FEES 102 Information and communication technology.pdf

Overview of Computer 2.1.1 Basic Components of a Computer Although computers differ widely in their details, all of them follow a basic organizational structure as shown in the figure below. 1. Input unit 2. Processing unit 3. Output unit 4. Ext...

Overview of Computer 2.1.1 Basic Components of a Computer Although computers differ widely in their details, all of them follow a basic organizational structure as shown in the figure below. 1. Input unit 2. Processing unit 3. Output unit 4. External storage unit In order to carry out the basic three operations, namely, input, process and output, a computer includes the following hardware components: 2.1.2 Input Unit An input unit presents data to the processing unit in machine-readable form. Although the keyboard is a common input unit for a small computer, a system may also support one or more of the input units. Common input devices No. Device Medium of data Remarks storage 1. Optical character Special paper Only input reader (OCR) document 2. Magnetic ink character Special paper Only input recognizer (MICR) document 3. Mark sense reader Special paper or card Only input 4. Graphics tablet Document Only input 5. Mouse Document Only input 6. Floppy drive Floppy disk Input, output, storage 7. Hard disk (Winchester) Magnetic disk Input, output, storage drive 8. Tape drive Magnetic disk Input, output, storage 9. D-ROM drive Optical disk Input, storage 2.1.3 Processing Unit Processing units receive data and instructions, store them temporarily and then process the data as per the instructions. The processing units include:  Memory unit (MU)  Arithmetic logic unit (ALU)  Control unit (CU) All the three units together is known as the central processing unit (CPU) 2.1.4 Memory Unit The memory unit holds (stores) all data, instructions, and results temporarily. The memory consists of hundreds of thousands of cells called storage locations each capable of storing one character of information. The memory unit is called by different names, such as internal storage, primary storage, main memory or simply memory  2.1.5 Arithmetic logic unit This unit is used to perform all the arithmetic and logic operation s, such as addition, multiplication, comparison, and the like. For example, consider the addition of two numbers A and B. The control unit will select the number A from its location in the memory and load it into the arithmetic logic unit. Then it will select the number B and add it to A in the arithmetic unit. The result will then be stored in the memory or retained in the arithmetic unit for further calculations. 2.1.6 Control unit This unit coordinates the activities of all the other units in the system. Its main functions are:  to control the transfer of data and information between various units; and  to initiate appropriate actions by the arithmetic unit. The program provides the basic control instructions. Conceptually, the control unit fetches instruct ions from the memory, decodes them, and directs various units to perform the specified tasks. 2.1.7 Registers  Instruction register  Instruction address register  Memory address register  Data register  Accumulator Instruction register holds the instruction currently being executed. Instruction address register holds address of the next instruction to be executed. Memory address register holds the address of the data on which the current instruction is being performed. Data register receives data from memory for use in calculations or logical operations. Data register also receives the resulting data (after operations) before sending them to the storage. Accumulator is a register in the ALU unit where arithmetic and other operations take place. The results are stored here temporarily and are transferred to the data registers. Accumulators are part of ALU while other registers are part of the control unit. Much of what the CPU does takes place in what are known as registers. Registers serve as temporary storage areas for data and instructions within the CPU. A register can receive information, hold it, and transfer it very quickly. Depending on the purpose for which a register is used, it may be classified under one of the following types. 2.1.8 Execution steps The Central processing unit performs the following three steps repeatedly while processing data:  Fetching an instruction to the instruction register.  Instruction decoding to determine the operation to be performed and the storage address of data to be used in the operation.  Instruction execution to perform the specified operation and place the results in the accumulator. The sequence of steps that the CPU executes is called the instruction cycle. The data path through which data are transmitted between the various units in the CPU is called the data bus. The overall plan and design of various components of a computer system is referred to as the computer architecture. 2.1.9 Output Unit Output devices receive information from the CPU and present it to the user in the desired form. Although a printer is the most commonly used output unit, devices such as plotters are also becoming popular. Some common output devices are given in below: Common output devices Device Medium of Remarks Presentation Printer Paper Only output Plotter Paper Only output Visual display unit (VDU) Display screen Only output  Floppy drive Floppy disk Input, output storage Disk drive Magnetic disk Input, output storage Tape drive Magnetic tape Input, output storage 2.1.11 External storage unit The purpose of external storage is to retain data and programs for future use. For example, a program may be required at regular intervals. If such information is stored in an external storage media, then one can retrieve it as and when necessary, thus avoiding repeated typing. Any number of files containing information can be stored on external media. Since they are permanent (they are not erased when the power is turned off) we can store long files on external media, and later on work on them in sections, keeping all the sections in storage except the one currently in use. 2.1.12 Features of Memory Main memory (or simply memory) is used to store programs and data required for processing a given application. Memory can be thought of as a series of cells, each of which can store a piece of information (usually one character). This piece of information is usually arranged as a chain of 8 bits , known as a byte. Memories are often rated in terms of their information stooge capacity. Typically, capacities of memories are expressed as KB (kilobytes), MB (megabytes) or GB (gigabytes). IKB = 210 = 1024 bytes IMB = 220 = 1,048,576 bytes IGB = 230 = 1,073,741,824 bytes Memories are generally classified by the nature of accessibility of the information and are divided into two major categories, namely, random access memory (RAM) and read only memory (ROM ). ROM further has many types as illustrated below.  