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Introducing Computer Systems
What is Computer? Computer is an advanced electronic device that takes raw data as input from the user and processes these data under the control of set of instructions (called program) and gives the result (output) and saves output for the future use. It can process both numerical and non-numerical (arithmetic and logical) calculations. (OR) A Computer is an electronic device that can perform activities that involve Mathematical, logical and graphical manipulations. Generally, the term is used to describe a collection of devices that function together as a system. A computer has four functions: Input a. accepts data b. processes data c. produces output d. stores results Input (Data): Input is the raw information entered into a computer from the input devices. It is the collection of letters, numbers, images etc. Process: Process is the operation of data as per given instruction. It is totally internal process of the computer system. Output: Output is the processed data given by computer after data processing. Output is also called as Result. We can save these results in the storage devices for the future use. Processing Output Storage

Advantages of computers:

1. High speed: Computers have the ability to perform routine tasks at a greater speed than human beings. They can perform millions of calculations in seconds. 2. Accuracy: Computers are used to perform tasks in a way that ensures accuracy. 3. Storage: Computers can store large amount of information. Any item of data or any instruction stored in the memory can be retrieved by the computer at lightning speeds. 4. Automation: Computers can be instructed to perform complex tasks automatically (which Increases the productivity). 5. Diligence: Computers can perform the same task repeatedly & with the same without getting tired. 6. Versatility: Computers are flexible to perform both simple and complex tasks. 7. Cost effectiveness: Computers reduce the amount of paper work and human effort, thereby reducing costs.


A computer system has the following main components:

1) Input unit 2) Central Processing Unit (CPU). 3) Output unit.

Input unit:
Input unit accepts coded information from human operators through electromechanical devices such as the keyboard. The information received is either stored in the memory for later reference or immediately used by the Arithmetic and Logic circuitry to perform the desired operation. Finally the result is sent back to the outside through the output unit. Examples: keyboard, Joy stick, track ball, mouse (pointing devices), scanner etc.

\ Central Processing Unit:
Central Processing Unit (CPU) is the main component or “brain” of a computer. It performs all the processing of input data. Its examine and execute the instructions stored in the main memory of a computer. In microcomputers, CPU is built on a single chip or Integrated Circuit (IC) and is called a microprocessor. A CPU consists of the following distinct parts: • Arithmetic Logic Unit (ALU) • Control Unit (CU) • Registers • Buses • Clock

. Memory unit
Arithmetic Logic Unit (ALU): The arithmetic and logic unit of CPU is responsible for all arithmetic operations like addition, subtraction, multiplication and division as well as logical operations, such as less than, equal to and greater than. All calculations and comparisons are performed in arithmetic logic unit.

Control Unit

The control unit is responsible for controlling the transfer of data and instructions among other units of a computer. It is considered as the “central nervous system” of a computer as it manages and coordinates all the units of computer. It also performs the physical data transfer between memory and hardware devices. Registers: Registers are small high-speed circuits (memory locations). These are used to store data, instructions and memory addresses (memory location numbers) when ALU performs arithmetic and logical operations. Registers can store one word of data (1 word = 2 bytes and 1 byte = 8 bits) until it is overwritten by another word. Depending on the processor’s capability, the number and type of registers different from one CPU to another. Depending upon their functions, these can be divided into the following six categories: • General purpose registers • Pointer registers • Flags registers Buses: Data is stored as a unit of eight bits (bit stands for binary digit, i.e. 0 or 1) in a register. Each bit is transferred from one register to another by means of a separate wire. This group of eight wires that is used as a common way to transfer data between registers is known as a bus. It is actually a connection between two components to transmit signal between them. A bus can be of three major types. These types are as follows: 1. Data bus-- It is used to move data. 2. Control bus-- It is used to move address or memory location. 3. Address bus-- It is used to send control signals between various components of a computer. Clock: A clock is another important component of CPU. It measures and allocates a fixed time slot for processing each and every micro computer. The clock speed of CPU varies from one model to another in the range 4.77 MHz (in 8088 processor) to 266 MHz (in Pentium II). The speed of CPU is also specified in terms of Millions of Instructions Per Second (MIPS) or Million of Floating Point Operations Per Second (MFLOPS). • Segment registers • Index registers • Instruction pointer registers

Memory Unit:

Memory unit is that component of a computer system which is used to store data, instructions and information before and after the processing by ALU. It is actually a work area within a computer where CPU stores data and instructions. It is also known as a main memory or primary or internal memory.

