OPERATING SYSTEM (PART 2)

Usually different computers are dependent on a set of machine code that is standard and is different for different microprocessors. The operating system removes this difficulty and provides a set of standard commands to user, which are not machine dependent. Even though an operating system is usually invisible, it is essential for every computer system to have an operating system. The operating systems can be classified as:-

• Single User Operating Systems
• Multi User Operating Systems

The brief systems does the following jobs:-

• It provides an interface between the user and the computer
• It controls access to the machine
• It works with ROM to start up or "Boot" the computer
• It schedules on efficient use of the CPU while eliminating the need for human intervention
• It manages data and information in internal memory
• It manages files on secondary storage, making them available as needed while providing security to multiple user
• It provides a palte form to run other software programme
• It manages peripheral devices such as printer, disk drives, and tapes drives

A platform defines a standard for which software packages are developed to run. A platform is defined by two basic elements:-

1. The processor (for example intel Pentium or Power PC) and
2. The operating system (for example MS DOS or Windows)

Generally, software created to run on one platform is not compatible with any other platform and therefore cannot run on any other platforms.

A single system can however be configured to run under different platforms like MS DOS, Windows 98, 2000, 2007. The selection of a platform is important because it establishes boundaries for what you can and cannot do with your computer system. Before choosing a platform consider the following:

• Availability of oppropriate commercial applications software for the platform
• Compatibility of platform with existing hardware, software and expertise

SINGLE USER OPERATING SYSTEMS

Single user operating systems are used on individual systems or computer. This type of operating system is used only on personal computers or microcomputers and is installed on all individual machines. Each system is designed for a specific microprocessor. Some of the most widely used single user are:-

• MS-DOS or Microsoft DOS
• PC DOS
• Machintosh System
• DOS Shell
• OS/2
• Windows

All the above operating systems reside on disks and hence are called (DOS).

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WHAT IS OPERATING SYSTEM

A computer is a digital electronic device. Hence to perform vaoius operatings it can accept commands only in digital form. There is a need to have an organized collection of programme that controls the over all operations of a computer. Thus software that enables the communication between the user and the computer system as a whole is called an operating system. It interprets the users commands into a language which the machine can understand, and executes the instructions and helps the computer to handle vaious peripheral devices such as keyboard, monitor, floppy disk drive, hard disk drive, the printer and the other devices. For example, the operating system must monitor the keyboard to determine when a key has been pressed to provide input to the computer. It must also manage the monitor screen and printer to provide output from the computer. The operating system provides link between the user and the computer and enables the user to have complete control over the computer system by giving certain commands from the keyboard or any other input device. The operating system is a set of system programme, known as control programme or supervisor, which resides all the times in the memory (RAM) of a computer and controls all the comonents of a computer system as soon as computer is turned on. Through the use of operating system, the user delegates part of his burden of data processing to the computer itself to achieve high efficiencies of which the processing systems are capable.

FUNCTIONS OF THE OPERATING SYSTEM

According to the facilities provided by operating systems, these range from simple systems to complex ones. The philosophy underlying the operating system is that the computer should perform such jobs of an operator, which it can do faster and more accurately. The operating system remaining in the background performs tasks that are essential for the efficient functioning of the computer system such as:

• Controlling Basic Input and Output Devices
• Allocating System Resources
• Managing Storage Space
• Maintaining Security
• Detection of Equimpment Failure

Various types of operating system have been developed to provide the computer user with a workable system for different purposes such as multiprogramming, multiprocessing, batch processing, remote processing, etc.

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VARIETY OF SOFTWARES

Represents a recent entry into the prepackaged software race. As the name implies, integrated software packages combine or integrate, a number of individual software packages into a single larger programme having two types of integrating software

• Horizontal Integration and
• Vertical Integration

HORIZONTAL INTEGRATION

Describes the combination of application packages such as word processor, data manager, spreadseet and graphic pakcages into one pakcage that can share data. The ability to combine and pass data between application packages makes integrated software very useful.

VERTICAL INTEGRATION

Refers to the enhancement of a single package. For example adding a spelling programme dictionary or thesaurus to a word processor would be considered vertical integration.

BUSINESS SOFTWARE

A second major application software vategory, business software, helps organizations efficiently accomplish routine tasks. Often, buiness software is divided into two categories, horizontal market softwae and vertical market software. Horizontal market software refers to generic software packages that can be used by many different kinds of businesses. Accounting and payroll applications are good examples of horizontal market software. Every business needs to maintain a set of books to track income and expenses. Accounting software is designed to computerized the book keeping tasks typically required in most businesses. Almost every business has employees and needs to maintain payroll records. Payroll software keeps track of employee hours and produces the reports required by the government for income tax reporting.

ENTERTAINMENT SOFTWARE

Entertainment software is designed to entertain you. With entertainment software, you can play cards, fly a simulated jet, play 18 holes of golf, solve a Sherlock Holmes mystery, battle monsters, or explore nre worlds.

EDUCATION AND REFERENCE SOFTWARE

Education and reference software is designed to help you learn more about a particular tpoic. One type of education software called CAI (computer aided instruction), or tutorial software, can help you learn how to do things, for example, how to type, how to fix your car, how to use your word processor, how to speak French, or how to prepare for the GMAT exam. Educational simulations let you work with a computerized model of something in the real world, manipulate it, and see what happens. For example, some College and Universities use computerized simulations in introductory chemistry classes and students can experiment with chemical reactions without using real, and sometimes dangerous, mixtures. Reference software, such as electronic encyclopedias, can also help you lool up facts an any topic.

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WORD PROCESSING SOFTWARE

Helps your to produce documents such as reports, letters, papers and manuscripts. Word processing can involve a number of different activities, but generally these activities can be categorized as either text editing or print formating. In the text editing process, text is entered into the computer via a keyboard. You can edit your work using insert a deleted keys. You can also move text to improve the logical flow of concepts of you document.

The second category of word processing activity, print formating occurs when the word processor communicates with the printer through the computer system and tells the printer how to print the text. Some of the more common print formating features found in Word processors is margin settings, lines spacing, centering, automatic paging, character enhancements and headers and footers. Character enhancement features may allow the user to underline, italicizw, boldface, subscript or superscript tex within a document.

SPREDSHEET SOFTWARE

Helps you to work with numbers. An electronic spreadsheet is a computerized version of traditional spreadseet used by accountants for performing financial calculators and recording transactions. An electronic spreadsheet is a table of rows and cloumn used to store and manipulate any kind of numerical data.

The point shere a row nd a column meet is called a cell. Each cell is a unique location. You enter number and formula in the cells and the computer automatically performs the calculations.