2.1.13 Random Access Memory (RAM) RAM is a type of memory that can be read from or written to. This is therefore a read/write random access memory. This memory stores the input data and the program that users give to the computer for immediate execution. RAM is volatile , meaning that it loses its contents when the power is shut off. Random access means that the storage cells of a RAM memory are organized so that the access time of any cell is the same for all. This is achieved by arranging the memory cells in a two-dimensional array as shown below. 2.1.14 Read Only Memory (ROM) ROM is a type of memory that can be read from but cannot be written to. ROM is non- volatile, meaning that it retains its content seven when the power is shut off. ROM is used as a part of the main memory to store programs that perform some special tasks. These programs are written permanently into the memory during manufacture. Such programs are called the firmware. Firmware technology has led to the development of several types of ROM. They include:  Programmable ROM (PROM )  Erasable PROM (EPROM)  Electrically alterable ROM (EAROM) PROM This is a type of ROM that may be customized to the needs of the users. That is, data and programs can be written in it after manufacture but once written, it becomes permanently fixed. EPROM This is a type of ROM that may be reused. That is, the contents of EPROMs may be erased by a special process and then reprogrammed/rewritten again by the user. EAROM This state-of-the-art ROM chips that may be erased and reprogrammed using an electrical signal to change the bit pattern of the program. 2.1.15 Cache memory Cache memory is a very high speed memory built into the processor to enhance the processor speed. Data that are most likely to be needed next by the CPU are kept in this memory. Cache memory is usually in the range of 512 KB of storage.  2.1.16 Input Devices An input device presents data to the processing unit in machine-readable form. Although the keyboard is a common input device for a computer, the system may also support one or more the input devices listed in the illustration on the following page. Keyboard The keyboard is very much like that of a standard typewriter with a few additional keys. The basic QWERTY layout of characters is maintained to make it easy for the trained typists to use the system. The additional keys are included to perform certain special functions such as deleting a character or editing a text. These are known as function keys that vary in number from system to system. The function keys help operators to automatically perform many tasks that would be tedious and time consuming on an ordinary typewriter. 2.1.17 Optical Character Reader (OCR) Optical character readers are input devices that are used to read any printed text. They can interpret handmade marks, handwritten characters, machine printed characters and special symbols and codes. Optical Character Readers scan the text optically, character by character, convert them into machine readable codes and store the text in the system's storage medium. They can read at a rate of up to 2,400 characters per second. This makes it possible for organizations to reduce the input keying bottlenecks. OCR devices are expensive and are used only when there are large quantities of documents to be keyed in. Other advantages of using OCR are:  It can be used to reformat printed or typed documents by inputting through the reader and outputting on the systems printer.  It can be used to consolidate texts from various documents into one document  It can serve as a link between the word processing system and the conventional typewriter. Since OCR eliminates the human element in data entry, it improves data accuracy and timeliness of output. However, if the document is poorly prepared, it may create some difficulties.  2.1.18 Magnetic Ink Character Recognizer (MICR) It is a device that can identify characters printed with ink that contain particles of magnetic material. MICR is used mainly in the banking industry to read cheques. Since the MICR system can recognise only certain character styles, the characters have to be accurately formed. 2.1.19 Mark Sense Reader (MSR) This device also known as optical mark reader (OMR) can recognise marks made by a pencil or pen on a specially designed card or form. These cards and forms are pre-printed for special purposes so that a mark in a particular position may represent certain meaning. Such forms are used in processing selection tests, survey questionnaires, and the like, where answers are recorded as dark pencil marks in certain boxes. 2.1.20 Graphics Tablet Graphics tablet is a computer-based terminal with additional features for creating, storing and printing pictures. We can create an image of a picture by simply moving a stylus on the picture. As the stylus moves the picture is created on the screen for checking. The picture can be stored or printed out on a plotter. A graphics tablet can be used for tracing existing documents such as drawings, company logos, maps or cartoons for integrating them with reports, or for creating new pictures. 2.1.21 Mouse Mouse is an input device and is used to position the cursor on the screen. It is a palm- sized box. Its manipulation on a flat surface moves the cursor in the same direction as the movement of the mouse. The box contains a ball underneath which senses the movement and transmits it to the computer over the cord.  2.1.22 Trackball A trackball is a pointing device that works like an upside-down mouse. It consists of an exposed ball which can be rolled with the help of our thumb to move the cursor around the screen. Trackballs are popular with the notebook computers. 2.1.23 Touch Screen The monitor screen can be designed to be touch-sensitive so that we can input instructions by merely touching on-screen prompts. Such screens are especially helpful when people are unfamiliar with computers. 2.1.24 Light Pen A light pen is a handheld, light-sensitive stylus attached to a computer that accepts the stylus position on the monitor as an input. We can move the pen across the screen to make menu selections or draw curves. 2.1.25 Bar Code Reader Bar code is a special bar chart created on products to represent product number, cost and other details. When a bar code is scanned, the bit patterns represented by the marked bars are translated into meaningful information. Bar codes are extensively used in libraries, departmental stores, post offices, inventory an d sales departments and so on. 2.1.26 Sensors In many scientific applications, data input is done through a sensor. These devices allow direct input of physical data to a computer for analysis. This method provides a timely and accurate data acquisition.  2.1.27 Digital Camera A digital camera can capture a scene, digitize the image and store the digitized image on a small disk within the camera. The disk can be removed and read by a computer. 