Output unit:
Output unit accept binary data from the computer and decodes it into original form and supplies this result to the outside world. E.g. monitor, Printer, Video terminals (provides both input & output functions), graphic displays etc GENERATION OF COMPUTERS (History of computers): The first electronic computer was designed and built at the University of Pennsylvania based on vacuum tube technology. Vacuum tubes were used to perform logic operations and to store data. Generations of computers has been divided into five according to the development of technologies used to fabricate the processors, memories and I/O units. I Generation: 1945 – 55 II Generation: 1955 – 65 III Generation: 1965 – 75 IV Generation: 1975 – 89 V Generation: 1989 to present

First Generation (1945 – 55):
1. In this generation “Vacuum tubes” were used for basic arithmetic operations. 2. Took few milliseconds for processing Bulky data. 3. This computer cost is very high and speed is low and consumes more power with limited performance. 4. In this time assembly language is used to prepare programs. These were translated into machine level language for execution. 5. Punched cards and paper tape were invented to store programs and data and to get results. 6. Magnetic tape / magnetic drum were used as secondary memory storage devices. 7. It was mainly used for scientific computations. 8. 1st generation computers are called:

(ENIAC - Electronic Numerical Integrator and Calculator EDSAC – Electronic Delay Storage Automatic Calculator EDVAC–Electronic Discrete Variable Automatic computer UNIVAC – Universal Automatic Computer IBM 701)

Second Generation (1955 – 65):
1. In this generation “Transistors” were used in place of vacuum tubes. (Invented at AT&T Bell lab in 1947). 2. The size of computers is small and less power consumption and better performance, Lower cost. 3. In this generation, High level languages such as FORTRAN, COBOL etc were used for prepare programs, the high-level program translate into corresponding assembly language program which was then translated into machine language using with Compilers. 4. The Magnetic ferrite core memories were used as main memory which is a “randomaccess non-volatile memory”. 5. Magnetic tapes and magnetic disks were used as secondary memory, and also Punched cards continued during this period. 6. Hardware for floating point arithmetic operations was developed and Index registers were introduced which increased flexibility of programming. 7. Separate input-output processors were developed that could operate in parallel with CPU. 8. It was used in business, industry and commercial organizations for preparation of payroll, inventory control, marketing, production planning, research, scientific & engineering analysis and design etc. 9. In 2nd generation computers are called: (Manufacturers – IBM 7030, Digital Data Corporation’s PDP 1/5/8, Honeywell 400).

Third Generation (1965 – 75):
1. In this generation “IC” (Integrated circuits) was used for basic operations. 2. Small Scale Integration and Medium Scale Integration technology were also implemented in CPU, I/O processors etc.

3. The size of computer is Smaller & better performance and lesser cost and Faster processors. 4. In the generation magnetic core memories were replaced by semiconductor memories (RAM & ROM) 5. The High level languages were standardized by ANSI eg. ANSI FORTRAN, ANSI COBOL. 6. Introducing Microprogramming, parallel processing, pipelining, multiprocessor system, multiprogramming, multi-user system (time shared system) etc. 7. The Operating system software was introduced (efficient sharing of a computer system by several user programs). 8. Cache and virtual memories were introduced (Cache memory makes the main memory appear faster than it really is. Virtual memory makes it appear larger) 9. It was used in Database management, multi-user application, online systems like closed loop process control, airline reservation, interactive query systems, automatic industrial control etc. 10. In 3rd generation computers are called: (System 360 Mainframe from IBM, PDP-8 Mini Computer Corporation). from Digital Equipment