DATABASE MANAGEMENT SOFTWARE

Helps you to work with facts and figures. Data managers are application software packages that computerize everyday tasks of recording and filing information. With data manager, data is recorded with the aid of a computer and is stored on the computer's secondary storage devices where it can accessed. They can be used to keep a personal property inventory, creating a list of important documents, creating a mailing list and keeping a computerized address book and phone listing. They offer a way to keep organized, readily accessible records.

GRAPHIC SOFTWARE

Draw a pictures, 3-D images and animatins. Graphic software allows you to convert data into meaningful diagram or graphs such as pie chart, graphs and bar charts, graphics software ranges from business to artistic user. If you have limited artistic ability, you can use graphic software to retrieve pre-drawn clip art images, which you can modify.

COMMUNICATION SOFTWARE

Allows your computer to talk to other computers. Communication software is the key that unlocks the door to a new world of information and information related services. With communication software, a microcomputer become more than a small stand alone computer. It becomes capable of interacting with a remote computer may be located in the next room of in New York. It provides you computerized mailbox that collects documents or mail you receive from other computers. It enables the transfer of files between a PC and another computer. It also allows you to send/receive faxes directly.

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APPLICATION SOFTWARE

System software performs essential support functions. Among these software products are operating systems, language translators, utility programms, performance monitoring programms.

However, application software also referred asproductivity software are developed to help the user to increase his/her productivity. Originally application software was developed by the organizations that owned computers. They have their own programmers to create programms according to their needs. Later, the companies that manufactured the computers began to develope and sell software packages along with their computers. These solutions packages were designed for the kinds of data management tasks that most organizations had, business related activities such as record keeping, inventory control, and payroll accounting.

Eventually, some firms began to sell software that deal with common computer tasks. There has been a tremendous increase in the use of application software packages with the development of microcomputers. Two factors are largely responsible:

1. The increased cost of developing one's application software.
2. The irse in the popularity of microcomputers.

Individuals and small businesses wanted specialized programms for creating letters and memos or for calculating their monthly budget. At first PC application programms were designed to create letters and memos and the electronic spreadsheet was created to manage numerical data. But as PC programmers deicovered new ways to use their computers, they began to design and sell an endless variety of specialized software for every type of computer user. Application programms are widely used for entering and editing text (Word processing desktop publishing programms) for entering and manipulating numeric data (spread sheet and many other business programms) and for record keeping (database management programms). These types of programms are sometimes called as general purpose applications.

Application software can be divided in four major classes. These are:

1. Productivity Software
2. Business Software
3. Education Software
4. Entertainment Software

PRODUCTIVITY SOFTWARE

The most important type of the application software is productivity software. It helps you to work most effectively. Productivity software includes:

• Word Processing Software
• Spreadsheet Software
• Database Management Software
• Graphic Software
• Cpmmunications Software and
• Integrated Software

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PROGRAMMING LANGUAGE TRANSLATOR

The earliest computers were programmed by arranging various wires and switches within the computer components. In the ENIAC, upto six thousand switches could be set to execute programmes. When a new programme was to be run, all the switches had to be rest. The EDSAC, the first stored programme computer allowed instructions to be entered into primary storage without switches having to be rewired or rest. Special codes that later becam known as machine language were used to enter the instructions. In the mid 1950s the development of FORTRAN signaled the beginning of a trned towards more sophisticated ways of giving computers instructions, these high level programming languages emphasized procedured and problem solving rather than the operation of the computer hardware. The lowest level language is machine language, the language of the computer. Machine language is the only language that a computer can recognize and act on directly. Machine language is the most efficient language that allows the programmer of fully use the computer's potential for processing data. But it is difficult for a programmer to learn and remember. Assembly languages were developed to overcome the disadvantages of machine languages. The programmer uses mnemonics instead of 1's and 0's to specify machine operations. The mnemonics are English like abbreviations for the machine language instructions. For example LDA stands for load in register A and STB stands for store in register B. Assembly languages programmes are highly efficient in terms of storage space use and fast processing. During the past fifty year nearly five hundred computer languages have been developed. These language have been categorized as Machine languages. Assembly languages and symbolic languages.

SERVICE SOFTWARE

Service programmes make life as a user a little easier. It can be further subdivided into

• Utlity Programms
• Library Programms
• Device Drivers

Hundreds of utility programms help us a better manage available hardware, software and data resources. Although an operating system usually works behind the scenes. there are times when you need utilities or utility software, to do things such as prepare disc to store data, detect computer viruses on a disc, prepare, copy, merge, sort out and update files. Protects monitor screen from burning during idle monitors by screen saver utility programme thus prolonging the life of monitor. A text editor utility programme such as Wrodstar, Wordperfect, Norton's editor etc., is used to create and modify a document another popular utility programme called data compression, help us reduce the size of a stored image/text for more efficient transmission over a data communication link.

Library programme maintains a directory of frequently used software module and their locations. These programmes migh consist of manufacturer supplied or user written routines or complete programms to computer mathematical functions such as root, exponential function and matrix inversion, control input/output devices, maintain appointments and so on. The library programms makes these routines available when requested by the user, the operating system or an application programme.

The type of programme require to run peripheral devices such as mouse, keyboard, floppy disk, CD-ROM modem, printer etc., reqire to install software that tells your computer how to use the device. This type of software is called device driver. When you purchase a hardware device, you also receive a disk containing a device driver.

A system programme may, for example allocate storage for data being entered into the system. The programme varies from computer to computer and cannot be used on different machines unless they are modified. System software is usually provided by the computer manufacturer or a specialized software developing firm.

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OPERATING SYSTEM

• Operating System
• Programming Language Translator
• Service Software

OPERATING SYSTEM

An operating system is a collection of system programmes that control and coordinate the overall operation of a computer system. These programmes act as an interface between the hardware and the applications software. It manages the resources of the computer namely the central processing unit, memory and input/output devices. Normally transparent to the user, operating system performs its task without intervention. The user is usually unaware that the operating system is quietly at work assisting the user in vaious ways. Examples are Disc operating system (DOS), Macintosh, and Unix. Dos is a command driven interface discussed in the next pages.