2.1.28 Voice Recognition Voice-recognition devices are available that can b e used to provide voice input directly to a computer. The computer recognizes the spoken words and converts them to output, either in audio form or in printed form. Since these devices understand a limited vocabulary, they are not widely used at present. VOICE RECOGNITION AND SYNTHESIS Computer recognition of human speech and transformation of the recognized words into computer readable digitized text or instructions is known as voice recognition. Computers, at present, have limited capability of recognizing spoken words. The process of obtaining the audible output of computer-based text in the form of synthesized speech that people can recognize and understand is known as voice synthesis. Voice synthesis is much easier to achieve than voice recognition. This capability has helped blind people gain increased access to written works. 2.1.29 Output Devices Output devices receive information from the CPU and present it to the user in a desired form. Although the printer is the most commonly used output device, other devices such as visual display units (VDU) and plotters are becoming popular. The types of output devices used today are listed in the illustration below. 2.1.30 Display Screen When a text is keyed in, the screen (which is similar to a television screen) displays the characters. The user can read the text line by line and make corrections before it is stored or printed on a printer. It is also possible to display on the screen a portion of text stored in the external storage for purposes of reviewing and editing. The operator normally uses a cursor to pinpoint a particular position on the screen. The cursor can be moved along a line (in both the directions) or vertically from one line to another. This facilitates quick and easy editing. Screen sizes differ from system to system. The standard size is 24 lines by 80 characters. Most systems have provision for scrolling. This facilitates the user to move the text vertically on the screen, thus bringing to the screen the stored text. It is also possible to scroll horizontally. Thus the user can scan through the entire file either to review or to select a particular portion. The cursor on the screen is controlled by cursor keys on the keyboard. The size and colour of the displayed characters and the colour of the screen should be carefully selected to avoid any glare and strain to the eyes. 2.1.31 Printers The final output is usually obtained from printers. The paper copy obtained from a printer is often referred to as printout or hard copy. While continuous stationery (paper) is commonly used, single sheet feeding is often possible. Some systems have provision for background printing. This allows the operator to continue to work on the keyboard while the printer is printing another output. Printers are available with a variety of printing mechanisms and speeds and varying quality. We could choose a particular printer depending on our requirements. The common categories of printers available in the market are: 1. Impact printers  Daisy wheel printers  Dot matrix printers  Line printers 2. Non-impact printer  Laser printers  Ink-jet printers  Thermal printers Impact printers which have been around for a number of years print by striking the type against the ribbon and the paper (similar to a typewriter). Printers that can print only one character at a time are called character printers as contrary to the line printers which printe an entire line at a time. They can produce carbon copies, if necessary. Non-impact printers are practically noiseless and create images only on one copy. They also require special paper but can provide better quality output than impact printers.  2.1.32 Impact Printers Line printer It prints an entire line at a time and is the fastest printer. Its speed varies from 300 to 3,000 lines per minute. It has been used in data processing applications for many years. Line printers normally have 132 print positions per line. This printer generally produces only upper case (capital) letters, though some have provision for both upper and lower case letters. The quality of printing in terms of style and density is poor for office use. However, they are used for producing large volume of reports in big organization. Dot matrix printer It is another impact printer which prints one character at a time. The print head comprises a matrix of tiny needles, usually of seven rows and five columns. They are electrically driven and punch characters in the form of patterns of tiny dots as shown in Fig. 2.8. The pattern of dots for each character is dictated by the information held electronically in the printer. Dot matrix printers can produce upper as well as lower case letters. Some printers are capable of producing compressed or expanded letters. Their speed varies in the range from 40 to 600 characters per second. The printing is poor for business reports. Nevertheless, they are used extensively on small computers because of their low cost. Matrix printers are much cheaper than line printers. Daisy wheel printer It is a character printer and derives its name from the shape of the print wheel. The daisy shape wheel is made of metal or plastic and holds the characters on its petals. The wheel rotates at a high speed and when the required character is positioned over the ribbon, a tinny hammer strikes it against the ribbon thus transferring the character symbol to the paper. Daisy wheels are interchangeable and therefore letters of different sizes and shapes can be used. These are slow printers, the ir speed ranging from 17 to 200 characters per second. But they produce good quality printing and are often used in word processing applications. 2.1.33 Non-Impact Printers Inkjet printer These printers print characters by spraying electrically charged ink on to the paper. They are capable of producing characters of various shapes and sizes. The speed is around 100 characters per second. Laser printer These are high speed, high quality printers. A laser beam is used to bum character images on to a rotating drum and the heated surface area picks up the toner which is offset to paper. Laser printers can produce around 20,000 characters per second. Though they can produce good quality characters of different styles and sizes and print even logos for letter heads, they are very expensive and therefore are not popular among small business organizations. Thermal Printer A thermal printer uses heated dot-matrix wires to print the output on a specially treated paper. The output on a thermal paper fades quickly when ex posed to light. Some fax machines use this concept. Another similar printer, known as electrostatic printer, uses electricity (instead of heat) to create images. 2.1.34 Graphic Plotters Plotters are used to produce outputs containing graphics. With the availability of multicolour plotters, they are increasingly used for preparing financial documents, annual reports and engineering drawings. Plotters may use either the pen or inject approach. Pen plotters are available in two forms: drum type and flat-bed type. In the drum plotters, both paper and pen move whereas in the flat-bed plotter, while the paper is fixed, the pen moves. The inject plotters use jets of ink with different colours and are able to produce large drawings containing many colours. 