Fourth Generation (1975 – 89):
1. VLSI (Very Large Scale Integrated Circuits) was used for basic operations. 2. VLSI design with Tens of thousands of transistors can be placed in a single chip. 3. It consumes less power with high performance, lower cost and very compact and much increase in the speed of operation. 4. In this time Semiconductor memory chips were used as the main memory. 5. CRT screen, laser & ink jet printers, scanners are also developed. 6. The Secondary storage memories, i.e. hard disks, Floppy disks & magnetic tapes were used for backup memory. 7. The Parallelism, pipelining cache memory and virtual memory were applied in a better way. 8. In this generation LAN and WANS were developed used for desktop work stations interconnected.

9. C language, Unix OS, and Graphical User Interface was introduced. 10. In 4th generation computers are called: (Intel’s 8088,80286,80386,80486 .., Motorola’s 68000, 68030, 68040, Apple II, CRAY I/2/X/MP etc).

Fifth Generation (1989 to present):
1. Introduced ULSI (Ultra Large Scale Integration) technology is developed for operations. For example Intel’s Pentium 4 microprocessor contains 55 million transistors millions of components on a single IC chip. 2. In this generation also developed Superscalar processors, Vector processors, SIMD processors, 32 bit micro controllers and embedded processors, Digital Signal Processors (DSP) etc. 3. The Object oriented language like JAVA for developed internet programming and new operating systems Windows 95/98/XP/…, LINUX, etc were developed. 4. The Computers based on artificial intelligence and Portable note book computers introduced in this time. 5. Computers are used base on extensive parallel processing, multiple pipelines, multiple processors, massive parallel machines and extensively distributed system connected by communication networks etc. 6. In this time also Introduced World Wide Web and other existing applications like e-mail, e-Commerce, Virtual libraries/Classrooms, multimedia applications etc. 7. Memory chips up to 1 GB, hard disk drives up to 180 GB and optical disks up to 27 GB are available (still the capacity is increasing). Storage technology and advanced large main memory and disk storage available. 8. The Quantum mechanism and nanotechnology will radically change the phase and variation of computers. 9. In this generation computers are called: (IBM notebooks, Pentium PCs-Pentium 1/2/3/4/ Dual core/Quad core... SUN work stations, Origin 2000, PARAM 10000, IBM SP/2).

COMPUTERS FOR INDIVIDUAL USERS (or) TYPES OF COMPUTERS (or) CLASSIFICATIONS OF COMPUTERS (or) COMPUTERS FOR ORGANIZATIONS: Most computers arc meant to be used by only one person at a time. Such computers are often shared by several people (such as those in your school’s computer lab), but only one user can work with the machine at any given moment. The primary types of computers in this category are: 1. Mainframe Computers 2. Minicomputers: 3. Supercomputers Mainframe Computers: Mainframe computers are used in large organizations such as insurance companies and banks, where many people frequently need to use the same data. In a traditional mainframe environment, each user accesses the mainframe’s resources through a device called a terminal. There are two kinds of terminals, A dumb terminal does not process or store data; it is simply an input/output (I/O) device that functions as a window into a computer located somewhere else. An intelligent terminal can perform some processing operations, but it usually does not have any storage. In some mainframe environments, however, workers can use a standard personal computer to access the mainframe. Examples of mainframe computers are: Univac 1180, and DEC. Minicomputers: It was first released in the 1960s. Minicomputers are small size compared to other computers of the day. The capabilities of a minicomputer are somewhere between those of mainframes and personal computers. For this reason, minicomputers are often called “midrange computers”. Minicomputers can handle much more input and output than personal computers. Although some mini computers are designed for a single user, the most powerful minicomputers can serve the input and output needs of hundreds of users at a time. Users can access a central minicomputer through a terminal or a standard PC. Examples of mini computers are Digital Equipments PDP 11/45 and VAX 11. IBM 3000 series, Burroughs B7900,