The trend today is towards more user friendly, graphics oriented environment called a graphic user interface (GUI), the user can interact with DOS through a series of pull down menu. Software that provides a GUI instead of command driven interface is called a shell. Dos shell provides a Graphical user interface (GUI) and was first introduced with DOS 4.0 version. Operating system 2 (OS/2) is an operating system devloped for IBM compatible microcomputers. UNIX is another operating system that was once used only on mini and mainframe computers, but has now been adapted for microcomputers as well. Unix is a protable multitask multi user, layered operating system. It is written in C language, which is a high level language. Hence it is less hardware dependent than those written in lower level machine language. ZENIX or XENIX is another operating system developed by Microsoft for microcomputers. It has multiprogramming capabilities with many useful utilities and GUI and is gaining rapid acceptance Windows is an operating system developed by Microsoft for microcomputers using Intel microprocessors. A true Windows operating system is Windows NT (New Technology). It is intended to work on computers for multiple manufacturers. Windows NT provides a whole new set of applications. Applications that once ran on large computers can now run under Windows NT on PCs. Windows NT can run any programme written for the earlier MS DOS, Windows, and OS/2 operating systems. Macintosh operating system is an operating system particularly used in Apple Computers. It has an extremely user friendly GUI.

continue..DOS, OS/2, IBM, UNIX, ZENIX, XENIX

COMPUTER SOFTWARE

COMPUTER SOFTWARE

A computer is the most versatile machine in the history that has ehanced the mental capability of human and has relieved him from mental labour. A computer can produce typeset documents, manipulate data, translate Japanese into English, compose music, design spacecrafts, control and regulate industrial plants, diagnose diseases produce piece of art, forecast weather, control flights, keep track of reservation, provide world wide data communication become partner to play games and much more.

All this versatility that a computer acquires is due to the software. Software can be as the driving force of computer without which it would be nothing except a well constructed combination of electronic device and circuits.

We interact with software to direct the overall activities within a computer. For example, if you wish to print a spreadsheet, you choose print command from your spreadsheep software's menu options. The print routine programme is executed, performing the internal operation needed to print your spreadsheet. Computer hardware is useless without software, that runs the computer for specific task. Similarly software is useless without hardware.

A computer programme is a set of detailed, step by step instructions tell a computer how to solve a problem or carry out a task. The step in a programme is written in a language that the computer can interpret or understand. It is the programme that controls the processing activities of a computer and tells the computer what to do and is referred as software. In order to produce useful output, the hardware and software must work together.

TYPES OF SOFTWARE

Software can be divided into two main groups:

• Sytem Software: Which coordinate the operation of the computer and helps the computer to carry out its basic tasks.
• Application Software: Which solve particular user problems and helps the user to carry out a task.

System Software and Application Software can be further subdivided into the following groups:

SYSTEM SOFTWARE

System software includes the programms that directly affect the operation of the computer. They are designed to facilitate the use of the hardware and to help the computer system run quickly and efficiently. They direct the fundamental operations of your computer such as displaying information on the screen, storing data on disks, sending data to the printer, interpreting the typed commands and communicate with peripheral devices. In addition system software provides a foundation so that application software operates correctly and use peripheral devices properly. System software can be divided into three main groups.

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BOOLEAN ALGEBRA

BOOLEAN ALGEBRA

A digital computer is made up of various types of simple circuits arranged and grouped according to the rules of symbolic logic. Logic is the science that establishes the validity of thought or reason. As a result of which what is true in one statement will be true in all equivalent statements Gottfried W. Leibnitz (1646 -1716) was the first who tried to develop a general method in which all truths of reason would be reduced to a kind of calculation. But the founder of symbolic logic was George Boole (1806 - 1876) who gave the world a form of mathematical logic now known as Boolean algebra. The significance of these developments was not realized at that time. Boolean algebra find its application to problems in the form of staements having answers either ture or false. It was the binary nature of these true/false problems which linked it with computer developments. The fundamental and simple ideas of logic outlined by Boole and his preducessors provide the foundation for many of the circuit applications in digital computer. But logical design is not limited to this area alone. The same principles have a direct bearing on and an increasing importance in control circuits for other electronic devices, such as digital watches, microwave oven, automatic cameras and instruments used for automatic control of manufacturing processor.

Blooean algebra like any algebra uses algebraic notation to express logical relationship in the same way that ordinary algebra is used to express mathematical relationships. In ordinary algebra an expression such as

x + y = a

consists of variables x, y and a which take numbers as values and symbols for mathematical operations such as (+) for addition. In Boolean algebra the same types of expression used are represented by set of elemetns, a set of operators and a set of rules satisfied by these two logical operators.

Since all electronic devices circuits have two discrete states ON and OFF. therefore we deal with two valued Boolean algebra.

A two valued Boolean algebra has a set with only two elements 0 and 1. A two valued Boolean algebra is also called switching algebra or binary logic. In our subsequent discussion we shall use Boolean algebra for two valued Boolean algebra.

ELEMENTS OF BOOLEAN ALGEBRA

In Boolean algebra an expression is constituted using constants and variables. Boolean Constants, Boolean variables.

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TYPES OF DATA

TYPES OF DATA

As we know that data means collection of facts, therefore, we need symbols for their representation. These symbols may be letters, words or figures. For example 3 books, 25 students, 23rd March, 40 Watt, PIA, 140847, each item conveys different meanings. In computers data items can be classified into following three types.

1. Numeric Date
2. Alphabetic Data
3. Alphanumeric Data

NUMBER SYSTEM

Digital computers are machines which responds numbers rather than letters. All the data and instructions must, therefore, be supplied in the coded numeral format. These numerals have specific meaning for computers. Thus number systems are very important to understand because the design and organization of a computer is dependent upon the number systems. The decimal system for counting has been so widely adopted that we rarely consider the possibilities of other number system. However, the binary number system, has proved the most natural and efficient system for machine use.

THE DECIMAL NUMBER SYSTEM

The decimal number system consists of 10 digits namely 0 to 9. A number written using these digits is called a decimal number. The value of each digits in a number depends upon the following:

• The face value of the digit, i.e. the digit itself
• The base of the system.
• The position of the digit in the number

THE BINARY NUMBER SYSTEM

In representing data by ON-OFF switch position there are only two possibilities and the corresponding numbers are 1 and 0. Binary number system is a very important number system so far as the digital computers are concerned. The binary digits are abbreviated to BITS. The word BIT stand for either of the binary digits 0 or 1. Since the system has only two digits, therefore, in the binary system the base is 2. Thus all the numbers in the binary system are written with the help of the digits 0 and 1. The positional value or place value of each digit.

NUMBER CONVERSION

Human normally deals with decimal number system while binary is needed for porcessing in digital computers other number systems are used for coding and storage purposes. Thus inter-conversion of number from one number system into other number system becomes important for compulation and data processing in digital computers.

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DATA REPRESENTATION AND INFORMATION

DATA REPRESENTATION

The most striking feature of a digital computer is its gnerality. It can follow a sequence of instructions, called a programme that operates on a given data. The user can specify and change programme and/or data according to the specific need. As a result of this flexibility, general purpose digital computers can perform a wide variety of information processing jobs. The effectiveness of computers and the potential benefit to society lies in their versatility and processing speed. Digital computers have made possible many scientific, industrial and commercial advancements which otherwise would have been unattainable.