2.1.35 Micrographic Equipment Micrographic refers to the technology by which images of text are photographically reduced and stored on films. The processed outputs are often called microforms. A large volume of information can be stored in a relatively small area. Microform readers can be used to display the information for reading as well as to produce printer outputs. Two kinds of microforms are popular: microfilm and microfiche. Microfilm is a continuous roll of film, either 16 mm or 35 mm. Microfiche is a sheet of film, usually 4 inches by 6 inches. The number of pages of text that could be stored in each frame of microfilm or in a microfiche depends on the degree of reduction. The reduced images are arranged in sequence in case of microfilm and in rows and columns in case of microfiche. The information on the film or microfiche is read by users using microfilm-viewing devices either manually or with the help of a computer. In the computer as sited retrieval (CAR) the user can ask for a particular page and the computer displays it on the screen. If the paper copy of the document is needed, a reader-printer is used to provide printed outputs. Voice Output Computers can employ speech synthesizers to provide voice output. Speech output has been used to provide information about flight departure times, stock market prices, etc. In recent cars, these systems remind the driver to turn-off headlights or add fuel. These systems will be of great value to people with visual handicaps and reading disabilities. See the Box on Multimedia. 2.1.36 External Storage Devices Storage media hold data before processing and store the results of processing. The following illustration shows various types of storage devices used in modem computers. 2.1.37 Magnetic Tapes Magnetic tapes are sequential access secondary storage media. Types of magnetic tape include: 1. Reel-to-reel tapes 2. Cassette tapes 3. Cartridge tapes Reel-to-reel tapes Large computers use tape drives to store and read data on reels of tape as shown in the illustration. A reel of magnetic tape is half inch in width and can store 1,600 to 6,000 characters per inch along its 2,400 feet length. Cassette and cartridge tapes Tape drives attached to early personal computers used cassette tapes similar to the familiar audio cassettes. Cassettes and reels are now being replaced by cartridge tapes. These cartridges are self-contained in hard plastic cells that are easy to mount in a tape drive and store when not in use. They can hold between 40 MB and 10 GB of data.  2.1.38 Magnetic disks Magnetic disks are the most commonly used storage media because they support direct access to data. Magnetic disk products include:  Floppy disks  Winchester disks  Removable disk packs  Disk cartridges Floppy disk The most common storage medium used on small computers today is a floppy disk. It is a flexible plastic disk coated with magnetic material and looks like a phonograph record. Information can be recorded or read by inserting it into a disk drive connected to the computer. The disks are permanently encased in stiff paper jackets for protection and easy handling. An opening is provided in the jacket to facilitate reading and writing of information. Floppy disks are available in two standard sizes. One is 5.25 inch and the other is 3.5 inch floppy disk. The 3.5 inch floppy disk which was introduced after the 5.25 inch disk can store more information than the previous one. The illustration show s both the types of disks. Table 2.3 lists instructions for handling floppies. Winchester disk Another magnetic media suitable for storing large volumes of information is the hard disk , popularly known as the Winchester disk. A Winchester disk pack consists of two or more magnetic plates fixed to a spindle, one below the other with a set of read/write heads. The disk pack is permanently sealed inside an easing to protect it from dust and other contamination, thus increasing its operational reliability and data integrity. Winchester disks possess a number of advantages compared to floppy disks:  They can hold much larger volume of information than floppies.  They are very fast in reading and writing.  Hard disks are not susceptible to dust and static electricity. Winchester disks are available in different sizes and capacities. Standard sizes are 3.5 inch and 5.25 inch , Storage capacities 80GB, 120 GB, 160 GB, 250GB, 500 GB and 1TB (terabyte ) are typical for a personal computer to day. Removable disk pack These are similar to Winchester disks but are removable. Removable disk packs lend versatility to disk use. Because disk packs can be inserted and removed, we can use any number of disk packs with a drive. They are portable an d interchangeable.. Disk cartridge Disk cartridges are small hard disks that can be mounted and removed from disk drives. They are compact, measuring only 3.5 inches in diameter, and store more than 20 MB of data. They are portable and interchangeable. 2.1.39 Optical Disks In optical disks, a laser beam is used to write and read data on this reflective disk. Optical disks support direct access to data and have more storage capacity than magnetic disks. Optical disks are popularly known as Compact Disks (CDs). Optical disks are available in the following forms:  CD-ROM (Compact disk-read-only memory)  D-R (Compact disk-recordable)  Magneto-optical (MO) disks  Write once, read many (WORM) disks CD-ROM disks These are used to distribute large volume of data, such as online encyclopaedia or multimedia presentations. Computer programs and user manuals are often distributed on CD-ROMS. CD-R disks These are recordable compact disks. They are less expensive and are used to record the contents of a CD-ROM to crea te master copies. They are also used by business houses to store certain permanent records. Magneto-optical (MO) disks In these disks, bit patterns are aligned magnetically and the disk and therefore they are known as magneto-optical disks. They are era sable disks. We can overwrite data on these disks. They are the most flexible type of optical disks. Write once, read many (WORM) disks Further advances in CD technology lead into developing a writable optical technology that resulted in the write once, read many (WORM) drive. As with the CD, once data have been etched into the surface of the WORM disk, they cannot be changed. WORM disks a re used for storing permanent records. Organizations such as banks use WORM disks to store a record of each day's transactions that can be read later, but cannot be changed 2.1.40 Media Input Devices Media input devices are used to record information (text and data) directly on magnetic media. An operator can key the information electronically