Super computers Supercomputers are the most powerful computers made, and physically they are some of the largest. These systems can process huge amounts of data, and the fastest. Super computers can perform more than one trillion calculations per second. Some supercomputers can contains thousands of processors. Supercomputers are ideal for handling large and highly complex problems that require extreme calculating power. For example, supercomputers have long been used in the mapping of the human genome, forecasting weather, and modeling complex processes like nuclear fission. Examples of super computers are CRAY Research :- CRAY-1 & CRAY-2, Fujitsu (VP2000), Hitachi (S820), NEC (SX20), PARAM 10000 by C-DAC, Anupam by BARC, PACE Series by DRDO. Micro Computers: They are designed to be used by only one person at a time. A microcomputer uses a microprocessor as its central Processing Unit (CPU). Microcomputers are tiny computers that can be different in size from a single chip to the size of a desktop model. It’s having Small to medium data storage capacities 500MB – 2GB.The common examples of microcomputers are chips used in washing machines, TVs, Cars and Note book/Personal computers. Examples of super computers are: IBM PC, PS/2 and Apple Macintosh This can be divided based on sizes as fallows: a. Desktop computers. b. Workstations. c. Notebook computers. d. Tablet computers. e. Handheld computers. f. Smart phones. Desktop computers: The desktop computer is a Personal Compute. That is designed to sit on (or under) a desk or table. These are the systems you sec all around you, in schools, homes, and offices. Desktop computers are more powerful and are used for an amazing array of tasks or jobs with greater easy and efficiency. But they can be also used to communicate, produce music, edit photographs

and videos, play sophisticated games. But it was much more used by everyone from preschoolers (L.K.G) to nuclear physicists; desktop computers are indispensable for learning, work, and play. Workstations: A workstation is a specialized; single-user computer that typically has more power and features than a standard desktop PC. These machines are popular among scientists, engineers, and animators who need a system with greater-than-average speed and the power to perform sophisticated tasks. Workstations often have large, high-resolution monitors and accelerated graphics handling capabilities, making them suitable for advanced architectural or engineering design, modeling, animation, and video editing. Notebook Computers: Notebooks computers are approximate the shape of notebook and easily fit inside a briefcase. Because people frequently set these computers on their lap, they are also called laptop computers. Notebook computers can operate on alternating current or special batteries. These computers is generally weigh less than eight pounds, and some of weigh less than three pounds based on a thin monitor and a keyboard and easy storage. Notebooks are fully functional microcomputers; the people who use them need the power of a full-size desktop computer. Tablet PCs: The tablet PC is the newest development in portable, full-featured computers). Tablet PCs offer all the functionality of a notebook PC, but they are lighter and can accept input from a special pen called a stylus or a digital pen. That is used to tap or write directly on the screen. Many tablet PCs also have a built-in microphone and special software that accepts input from the user's voice. A few models even have a fold-out keyboard, so they can be transformed into a standard notebook PC. Tablet PCs run specialized versions of standard programs and can be connected to a network. Some models also can be connected to a keyboard and a full size monitor. Handheld PCs: Handheld personal computers are computing devices small enough to fit in your hand. A popular type of handheld computer is the personal digital assistant (PDA). A PDA is no larger than a small appointment book and is normally used for special applications, such as taking notes, displaying telephone numbers and addresses, and keeping track of dates or agendas. Many PDA’s can be connected to larger computers to exchange data. Most PDAs come with a pen that

lets the user write on the screen. Some handheld computers feature tiny built-in keyboards or microphones that allow voice input. \Smart Phones: Some cellular phones double as miniature PCs. Because these phones offer advanced features not typically found in cellular phones, they are sometimes called smart phones. These features can include Web and c-mail access, special software such as personal organizers, or special hardware such as digital cameras or music players. Some models even break in half to reveal a miniature keyboard.