Digital computers are the machines which responds numbers rather than letters. All the data and instructions must, therefore, be supplied in the coded numeral format. These numerals have specific meaning for computers. Thus number system are very important to understand because the designe and organization of a computer is dependent upon the number systems. The decimal system for counting has been so widely adopted that we rarely consider the possibilities of other number system. However, the binary number system, has proved the most natural and efficient system for machine use.

Computer are often called data processors. The numbers, facts, names etc, that are represented by symbols. The symbols may be printed characters. Since computeris a physical device, the symbol it handles must be in some physical form. In the abacus numbers are represented by the position of beads, we assign a number to a bead. In this way we represent a particular number. Digital compurters are built from components that have two states ON and OFF like electric switches.

DATA AND INFORMATION

The term we use to describe the signals with which computers work is data. The word data and information are often used interchangeably. There is an important distinction between them. Data is the words, numbers and graphics that describe people, events, things and ideas. Data becomes information when you use it as the basis for initiating some action or for making a decision. Information then is defined as the words, numbers and graphics used as the basis for actions and decisions. A representation of facts, concepts or instructions in a formalized manner suitable for communication, interpretation or processing by humans or machines is called data. In short data means collection of raw facts.

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FIXED MAGNETIC DISKS (HARD DISKS)

FIXED MAGNETIC DISKS (HARD DISKS)

The increasing complexity of the software requires additional storage space. The user must switch floppies in and out as by the software packages. To meet this growing need for large secondary storage, the hard disks are developed. Hard disks or fixed disks, also called winchester, are premanently installed or fixed within the system unit. All fixed disks are rigid are usually made of aluminum with a surface coating of easily magnetized elements, such as iron, cobalt. Chormium or nichel a fixed disks or hard disk is a high speed, target capacity disk and is referred as a mass-storage magnetic medlum that provides computer system with the ability to read or retrieve large amount of information swquentially or randomly under the direction of a controller board. A hard disk contains several disk platters stacked on a single rotating spindle. Data are stored on all reconding surface.

For a disk with four platters, there are eitght recording surfaces on which data can be stored. The disks spin continuously at a high speed (from 3600 rpm to 6000 rpm high speed) within a sealed enclosure. The enclosure keeps the disk-face surface free from contaminations, such as dust and cigarette smoke. This contamination-free enviironment allows hard disk to have greater density of data storage than the interchangeable diskettes. In contrast to a hard disk, a diskette is set in motion only when a command is issued to read from or write to the disk. An indicator light near the disk drive is illuminated only when the floppy diskette is spinning.

COMPACT DISK

Compact disk read only memory. Systems are becoming popular, especially for use with microcomputers. These systems use a disk that is less than 5 inches in diameter and yet can hold more than 600 megabytes of data. Although these disks look just like the well known music CDs, they are used to store the kind of digital information used in computers.

The large colour graphic and video image files that are used with multimedia are often stered on optical disks. In addition, these disks can store two channels of sound just like the popular music CDs. Infact, a computer controlled CD-ROM drive can be used to play the two music channels on a standard music CD, and CD-ROM disks are now available with both the music and comuterized information about the music.

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SEQUENTIAL ACCESS, DIRECT ACCESS STORAGE

SEQUENTIAL ACCESS

Access through sequential access storage is non-addressable that is an operator cannot refer directly to the contents of a particular storage location such as that used with magnetic tape. This involves examining sequentially all recorded data. This form of storage necessitates tape searching by starting at the beginning of the tap and continuing to search through all records until the desired information area in found.

DIRECT ACCESS STORAGE

Direct Access Storage is addressable that is a given item can be selected from any where in storage by simply specifying the address where it is located Direct Access devices such as magnetic disks provide immediate access to individual records and do not require reading from the beginning of a file to find a particular record.

Auxiliary storage is also called the backing storage because it is used to supplement the capacity of main storage. Backing storage is essential not only due to the limited size of the random access memory (RAM) but also because RAM is only a temporary memory and moreover it is expensive a well. Auxiliary storage provides massive capacity for storing large number of programme and vast amount of daa of the order of hundreds of megabytes. Hence the name mass storage is also given to backing storage. These permanent storage devices come in the following forms:

• Magnetic Tapes
• Magnetic Disks
• Laser Holographic Storage

MAGNETIC TAPE DEVICES

A popular form of secondary storage for both mainframe and personal computers is magnetic tape. Megnetic tape is a thin Mylar tape from 1/4 inch to 1 inch wide. The tape is coated with ferrous oxide, on which information is recorded in binary form. Magnetic tape is available in two primary forms, reel-to-reel tap and data-cassett/cartridge tape.

The information is transferred to from the tape with a read/write head, which encodes the magnetic signals on the ferrous oxide tape coating as the tape moves across the read/write head. Most computer centers have a tape library run by special librarian who manages tape storage. These tapes are cheap and reliable but they take considerable time to load and to loacte a programme.

MAGNETIC DISKS

A magnetic disk is a metal or plastic disk coated with ferrous oxide. A bit pattern may be magnetically coded onto the ferrous oxide. Because these disks spin in the disk drives, the read/write heads can reach any data on the disks directly rather than sequentially. Because of its random and sequential processing capabilities, magnetic disk storage is the overwhelming choice of computer users. A variety of magnetic disk drives (the hardware devices) and magnetic disks (the media) are manufactured for computer use. There are two fundamental types of magnetic disks, interchangeable disks calles the floppy disks and fixed disks called the hard disks.

Nearly all PCs sold today are configured with at least one fixed disk drive and one interchangeable disk drive. Having two disks increases system flexibility and throughput. The interchangeable disk drive provides a means for the distribution of data and software and for backup and archival storage. The high capacity hard disk storage has made it possible for today's PC users to enjoy the conenience of having all data software readily accessible at all times.

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FLASH & CACHE MEMORY, SECONDARY MEMORY

FLASH MEMORY

Flash memory is a type of non-volatile memory that can be altered easily by the end user. Flash memory is a feature of many new I/O and storage devices. The intelligence of these devices can be upgraded by simply downloading new software from a vendor-supplied disk to flash memory. Upgrades to early inptu/output and storage devices required the user to replace the old circuit board or chip with a new one. The emergence of flash memory has eliminated this time consuming and constly method of upgrade. Look for non-volatile flash memory to play an increasing role in computer technology as its improvements continue to close the gap between the speed and flexibility of CMOS RAM.