using a keyboard. The information is displaced on a screen for correcting any errors and then transferred to the magnetic medium. The medium is physically transferred to the computer system for further processing and producing outputs on paper. When changes are required, the medium is returned to the input device for editing. These devices are not electrically connected to the computer and are therefore referred to as off -line data entry systems. The cost of data entry terminals is much less compared to a computer system. If we use off-line data entry systems, we can avoid the use of the computer system for entering data, thus saving CPU time. Depending on the configuration of the main system, one or more of the following off-line data entry systems may be used: 1. Key-to-floppy 2. Key-to-cassette 3. Key-to-tape Among these three, the most popular data entry system used in offices is the key-to-flopp y system. Most data entry systems include facilities for defining, storing, verifying and validating large volumes of data. The validation may include type verification on fields, check digit incorporation and hash totaling. MULTIMEDIA Multimedia is a computer -based method of presenting information by using more than one medium of communication, such as text, graphics, and sound and emphasizing interactivity. Multimedia is changing the way we store, retrieve and read the information. Instead of linear presentation of text (as printed in books), multimedia makes reading dynamic by the ability to branch to any place in the text and by giving life to it with sound, pictures, music and video. Multimedia is extensively used in teaching, learning and electronic publishing.  Introduction to the Software Software 3.1.1 The Software Computers need clear-cut instructions to tell them what to do, how to do, and when to do. A set of instructions to carry out these functions is called a programme. A group of such programmes that are put into a computer to operate and control its activities is called software. These programmes must reside in the internal storage (memory) to execute their instructions. For example, if we want to delete some data store d in memory, the system uses, one set of programme instructions. Similarly, if we want to sort a list of names, it uses another set of instructions designed to perform this task. Software is an essential requirement of computer systems. As a car cannot ru n without fuel, a computer cannot work without software. The software may be broadly classified into two major categories:  System software  Application software System software may include the following types of software:  Operating system  Language translators  Linker programmes  Loader programmes  Editor programmes  Utility programmes These are designed to help users to manage computer resources and to run their application programmes effectively. These are complex programmes and are devel oped and supplied by computer vendors. Application software includes, among others, the following most popular packages:  Word processing packages  Electronic spread sheet programmes  Database management systems  Graphics software These are packages developed to carry out specific applications that are used in many organisations.  3.1.2 Operating System The software that manages the resources of a computer system and schedules its operation is called the operating system. The operating system acts as an interface between the hardware and the user programmes and facilitates the execution of the programmes. The principal functions of operating system include: · To control and coordinate peripheral devices such as printers, dis play screen and disk drives. · To monitor the use of the machine's resources. · To manage disk files where data and programmes are stored. · To help the application programmes execute its instructions. · To help the user develop programmes. · To deal with any faults that may occur in the computer and inform the operator. The operating system is usually available with hardware manufacturers and is rarely developed in-house owing to its technical complexity. Small computers are built from a wide variety of microprocessor chips and use different operating systems. Hence, an operating system that runs on one computer may not run on the other. Popular operating systems include, among others, MS- DOS, Windows 95, Windows 98, Windows NT, Windows 2000 and UNIX. 3.1.3 Utility Programmes There are many tasks common to a variety of applications. Examples of such tasks are:  Sorting a list in a desired sequence.  Merging of two programmes.  Copying a programme from one place to another  Report writing. One need not write programmes for these tasks. They are standard and normally handled by what are known as utility programmes.Like operating systems, utility programmes are prewritten by the manufacturers and supplied with the hardware. They may also be obtained from standard software vendors. A good range of utility programmes can make life much easier for the user.  3.1.4 Language Translators Computers can understand instructions only when they are written in their own language called the machine language. Therefore, a programme written in any other language should be translated into machine language. Special programmes called language processorsare available to do this job. These special programmes accept the user programmes and check each statement and, if it is grammatically correct, produce a corresponding set of machine code instructions. Language processors are also known as translators. There are three forms of translators:  Compilers  Interpreters  Assemblers 3.1.4.1 Compiler A compiler checks the entire user-written programme (known as the source programme ) and if error-free, produces a complete programme in machine language (known as object programme ). The source programme is retained for possible modifications and corrections and the object programme is loaded into the computer for execution (see the illustration below). 3.1.4.2 Interpreter An interpreter does a similar job but in a different style. The interpreter (as the name implies) translates one statement at a time, and if error-free, executes it. This continues till the last statement. Thus an interpreter translates and executes the first instruction before it goes to the second, while a compiler translates the whole programme before execution. The major differences between the two are: Error correction (called debugging ) is much simpler in the case of the interpreter because it is done in stages. The compiler produces an error list for the entire programme at the end. Interpreters take more time for the execution of a programme compared to compilers because a statement has to be translated every time it is read.  3.1.4.3 Assembler As the computer understands only machine-code instructions, a programme written in assembly language must also be translated into machine language before the programme is executed. This translation programme is called the assembler. Language translators a re usually written and supplied by the hardware vendors. Since a compiler (or an interpreter) can translate only a particular language for which it is designed, we will have to use a separate translator for each language. 