The Parts of a Computer System ?
Computers come in many varieties, from the tiny computers built into household appliances, to the astounding supercomputers that have helped scientists map the human genome. But no matter how big it is or how it is used, every computer is part of a system. A complete computer system consists of four parts: 1. Hardware 2. Software 3. Data 4. User Hardware: The mechanical devices that make up the computer are called hardware. Hardware is any part of the computer you can touch. A computer’s hardware consists of interconnected electronic devices that you can use to control the computer’s operation, input, and output. (The generic term device refers to any piece of hardware.) Hardware refers to any physical component of computer. For example, CPU, monitor (VDU), keyboard, hard disk, floppy disk, printer etc. are physical components and, thus, are all hardware. Software: Software is a “set of instructions that makes the computer perform tasks and software tells the computer what to do”. (The term program refers to any piece of software.) Some programs exist primarily for the computer's use to help it perform tasks and manage its own resources. Other types of programs exist for the user, enabling him or her to perform tasks such

as creating documents. Thousands of different software programs arc available for use on personal computers. For example: DOS (Disk Operating System), BASIC, COBOL, dBase, Accounting Software etc. Data: Data consist of individual facts or pieces of information that by themselves may not make much sense to a person. A computer’s primary job is to process these tiny pieces of data in various ways, converting them into useful information. Users: People are the computer operators, also known as users. It can be argued that some computer systems are complete without a person’s involvement; however, no computer is totally autonomous. Even if a computer can do its job without a person sitting in front of it, people still design, build, program, and repair computer systems. This lack of autonomy is especially true of personal computer systems, which arc the focus of this book and are designed specifically for use by people.

Keyboard 1. Keyboard (similar to a typewriter) is the main input device of a computer. 2. It contains three types of keys: Alphanumeric keys, Special keys. Function keys. The Alphanumeric Keys: The alphanumeric keys are keyboard that looks like a typewriter's keys. That are arranged the same way on almost every keyboard. Sometimes this common arrangement is called the QWERTY layout. Because the first six keys on the top row of letters are Q, W, E, R, T, and Y. Along with the keys that produce letters and numbers. The alphanumeric key group includes four keys having specific functions. The tab, CAPS LOCK, backspace, and enter keys are described Alphanumeric keys are used to type all alphabets, numbers and special symbols like $, %, @, A etc. Special keys:

Special keys such as <Shift>, <Ctrl>, <Alt>, <Home>, <Scroll Lock> etc. are used for special functions.

Function keys such as <Fl>, <F2>, <F3> etc. are used to give special commands depending upon the software used. The function of each and every key can be well understood only after working on a PC. When any key is pressed, an electronic signal is produced. This signal is detected by a keyboard encoder that sends a binary code corresponding to the key pressed to the CPU. There are many types of keyboards but 101 keys keyboard is the most popular one.

The monitor is the most commonly used output device on most personal computer systems. When you are typing a letter, copying files, or surfing the Internet that are looking at your monitor. They want to sec whether the image is crisp and clear and how well graphics are displayed on the monitor. Two important hardware devices determine the quality of the image you see on any monitor: (1) The monitor (2) The video controller. Color monitors can display between 16 colors and 16 million colors. Today, most new monitors display in color. Many color monitors can be set to work in monochrome or grayscale mode. The monitors can be divided into to types based on color: Monochrome monitors display only one color that is black. (Such as green, amber, or white) against a contrasting background. These monitors are used for only displays “text”. where the user does not need to see color graphics. Grayscale monitors display varying intensities of gray from a very little gray to black against a white or off white background. And these are essentially a type of monochrome monitor. Grayscale flat panel displays are used in low-end portable systems. There are two types of monitors are used with PCs. A cathode ray tube (CRT) A flat-panel display 1). A cathode ray tube (CRT): 1. The cathode ray tube (CRT) monitor comes with most desktop computers. 2. It looks a television screen. This type of monitor uses a large vacuum tube for display. 3. The CRT monitor works from back o f a monitor's housing is an electron gun. 4. The gun shoots a beam of electrons through a magnetic coil called a yoke. This aims the beam at the front of the monitor. 5. The back of the monitor’s screen is coated with phosphors, chemicals that glow when they are struck by the electron beam. 6. The screen’s phosphor coating is organized into a grid o f dots. 7. The smallest number of phosphor dots that the gun can focus on is called a pixel.