CASHE MEMORY

Cashe Memory Proramms and data are loaded to RAM from secondary storage because the time required to access a programme instruction or piece of datum from RAM is significantly less than from secondary storage. Thousands of instructions or pieces of data can be accessed from RAM in the time it would take to access a single piece of datum from disk storage RAM is essentially a high speed holding area from data and programms. Infact, nothing really happens in a computer system until the programme instructions and data are moved from RAM to the processor. This transfer of instructions and data to the processor can be time consuming, even at microsecond speeds. To facilitate an even faster transfer of instructions and data to the processor, some processors are designed with cache memory. Cache memory is used by computer designers to increase computer system throughout. Throughout refers to the rate at which work can be performed by a computer system.

Like RAM, cache is a high speed holding area for programme instructions and data. However, cache memory uses a technology that is about 10 times faster than RAM and about 100 times more expensive. With only a fraction of the capacity of RAM, cache memory holds only those instructions and data that are likely to be needed next by the processor.

SECONDARY MEMORY

Secondary memory, or Secondary storage is memory also referred as backing storage is used to supplement the capacity of main storage. This memory stores a bulk of information. It is also called auxiliary storage or mass storage. The information stored in this memory is used by the CPU by first bringing it to main memory. Examples of auxiliary memory are magnetic disks, magnetic tape and mass cartridge systems. Auxiliary storage is of two types: Sequential and Direct Access.

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SIMMs and DIMMs & READ ONLY MEMORY (ROM)

SIMMs and DIMMs

The capacity of RAM in a computer affect computer's power. More capacity of RAM means the computer can use larger an more powerful programms and those programms can access bigger data files. More RAM can make computer run faster. The computer does not necessarily have to loa an entire programme into memory to run it, but the more of the programme it can fit into memory, the faster the programme will run. For example, a PC with 8 MB of RAM is capable of running Microsoft Windows even though the programme actually occupies about 40 MB on the disk storage space. When you run Windows, the programme does not to load all the files of windows into memory to run properly. It load only the essential parts into memory. When the computer needs to access other parts of the programme on the disk, it can unload nonessential part from RAM back to the hard disk and load the programme code or data it needs. However if your PC has larger RAM, windows will run faster because CPU will need to unload programme instructions between RAM and the hard disk less often.

Instead of (dual in line Pin) DIP package, today's computers use SIMMs (Single in-line Memory modules) and plug it in your computer easily. SIMMs and DIMMs provide large memory capacity but require very little space. They are available in many several configurations and in variety of speed. Each SIMMs or DIMMs can hold 4 MB to 128 MB of RAM. You can expand RAM capacity of your computer by plugging in more SIMMs or SIMMs chips.

When you purchase additional memory for your computer, you must make sure that the memory modules you purchase are the current type, and sould match with the configuration and speed for your computer system.

READ ONLY MEMORY (ROM)

A special type of internal memory contained in the computers is the Read Only Memory (ROM). This is a permanent of the computer designed by the manufacturers of the computers and cannot be altered by the programmer. The contents of ROM, a non-volatile technology, are hard wired (i.e., designed into the logic of the memory chip) by the manufacturer and can be read only. It is used to store the instructions required to control the basic input outpur operations of a computer whenever the computer system is turned on when you turn on a microcomputer system, a programme in ROM that resides in it automatically readies the computer system for use and displays the initial display-screen prompt. ROM is a semiconductor chip programmed at the time of its manufacture and is not reprogrammable by the user. Programmes sotred in ROM are called firmware. ROM is a non-volatile memory, i.e. the contents of this type of memory are not lost when the computer is turned off.

A variation of ROM sis programmable read-only memory (PROM). FROM is ROM into which you, the user, can load read-only programms and data. Some microcomputer software packages, such as electronic spreadsheets, are available as PROM units as well as on interchangeable disks. Generally, once a programme is loaded to PROM, it is sel-dom, if ever, changed. ROM and PROM are used in a variety of capacities within a computer system.

MAIN & RANDOM ACCESS MEMORY (RAM)

MAIN MEMORY

Main memory is the computer's primary storage. It is an extenstion of the Central Processing Unit (CPU) and is directly accessible to it. Its function is controlled by the Control Unit (CPU), which sends READ or WRITE signal for the appropriate memory location. Main memory accepts data and instructions from the input unit, exchanges data with the supplies instructions to other parts of the Central Processing Unit (CPU). This memory unit also stores those instructions and data, which are currently being used by the CPU or those instructions and data, awaiting immediate execution. Its capacity must be sufficient to retain both the usable amount of data and instructions needed for processing it. Some operations require more instructions or data than can be held is main memory at one time. In such cases, main memory is supplemented with auxiliary storage.

The capacity of a storage device is expressed as number of bytes, characters or bits. One byte is a group of eight bits that forms one character. Thus if the memory of a computer is 32000 bytes, it means that it can sote 32000 characters. Semiconductor storage (intergrated circuits) and magnetic cores are the most widely used forms of main storage.

Besides reduced size increased speed, semiconductor storage offers the advantage of non-destructive read. There are various storage types available, which include RAM, ROM, PROM, EROM, and EAROM. The most common kind of semiconductor storage is RAM (Random Access Memory) and ROM (Rad Only Memory).

RANDOM ACCESS MEMORY (RAM)

RAM is a memory in which each element of information has its own address (location) and from which any element can be retrieved easily and quickly by using that address. It is a temporary memory of a computer used to store the data and the instructions of the user when the computer is on. It is also called as Read-And-Write Memory (Ram) since the computer can store (write) data at any selected location (address) and at any subsequent time, retrieve (read) the data. The contents are lost when the power to the computer is shut off. Thus RAM is active only when computer is on and also referred as a volatile memory because the data stored in it disappears whenever he computer is switched off. However the RAM has the advantage of much better access time, higher transfer rate and simple interfacing. Generally RAM is meausured in K bytes or M bytes. Large RAM sizes provides larger amount of information which a computer can hold for processing and hence increases the processing speed.

Unlike magnetic secondary storage devices, such as tape as disk, RAM (primary storage) has no moving parts. RAM, a read-and write- memory, enables data to be both read and written to memory. Data can be accessed from RAM without any mechanical movement at electronic speed, close to the spee of light. Most of today's computers use CMOS (Complementary Metal-Oxide Semiconductor) technology for RAM. A CMOS memory chip less than one fourth the size of a postage stamp can store more than 4,000,000 bits, or more than 400,000 characters of data. Physically, memory chips are installed on single in-line memory modules or SIMMs. SIMMs are circuit boards with links directly to the processor.

The most common RAM technologies are dynamic RAM (DRAM) and static RAM (SRAM). DRAM is used in more computer than SRAM. However, SRAM is faster than DRAM because it does not have to be refreshed hundreds of time a second like DRAM. It is also much more expensive and requires more space and power than does DRAM.