3.1.5 Linker and Loader Programmes A linker is a system programme that links modules or programme segments together so that they can refer to one another. It produces a single load module that can be stored in a programme library on disk or run as a programme to produce results. The linker is respon sible for producing an executable code by linking the object code with other library functions (see the illustration below). A loader is also a system programme that loads programmes stored in a programme library into memory for execution. An example of a loader is the bootstrap loader that brings the operating system from the disk and loads it into the memory. 3.1.6 Editor programmes An editor is a system programme that is used to type in and edit text or programme statements. All operating systems contain their own editors. Word processing packages can also be used as editors. 3.1.7 Application Programmes While an operating system makes the hardware run properly, application programmes make the hardware do useful work. Application programmes are specially prepa red to do certain specific tasks. They can be classified into two categories.   Standard applications  Unique applications Some applications are common for many organizations. Ready-to- use software packages for such applications are available from hardware and/or software vendors. Standard packages include among others are:  Word Processing (WP) systems Electronic Spread sheet  Database Management Systems (DBMS)  Graphic Packages (CAD/CAM)  Sales Ledger  Purchase Ledger  Statistical Analysis  Pay Roll  PERT/CPM  Production Planning & Control  Inventory Management  Linear Programming The most commonly used packages are word processing package, electronic spread sheet and database systems. These packages are popularly known as decision support systems. There are situations where we may have to develop our own programmes to suit our unique requirements. Such programmes fall into the category of unique application packages. 3.1.8 Programming Languages We have stated earlier that the functioning of a computer IS controlled by a set of instructions (called a computer programme ). These instructions are written to tell the computer:  What operation to perform?  Where to locate data?  How to present results?  When to make certain decisions? and so on. The language used in the communication of computer instructions is known as the programming language. The computer has its own language and any communication with the computer must be in its language or translated into this language. Three levels of programming languages are available. · Machine languages (low level languages) · Assembly (or symbolic) languages · Procedure-oriented languages (high level languages)  3.1.8.1 Machine Language Computers are made of two-state electronic components which can understand only pulse and no-pulse (or 'I' and '0') conditions. Therefore, all instruct ions and data should be written using binary codes 1 and O. The binary code is called the machine code or machine language. Computers do not understand English, Hindi or Tamil. They respond only to machine language. Added to this, computers are not identical in design. Therefore, each computer has its own machine language. (However, the script 1 and 0 is the same for all computers). This poses two problems for the user. First, it is a cumbersome experience to understand and remember the various combinations of 1s and is representing numerous data and instructions. Also, writing error-free instructions is a slow process. Secondly, since every machine has its own machine language, the user cannot communicate with other computers (if he does not know its language). Imagine a Tamilian making his first trip to Delhi. He would face enormous obstacles as soon as he moves out for shopping. A language barrier would prevent him from communicating. 3.1.8.2 Assembly Language An assembly language uses mnemonic codes rather than numeric codes (as used in machine language). For example, ADD or A is used as a symbolic operation code to represent addition and SUB or S is used for subtraction. Memory locations containing data are given names such as TOTAL, AMRKS, TIME, MONTH and the like. The assembly language is again a machine-oriented language and hence the programme has to be different for different machines. The programmer should remember ma chine characteristics, when he prepares a programme. Writing a programme in assembly language is still a slow and tedious task.  3.1.8.3 Procedure-Oriented Language (POL) These languages consist of a set of words and symbols and one can write programmes using the se in conjunction with certain rules like 'English' language. These languages are oriented towards the problem to be solved or procedures for solution rather than mere computer instructions. These languages are more user-centered rather than machine- centred. They are better known as high level languages. The most important characteristic of a high-level language is that it is machine- independent and a programme written in a high-level language can be run on computers of different makes with little or no modification. The programmer need not know the characteristics of that machine. However, such programmes are to be translated into equivalent machine-code instructions before actual implementation. A programme written in a high-level language is known as the source-programme and can be run on different machines using different translators. The translated programme is called the object programme. The major disadvantage of high-level languages is that they take extra time for conversion and are thus less efficient compared to the machine- code languages. The illustration given below shows the system of implementing the three levels of languages. There has been a continuous improvement in the methods of communication with the computers. The improvements have been as significant as that of hardware. The developments in languages have been classified under four phases. They are termed as follows:  First generation: Machine languages  Second generation: Assembly languages  Third generation: High-level languages  Fourth generation: DBMS, lESS, WP, and Graphics packages 3.1.9 Common High-Level Languages Many high-level languages have been developed during the last four decades: The most common high-level languages are FORTRAN, BASIC, COBOL, C, PL/1, C++ and Java. Although they are less efficient than the machine or assembly languages, they relieve the programmers of the tedious task of remembering numeric codes for storage locations, operations and the like. In addition, these languages are easier to learn and use. The choice of a language depends upon many factors such as the knowledge of the