on the headset. The computer must generate views many times a second so that the user does not receive a view that jumps and appears to lag behind their own movements. Even though computers have increased dramatically in power, virtual world models still need to be kept rather simple so that the computer can update the visual imagery fast enough at least ten times a second. These virtual worlds, because of their simplicity, are still easily distinguishable from physical reality. Users can hear sounds in a virtual world through earphones contained in the headset. The information that is reported by the position tracker on the headset can also be used to update audio signals. When a sound source is not directly in front of or behind the user, the sound will arrive at one ear a little earlier or later than the other. The sound will also be a little louder or softer and slightly different in pitch. The brain compares the sound signals that arrive at each ear and uses these differences to locate sound sources in space. The computer uses the position information from the headset to transmit sounds through the earphones that seem to come from a definite location in virtual space. Haptics refers to a person's sense of touch. The haptic interface in virtual reality is the least developed and perhaps the most challenging for researchers. With the use of a