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LASER HOLOGRAPHIC, STORAGE DEVICES

LASER HOLOGRAPHIC STORAGE

With recent advances in holography and laser beam technology, computer developers are studying optical memories with capacity greater than 100 megabits of data and random access time may be as short as one microsecond. Hologrammes make use of high-energy laser beam to store or display three dimensional images. The image produced by a hologram can easily be read by a photodetector, and information can be stored.

A holographic memory is made on a special recording medium similar to photographic film. Information is retrieved from the memory by projecting the data recorded on the hologram onto a light sensitive detector. The detector converts the optical information into electronic signals that computer can process.

STOREAGE DEVICES

Computer storage also referred as computer memory is actually an electronic file in which instructions and data are placed until needed. Computer storage is divided into two classes:

1. Main Storage or Main Memory
2. Secondary Storage or Secondary Memrory

MEMORY UNITS

The capacity of a storage device is expressed as number of byties, characters or bits. One byte is a group of eight bits that one character. Thus if the memory of a computer is 32000 bytes, it means that it can store 32000 characters.

BITS AND BYTES

The basic unit of a digital computer is a bit, which stands for binary digit. A bit is binary number and has value1 or 0 representing ON or OFF state. In microprocessor, bits are grouped into set of eight, called a byte, a four-bit sequence is called a nibble. The memory of a computer can be thought of cells. Each of these cells contains one bit of information. Each cell will have either 0 or 1 state. The combination of eight such cells forms one byte. Each byte contains one character sucl, as A, B,......etc. A group of bits representing data or instruction that form the basic information unit of the computer is called a "word". A word may have a length of 8 bits, 16 bits, 32 bits or more. Thus a word of 16 bits size can be called as 2-byte word. Generally computers that handle larger words are considered to be more powerful than the computer that use smaller word sizes because greater amount of information can be processed in a single cycle. The storing capacity of a computer is measured in bytes. The basic unit of information in digital computers is a bit, which stands for binary digit. A bit is binary number and has value1 or 0 representing ON or OFF state.

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FLOPPY DISKETTES, HARD DISK

The floppy diskette is the most common and popular type of auxiliary storage. It provides computer system especially microcomputer system with the ability to read or retrieve information randomly. Diskettes are as real as paper files. Anything you can do with a paper file can be done with a disk file. This includes copying it. changing it, adding to it and changing its name. Floppy disks are flexible, made of oxide coated mylar and stored in a paper or plastic envelopes. The entire envelope is inserted in the disk unit, thus effectively protecting the contents of the disk surfaces. The disk surfaces are rotated inside the prtective covering. The disk head contacts the track positions through a solt in the covering. Recording is magnetic in conectric circles called tracks. These circles or track are divided into sectors. Each individual sector holds 512 bytes of data. Floppy or diskettes are available in three different sizes, the large size 8" diameter can hold information of the order of 360 kilohytes in the magnetic medium. The computer access the magnetic medium through the read/write opening. The index hole is used by the computer to properly align the diskette. Minifloppy diskette or 5.25" diameter floppy can hold information upto 1.2 megabytes while mircrofloppy diskette are of 3.5" diameter, and has capacity upto 1.44 megabytes. These drives and the diskettes used in them are referred to as "single sided/single density" or SS/SD, "single sided/double density" or SS/DD and also double sided/double density or DS/DD types. Generally microfloppy and minifloppy and assigned drive A and drive B respectively while the fixed hard disk drive is made the drive C. Various diskette deive configurations are used in addition to compact disk drive. They are housed in the system unit.

HARD DISK

Many computers contain a special type of disk called a fixed disk or hard disk. It is a high speed, large capacity disk and is referred as a mass storage magnetic medium that provides computer systems with the ability to read or retrieve informations sequentially or reandomly. The phsical characteristics of all magnetic recording material. Hard disks are faster and capable of storing much more information ranging in gigabytes. A typical hard disk unit consists or 6 circular disks fixed to a central shaft. Both surfaces of each plate are used to contain data except for the tep and bottom plaets. A set of disk plates can have 10 surfaces. For each recording surface there is a read/wirte head assembly. Each head can be positioned at any desired track/band on each surface. All the heads move together, therefore, the same recording band is selected on each recording surface at the same time. Thus for any single head movement the computer can access 10 bands of information.
LASER HOLOGRAPHIC STORAGE

COMPACT DISK AND DISC RECORDABLE

COMPACT DISK

CD Rom stand for Compact disc read only memory. These discs are cecoming popular now a days especially for use with microcomputers. The name implies to its application. Once inserted into the CD Rom driver, the text, video images and so on can be read into Ram for processing or display. However the data on the disc are fixed and cannot be altered this is in contrast to read/write capability of magnetic discs. CD Roms have tremendus storage capacity. The capacity of a single CD Rom is upto 680 Mbytes wquvalent to over 600 normal size books each of 500 pages or about 470 DS/HD diskettes. Magnetic disks store data in concentric tracks each of which is divided into sectors. The sectors on the inside tracks hold the same amount of information as those on the outside tracks. Although this wastes magnetic disk space, it optimized the speed at which data can be written and retrieved because a magnetic disk spins at a constant speed. In contras, CD Roms store data in a single track that spirals from the centre to the outside edge. The ultra thin track spirals around the disk thoudands of times. Data are recorded on the Roms reflective surface in the form of pits and lands. The pits or depressions are tiny reflective bumps that have been burned in with a laser beam. The pits are flat areas separating the pits. A land reflects the laser light into the sensor and a pit scatters the light. A spot that reflects the laser beam into the sensor is interpreted as a 1, and the absence of a reflection is interpreted as a 0. Together they record read only binary information that can be interpreted by the computer as text, audio, images and so on. Once the data have been recorded, a protective coating is applied to the reflective surface. The speed at which a given CD Rom spins depends upon the phsical location of the data being read. The data passes over the moveable laser detector at the same rate, no matter where the data are read. Therefore the CD Rom must spin more quickly when accessing data near the centre. Even though with the changing speed of the disk, reading data from an optical medium is a relatively simple matter, writing data is a different matter. The fact that you cannot write data to a CD Rom does not mean that this storage medium is not useful. Infact many applications rely on huge volumes of data that that rarely changes. For example, dictionaries, encyclopedias, medical, legal and other professional referance libraries, software packages, music and video all require termendous amount of data that you would not normally want to alter.

COMPACT DISC RECORDABLE

For large quantities, CD Rom disks can be produced by manufacturers with expensive duplication equipment. Nearly all the PCs have CR Rom drives. This rapid and universal acceptance of CD Rom has given rise to an exciting new technology CD R, compact disk recordable. A CD R disk is functionally equvalent to a pre recorded CD Rom. A locally developed CD R will paly in any CD Rom drive.