8. The Pixel is a contraction of the term picture element. Each pixel has a unique address, which the computer uses to locate the pixel and control its appearance. Some electron guns can focus on pixels as small as a single phosphor dot. 9. The electron gun does not just focus on a spot and shoot electrons at it. 10. It systematically aims at every pixel on the screen, starting at the top left corner and scanning to the right edge. Then it drops down a tiny distance and scans another line. 11. A CRT monitor contains a shadow mask, it is a fine mesh made of metal, fitted to the shape and size of the screen. 12. The holes in the shadow mask’s mesh are used to align the electron beams, to ensure that they strike precisely the correct phosphor dot. In most shadow masks, these holes are arranged in triangles. CRT monitors have long been the standard for use with desktop computers because they provide a bright, dear picture at a relatively low cost. 2). A flat-panel display: The Flat-panel monitors have been used primarily on portable computers. a new generation of large, high-resolution, flat-panel displays is gaining popularity among users of desktop systems. These new monitors provide the same viewable area as CRT monitors, but they take up less desk space and run cooler than traditional CRT monitors. There are several types of flat-panel monitors: I). Liquid crystal display (LCD): The LCD monitor creates images with a special kind of liquid crystal. That is normally transparent but becomes opaque when charged with electricity. LCD monitors is that their images can be difficult to see in bright light. For this reason, laptop computer users often look for shady places to sit when working outdoors or near windows. A bigger disadvantage of LCD monitor is their limited viewing angle. That is the angle from which the display's image can be viewed clearly. You can sec the image clearly even when standing at an angle to the screen with most CRT monitors. In LCD monitors the viewing angle shrinks and as you increase your angle to the screen, the image becomes fuzzy quickly. (There are two main categories of liquid crystal displays :)

II). The passive matrix LCD: The passive matrix LCD works based on transistors for each row and each column of pixels. A grid that defines the location of each pixel. The color displayed by a pixel is determined by the electricity coming from the transistors at the end of the row and the top of the column. Although passive matrix monitors are inexpensive to manufacture, they have a narrow viewing angle. Another disadvantage is that they don’t “refresh” the pixels very quickly. If you move the pointer too quickly, it seems to disappear an effect known as submarining. Animated graphics can appear blurry on a passive matrix monitor. Most passive matrix screens now use dual-scan LCD technology, which scans the pixels twice as often. Submarining and blurry graphics are less troublesome than they were before the dual-scan technique was developed. Types of Monitors Depending upon the resolution, monitors can be classified as follows: a. CGA: This stands for Color Graphics Adapter. It is a circuit board introduced by IBM and the first graphics standard for the IBM PC. With a CGA monitor, it is harder to read than with a monochrome monitor, because the CGA (320 X 200) has much fewer pixels than the monochrome monitor (640 X 350). It supports 4 colors. b. EGA: It stands for Enhanced Graphics Adapter. EGA is a video display standard that has a resolution of 640 by 350 pixels and supports 16 colors. EGA supports previous display modes and requires a new monitor. c. VGA: VGA stands for Video Graphics Array. This is a video display standard that provides medium to high resolution. In a text mode, the resolution of this board is 720 by 400 pixels. It supports 16 colors with a higher resolution of 640 by 480 pixels and 256 colors with 320 X 200 pixels. d. Super VGA: This is a very high resolution standard that displays up to 65,536 colors. Super VGA can support 16.8 million colors at 800 by 600 pixels and 256 colors at 1024 by 768 pixels. A high-priced super VGA allows 1280 by 1024 pixels. Larger monitors (17" or 21" and larger) with a high resolution of 1600 by 1280 pixels are available. VESA (Video Electronics Standards Association) has set a standard for super VGA.