programmer, the computer, the problem to be solved and so on.  3.1.10 Programme Execution Mode's The execution of a programme can be done in one of the following modes depending upon the computer system available and the task:  Batch mode  On -line mode  Time-sharing mode  Interactive computing 3.1.10.1 Batch Mode In this mode programmes are presented to the system in batches. The system executes them one after another. All the data required for a particular programme are gathered (in batches) and supplied together with the programme. Some applications, such as payrolls and results are processed in this mode. 3.1.10.2 On-Line Mode In this mode, the computer executes the programme instructions as and when the data is entered into the system. This requires two things: · the programme that is used to process the data should reside in the internal memory continuously, and · the computer system should be on-line. 3.1.10.3 Time-Sharing Mode The time-sharing mode refers to the use of a single computer system by many users simultaneously. The computer allocates small divisions of time to each user in turn. In this mode of operation, each user has one (or more) input/output device (called terminal) connected to the computer. The most common terminal device is the visual display u nit (VDU). The user can enter his programme and data at his own terminal and get the processed results either at his terminal or on a printer that is connected to the computer. Although many users share the computer, a user feels that the computer is attending to him all the time. This mode of operation is suitable for applications, such as inventory control, airline reservations, banking systems and the like, where the transactions are to be processed as soon as they occur. Such systems are also referred to as on-line, real-time systems. 3.1.10.4 Interactive Computing A major breakthrough in programming took place in the early 1960s when interactive languages' like BASIC were developed. With an interactive language, we can converse (interact) with a computer. Most of the modem languages including FORTRAN have incorporated interactive features. With the help of an interactive language, we may engage in a conversation with our computer like this: I am computing sum of two values  Please input value of X = 255.75  Please input value of Y= 120.50  Sum of X any Y = 376.25  Do you want me to do one more sum?  No Thanks  Bye then, See you again! The lower-case words are of the computer and the words underlined are ours. Such interactive computing would be useful in knowing certain intermediate results and taking actions depending upon the values. 3.1.11 Problem Solving and algorithms Problems that can be solved through the computer may range in size and complexity. Since the computer does not possess any common sense and cannot make any unplanned decisions, the problem, whether it is simple or complex, has to be broken into a well-defined set of solution steps. It should be remembered that 'computers do not solve problems; rather they are used to implement the solutions to problems. In every instance of problem solving, the computer cannot be used to solve the problem until a method of solution has been evolved and a detailed procedure has been prepared by the user. It is assumed that the user has a certain amount of background, knowledge, knows certain facts about the problem and possesses sufficient deductive and reasoning skills. Problem solving involves the following steps: 1. Studying the problem in detail, 2. Redefining or restating the problem, 3. Identifying output requirements, input data available and conditions and constraints to be used,  4. Comparing different methods of solution, 5. Selecting the method which is considered to be the best, 6. Preparing a logical and concise list of procedures or steps necessary for solving the problem, 7. Computing the results, and 8. Examining the results for correctness. Only in the seventh step, the computer's help may be necessary. All the remaining steps are to be performed by the user. It is this fact that poses a problem for a beginner. The logical and concise list of procedures for solving a problem is called an algorithm. It describes the steps that lead to unambiguous results in a finite number of operations. An algorithm prepared for the first time might need review to:  Determine the correctness of various steps,  Reduce the number of steps, if necessary, and  Increase the speed of solving the problem. An algorithm should also include steps to identify any abnormal data or results and take corrective measures, if possible. In case of large problems, we can-break them into parts representing small tasks, prepare several algorithms and later combine them into one large algorithm. This is known as the modular approach. Developing computer programmes using the modular approach is known as modular programming. A module is a programme unit or entry that is responsible for a single task. Modules (known as sub-programme ) are arranged into a hierarchical structure (similar to organisational chart ) in which bigger modules are broken into smaller ones, such that they are small enough to understand and are easily coded using simple logic. top-down design 3.1.12 Flow Charting When organising a problem for computer solution, it is desirable to present the algorithm pictorially. A flowchart is a diagram that outlines the sequence of operations to be performed. The operating steps are placed in boxes that are connected by arrows to indicate the order of execution of steps. The important functions of a flowchart are:  It provides a graphic representation of the problem so that it is easier to understand the plan of solution.  It provides a convenient aid to writing computer instructions (programme).  It assists in reviewing and correcting the programme.  It helps in discussion of the solution logic with others.  3.1.13 Structuring the Logic Solution steps of all problems can be organised into one or a combination of the following three forms known as control structures.  Sequence structure  Branching structure  Looping structure A sequence structure is used when the solution does not involve any repetitive operations or options. This is known as straight-line logic (see the figure below). Branching refers to the process of following one of two or more alternate paths of computations. This happens at a point where a 'test' is performed to identify the conditions of certain variables in the process. The basis for selection of a particular path is state d within the decision box. The decision can be based on a comparison, on the value of a variable, on the sign of a variable, etc. Looping refers to the repeated use of one or more steps. There are two types of loops. One is known as fixed loop where the operations are repeated a fixed number of times. In. this case, the values of the variables inside the loop have no effect on the number of times that the looping operation is performed. The other is known as variable loop where the operations are repeated until a specified condition is met. Here, the number of times that the loop is repeated may vary. Searching for a particular item in a list is an example of variable loop. Loops are also referred to as backward Jumps. These jumps may occur either after me editing a specified condition in the process or after doing a certain computation. 