PRINTERS WORK WITH COMPUTER

DOT MATRIX PRINTERS

In dot matrix printer, each letter is formed with a series of dots. Standard characters are produced by a pin printer or 24 pin printer. A normal dot matrix printer has speeds varying form 50 to 600 characters per seconds. The carriage width of a printer also varies ranging from 80 columns to 132 columns. Thus standard width printer print 80 characters per line while the wide carriage printer prints 132 characters per line.

DAISY WHEEL PRINTERS

This printer uses a print mechanism in the form of a series of petals arranged on a petal wheel. A character is contained at the end of each petal. The wheel rotates to bring the desired character into position. An image formed when a hammer strike the desired character. Daisy wheels are available in several formats (bold letter, italics etc.). A daisy wheel printer is slower than a dot matrix machine because of the time required to turn the print wheel. However it produces high quality print.

LINE PRINTERS

Line Printers are used with minicomputers and mainframe computers. They print at very fast speed. Line printers print one complete line at a time and at speeds greater than 1000 lines per minute. Line printers may be further divided into drum printers and band or chain printers.

NON IMPACT PRINTERS

New methods are being used in the computer printer technology in order to attain better quality and higher speed. A number of non impact printer have been developed which produce a printed image without striking the paper. Non impact printer are much faster has impact printers and produce much less noise and print high quality characters. Some of the non impact type of printers are:

• Electrostatic Printers
• Electro Thermal Printers
• Ink jet Printers and
• Laser Printers

ELECTROSTATIC PRINTERS

Electrostatic printer are high speed line printers. On scanning by electric field, a charged image is formed on paper. This paper is passed through an inkfog. The ink adhered at charged spots, which is fixed on passing through the heated rollers, producing the final printed sheet.

ELECTRO THERMAL PRINTERS

Thermal printers are inexpensive, small and require low voltage or battery power to operate. Their drawbacks are that these are slow and can only print on a special heat sensitive waxy paper. The image formed on the waxy paper is burned by the printer. Moreover this burned in image fades with time on prolonged exposure to sunlight. Thermal printers were popular a few years ago and are still available because of their protable nature and low cost.

INK JET PRINTERS

Work in the same way as dot matrix printers except that ink jet printer have fine spray nozsles instead of tiny pins used in dot matrix print head. Nozzles spray a stream of ink onto the paper. Because the ink is put directly on the paper, therefore, these printers require ink in a reservoirs instead of ribbon. Since no mechanical movement is involved, hence these printers are much quieter and have speed of about 200 characters per second. Ink jet printers are also available which have more than one ink reservoir, each with a different colour. The colours can be chosen through software to allow the printer to print in a rainbow colour. Thus a full colour graphinc image from a computer screen can be obtained very quietly on paper.

LASER PRINTER

Laser printers are the fastes machines in the printer industry. These printers use laser and electrophotographic technology to achieve printing speeds greater than 20,000 lines per minutes. In laser printing, a software creates a bitmapped image of the page in the computer. This image is processed and transferred to the printer which activates a laser beam to secan image on positively charged photoconductive surface of a rotating drum. The spots written by the laser take up a negative electric charge. A positive charged toner material is attracted by the negatively charged area of the drum written by the laser beam. This develops an image on the drum. As the paper passes between the drum and a positively charged wire called the corona wire, the image is transferred on the paper. The paper then passed between two rollers where the dots of the toner forming the image get fused onto the paper under the combined effect of heat and pressure. The printed result from a laser printer is impressive and better than letter quality. An advantage of laser printers over daisy wheel printers is their ability to produce graphic. Depending upon the software graphic and text can be merged on one page. As far as noise is concerned, laser printers are quieter and much faster.

DOT MATRIX, DAISY WHEEL, LINE PRINTERS

WHICH FUNCTIONS OUTPUT DEVICES

OUTPUT DEVICES

Peripheral devices used to get information or result from a computer are called output devices. A number of output devices are available for use in computer systems and vaious cominations of them are found in common use. These include display units, printers. floppy drivers, hard disks and other display devices.

VIDEO MONITORS

The video display unit or video monitor is the most popular microcomputer output divice, primarily because it is fast, quiet and cheaper than a printing device. A video monitor consists of a cathode ray tube (CRT) which is a TV like secreen. The monitor is made specifically to display information and is very similar to a home TV except that it does not include the electronic circuits for receiving TV signals. A computer can communicate with the user through a video monitor by presenting the result/information in visual form. There are two types of monitors black and white or monochrome monitors and coloured monitors. Mononchrome monitors come in two varieties depending upon the shade of display, green and amber. There are basically two kinds of colour monitors composite and RGB. A composite monitor works the same way as colour TV set. An RGB monitor has three electron guns, a green gun and a blue gun. The image is sharper and more refined on an RGB than a composite monitor. Visual display devices fall mainly in two categories. Alphanumeric display units, which can display only text, others called graphic display devices. Visual display units also come in low resolutions medium resolution or high resolution graphic mode and can display graphics such as maps, charts, graphs, engineering drawings, animation figures and other types of pictorial information as well as alphabets and numerals.

PRINTERS

The printers are one of most useful output devices for use with a computer. Printers are used extremely mechanical peripherals to produce permanent output from the computer. These are classified into two main categories depending upon the type of print mechanism.

• Impact Printers
• Non Impact Printers

INPACT PRINTERS

Impact printers work like typewriters. These are further classififed into three types based on their spped of printing and number of characters perline. There are

• Dost Matrix Printers
• Daisy Wheel Printers
• Line Printers

continue...VIDEO MONITOR, PRINTERS

MICROPHONES AN VOICE RECOGNITION

MICROPHONES AN VOICE RECOGNITION

Everone's computer's fantasy is to be able to dictate a command to the computer through a microphone and have the computer execute the command on the screen. For this type of sound input, a digitized recording is required. All you need to make such a recording are a microphne and a sound card that translate theelectrical signal from the microphoe into a digitized from that the computer can sotre and process. Sound cards can also translate digitized sounds back into analog signals that can then be sent to the speakers. There is also a demand for translating spoken words into text much as there is a demand for translating handwriting into text. Translating voice to text is a capability known as voice recognition. With it you can speak to the computer rather than having to type and you can control the computer with simple commands such as "shut down" or "print". Voice recognition faces problems that includes the large number of words required for a commercially feasible system (5,000 to 10,000 words), individual differences in speech patterns between different conditions, background noise and determining when one word stops and another begins. For example it would be extermely difficult for a computer to understan the difference between "two" and "too", "check" and "cheque" because of the similarity in the way they are spoken. There are four basic steps involved in speech recognition:

1. Say the word when you speak into a microphone, each sound is braken down into its vareous frequency.
2. Digitize the word the sounds in each frequency are digitized so they can be manipulated.
3. Match the word the digitized version fo the word is matched against similarly formed templated in the system's electronic dictionary.
4. Display each words or paerform the command when a match is found, the word is displayed on a V.D.T or the appropriate command is performed. If no match is found, the speaker is asked to repeat the word.