A printer is an output device that produces a hard copy of data. The resolution of printer output is expressed as DPI. Printers can be classified into different types in several ways. First, the printers can be divided into three categories by the way they print. Serial Printers: Also called a character printer. Print a single character at a time. They are usually inexpensive and slow. Line Printers: Print a line at a time. They are expensive and very fast. Line printers use a band, a chain, etc. Page Printers: Also called a laser printer. Print a page at a time. They usually use a laser to produce page images. Quality is best. This is a little bit expensive, but the price of the personal laser printer is decreasing. The price range of the personal laser printer is around $400, today. Second, printers can be classified into two forms according to the use of a hammer. Impact Printers: Hammer hits ribbons, papers or print head. Dot-matrix and daisy-wheel printers are the example. Noisy. Non-impact Printers: They do not have the hammer and do not hit. An example is an ink-jet and laser printer. Third, classification can be made by the way they form characters. Bit-Mapped Printers: Images are formed from groups of dots and can be placed anywhere on the page. They have many printing options and good printing quality. They use PostScript as a standard language for instructing a microcomputer. Character-based Printers: Printer print characters into the lines and columns of a page. These printers use predefined set of characters and are restricted in position of characters. Microcomputers use five kinds of printers. They are daisy wheel printers, chain printers, dot-matrix printers, ink-jet printers, and laser printers. Daisy-Wheel Printer Daisy-Wheel is a printer mechanism that uses any kind of hub (wheel) having a set of spokes at the margin of the hub. The wheel can be removed to use a different character set. The end of each spoke is a raised image of a type character. When the wheel is turned and the required character is aligned to the print hammer, the character is then struck into a ribbon and

onto a paper with the hammer. Daisy-Wheel Printer prints typewriter-like very high quality characters. However, they are slower and less reliable than dot-matrix printers. Microcomputer users seldom use this printer, because the better dot-matrix printers and inexpensive laser printers are available today. Chain Printer: A chain printer uses a printing mechanism that uses character typefaces linked together in a chain. The chain spins horizontally around a set of hammers aligned with each position. When the required character is in front of the selected print position, hammer in that position hits the paper into the ribbon against the character in the chain. This printer is not commonly found around microcomputers, because it is a very expensive, highspeed machine designed originally for mainframes and minicomputers. Chain printers are very reliable and can speed up to 3000 lines per minute. Dot-Matrix Printer: Dot-matrix printers are printers that write characters and form graphic images using one or two columns of tiny dots on a print head. The dot hammer moving serially across the paper strikes an inked-ribbon and creates images on paper. Dot matrix printers are popular printers used with microcomputers, because the printers are highly reliable and inexpensive. They are used for tasks where a high-quality image is not essential. Many users, however, move from dot printers to laser printers, because the price of laser printers is falling down. Several kinds of dot matrix printers are available with print heads that have 7, 9, 18, or 24 pins. Ink-Jet Printer: Ink-jet is a printer mechanism that sprays one or more color of ink at high speed onto the paper and produces high-quality printing. This printer also produces color printing as well as highquality image. That is, ink-jet printers can be used for variety of color printing at a relatively low cost. Ink-jet printing has two methods: Continuous stream method and drop-on- demand method. Laser Printer A laser printer is a printer that uses the electrophotograpic method used in a copy machine. The printer uses a laser beam light source to create images on a photographic drum. Then the images on the drum are treated with a magnetically charged toner and then are transferred onto a paper. A heat source is usually applied to make the images adhere.

In 1984, Hewlett-Packard introduced the first desktop laser printer, called the LaserJet. The laser printer revolutionized personal computer printing and has spawned desktop publishing. The laser printer produces high-resolution letters and graphics quality images, so it is adopted in applications requiring high-quality output. Although a high-priced color laser printer is also available in the market, a less expensive, desktop gray scale laser printer is widely used. Recently, the laser printer is gaining its market share dramatically, mainly because the lowered price and the quality.

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