3.1.14 Using the Computer Computers can be used to solve specific problems that may be scientific or commercial in nature. In either case, there are some basic steps involved in us ing the computers. These are as follows: Problem analysis Identify the known and unknown parameters and state the constraints under which the problem is to be solved. Select a method of solution. Collecting information Collect data, information and the documents necessary for solving the problem and also plan the layout of output results. Preparing the computer logic Identify the sequence of operations to be performed in the process of solving the problem and plan the programme logic, preferably using a programme flowchart. Writing the computer programme Write the programme of instructions for the computer in a suitable language. Testing the programme There are usually errors (bugs) in it. Remove all these errors wh ich may be either in using the language or in the logic. Preparing the data Prepare input data in the required form. Running the programme This may be done either in batch mode or interactive mode. The computer performs the computations and the results are given out. The use of a particular input/output device depends upon the nature of the problem, type of input data and the form of output required. 3.2.1 The Operating System Every computer has a layer of special software that gives life to the hardware. When we turn a computer on, a firmware built into it runs this special software which then continues to operate as long as the computer is on. This special software is known as the operating system (OS). This is perhaps the most complex and important software running on a computer. The operating system, as its name implies, oversees all computer operations. It not only coordinates the operations of hardware units such as CPU, memory, keyboard, video display system, and disk drives but also provides an interface for users to communicate with the computer and run application programmes. 3.2.2 Major Functions of an Operating System  Providing user interface, either command-line or graphical for communicating with the computer.  Managing the hardware devices of the computers.  Managing and maintaining the disk files  Supporting the running of application programmes.   3.2.3 Components of os and their functions The operating system is a collection of programmes and sub-programmes ( known as routines ) that are designed to carry out various tasks. They include: · Bootstrap programme · Diagnostic programmes · Monitor programme · BIOS programme · Utility programmes · File maintenance programmes 3.2.3.1 Bootstrap Programme Bootstrap loader programme is usually contained in the ROM memory. Its task is to load the main portion of the operation system residing in the secondary memory into the main memory, when the computer is first switched on. This operation is known as booting the computer. 3.2.3.2 Diagnostic Programmes These programmes test the operation of the system components. For instance, one programme may check the conditions of the disk while the other checks the RAM. 3.2.3.3 Monitor Programme This programme monitors and controls the activities of various units and assigns tasks to them. It also monitors user requests and schedules the execution of application program.mes. The monitor programme is also known as the operating system executive.  3.2.3.4 BIOS Programme This programme ( Basic Input/output System ) contains a number of subprogram me that control some' basic operations of input output devices, such as reading from the keyboard and writing to the printer. 3.2.3.5 Utility Programmes These programmes are used to perform 'housekeeping' functions such as formatting a disk, copying the contents of a disk, determining the space available in a disk and so on. 3.2.3.6 File Maintenance Programmes These program.mes are used to maintain files on the secondary storage units attached to the computer. They enable users to create, read, and write files. 3.2.4 Types of Operating Systems Operating systems are designed not only to manage hardware but also serve the users in a variety of environments. Operating systems depending on how they handle user programmes may be categorised as follows:  Batch operating systems  Multi-user operating systems  Multitasking operating systems  Multiprocessing operating systems  On-line operating systems  Real time operating systems 3.2.4.1 Batch systems In batch processing, the system reads in a job, passes controls to this job, and when the job is completed, the controls to the next job. Most of the early computers used batch processing operating systems. Such operating systems were called resident monitors.  3.2.4.2 Multi-user systems A multi-user operation system is designed to allow more than one user to access and use a computer at the same time. In this case, the CPU time is shared by the users working simultaneously. Since the same processor caters the needs of many users (using time- slicing technique) the response time will decrease when the numbers of users increase. Such systems are also known as time-sharing systems. 3.2.4.3 Multitasking systems The ability of an operating system to handle more than one task at a time is called multitasking (or multithreading). This capability, for example, allows us to print documents in the background, or to sort a database while preparing a letter using a word processor. Multimedia applications require as with multitasking capabilities. Such systems which can run two or more independent programmes concurrently are also called multiprogramming systems. 3.2.4.4 Multiprocessing Systems An operating system that can manage a series of CPUs working together is known as multiprocessing operating system. Such systems increase the overall power of the computer. These systems are also called parallel processing systems. 3.2.4.5 On-line Systems In on-line operation, the computer system is on-line and the programme that is used to process the data resides in the internal memory continuously. The system executes the programme as and when the data is available. This kind of facility is very useful in implementing systems such as railway reservation system. An on-line system may be a single-user or multi user system. 3.2.4.6 Real-time Systems There are situations where computers are used to control some physical systems such as process control systems. In such systems, a computer operates in real-time that is, the computer receives signals, processes them instantly and takes control action immediately. Real-time operating systems have to respond within a specified time limit.