A spech recognition system contains a software, a generic vacabulary database, and a high quality microphone with noise concelling capabilities. The cocabulary may be only a few hundred words to enable navigation around windows (exist, copy, drag mouse and so on) and spread sheet. Once you have installed the software, you would need to train the system to recognize your unique speech pattern. To train the system, we simply talk to it for at least 20 minutes. The system uses artificial intelligence techniques to learn our speech pattern and update the vocabulary database accordingly. The challenge for voice recognition software is to reduce a sound's meaning correctly from its context and to distinguish meaningful sound from background noise.

VOICE RECOGNITION

 

HOW CAN WORK THESE IMPORTANT FUNCTIONS

TRACKBALL

The user controls the cursor on the screen by rolling a plastic ball with the fingertip or wrist. To execute commands with a trackball, one or more buttons are pressed, much as is done with a mouse. To move the cursor around the screen you roll the ball with your thumb or finger. Because you dont't move the whole device, a trackball requires less space than a mouse, so trackball is a solution when space is limited and is popular among users of laptop computers. Trackball may vary in size, but most are approximately the size of a standard desk calculator and may be amounted on either side of the keyboard. For handicapped people who may have difficulty pressing keys on a stadard keyboard or using a mouse, the trackball may be the answer since it is not necessary to move the entire arm to use it.

ELECTRIC LIGHT PEN

It consists of a photo cell placed in a small tube which can sense a position on displau tube when the end of the pen is held against the screen. In specially designed display seceen, data can be entered with a light pen as shown. When this electric light pen is brought near the screen, the light emitted is sensed by the photo cell. The electrical response is communicated to the computer to determine the part of the displayed item which is exciting the photo. A spot appears on the display screen at the location determined by the position of the pen. A computer programme compares the dot position generated by the device with computer generated data.. In this way new data can be entered or existing data can be modified figure below shows the use of a light pen in conjunction with a graphic display terminal to enter information into the computer. There are many other input devices like video digitizer, voice recognizer, magnetic ink character reader (MICR), optical character reader (OCR), remote terminal used for E mail, graphic table, etc. which can be linked with the computer. This is the most exciting part of using a personal computer.

DIGITAL CAMERA

Digital cameras operate on the same basic principles as a basic camera, but are more suitable for desktop publishing. The primary difference between a basic camera and a digital camera is that they use different media capturing images. The traditional camera uses film, while the digital camera uses a charged coupled device (CCD). Once captured, images can be uploaded to a PC and used in countless applications.

TRACKBALL, ELECTRIC LIGHT, DIGITAL CAMERA

MOUS, JOY STICK, SCANNERS

TYPES OF KEYBOARD

There are two basic styles of keyboard commonly found eith micro computers. PC/XT style keyboards and AT style. The PC/XT style keyboard was first one developed by IBM. Later AT style was developed with a slightly different keyboard having 101 keys. Different manufacturers have developed diferent keyboards with slight alternation with keys ranging from 84 to 108 using different style.

MOUSE

A mouse is an input device. Unlike the keyboard where the information is typed, mouse is a pointing device and is used to select various option. A mouse consists of a small hand held unit with one, two or three buttons and a small ball at the bottom on which it can roll. As the mouse is moved across the desk on a mouse pad, it guides a small locator symbol (often an arrow, a cross hairs or a cursor) which moves across the screen. A mouse can also be used to draw pictures on the screen and edit text.

JOY STICK

Joystick is an input device, a small box with moving stick and buttons. It is generally found with micro computer and is used primarily for games, educational sofware and computer aided design (CAD) system. You manipulate the handle of the joystick to position the cursor and click on a button to send impulses to the computer. The joystick does not interfere with your view of the screen nor does it require the movement of an object like a mouse. A joystick is fun to use because of the speed with which it moves the cursor on the screen.

 SCANNERS

Image scanners convert any image into electronic from by shining light onto the image and sensing the intensity of the reflection at every point. Colour scanners use filters to separat the components of colour into the primary additive colours red, green and blue at each point. Red, green and blue and known as primary additive colours because they can by combined to create any other colour. The image scanner is useful because it translates printed imagtes into an electronic format that can be sotred in computer's memory. The soted image can be transferred into a paint programme or directly into a word processor. A flatbed scanner easily scan documents books or periodicals. Flatbed scanners have higher quality reproduction and can scan a page in a single pass. Scanners difers in resolution: the more dot a scanned image contained, the sharper it is. An inexpansive scanner produces images at about 300 dots per inch or 600 dpi. The more expensive scanners produce images at a higher resolution.

PC/XT, (CAD)

SOME DIFFERENT KIND PORTS

PORTS

A system board with its processor and memory unit can work only when linked to input/output storage and communication devices to receive data and communicate results of processing. Peripheral devices such as a keyboard, mouse, monitor and a printer come with a cable and a multiple connector. To link a device to the PC, you plug its connector into a receptacle called a port in much the same way you plug a lamp cord into an electrical outlet. A port is one of the entry lines coming into the computer. A port provides a direct link to the microcomputer's common electrical bus. Most computer have several types of ports, each with different capabilities and uses. External peripheral devices outside the processor unit can be linked to the processor via cable through either a serial port or a parallel port. The system board is normally designed with at least one of each plus a dedicated keyboard port. Some have a dedicated port for the mouse. When a port is not available, you will need to install a circuit board that includes the port you need.

SERIAL PORTS

A  serical port provides a connection for transmitting data one bit at a time. A serial port connects your computer to a device such as a modem, which requires two way data transmission, or to a device such as a mouse, which requires only one way data transmission, or to a device such as a mouse, which requires only one way data transmission. To connect a peripheral device to your computer through a serial port, both the device and your computer must have a serial port.

MOUSE PORTS

A mouse port is a special serial port that connects a mouse to a computer. The mouse port can be built into the main board or mounted on an expansion card. Many computers include a mouse port in addition to standard serial ports.

INPUT/OUTPUT DEVICES

To communicate with a computer, there must be convenient ways of transferring the data and instructions into the computer and getting the results out in an acceptable form. This is accomplished by the input/output devices or I/O devices. Input devices receive information from the outside, convert it from a form that is easily understandable by the user into a form that is acceptable for the computer. Output devices retrieve information from the computer memory. These input/output devices are similar to our senses, our limbs and our ability to speak to our brain.

PORTS, SERIAL PORTS