Line Printer, in computer science, any printer that prints one line at a time, as opposed to one character at a time (as with a standard dot-matrix printer) or one page at a time (as with a laser printer).





Line printers typically produce the 11-by-17-inch “computer” printouts familiar to many. They are high-speed devices and are often used with mainframes, minicomputers, or networked machines rather than with single-user systems. Types of line printers include chain printers and band printers. The abbreviation LPT originally stood for “line printer”; on MS-DOS microcomputers, the same abbreviation is used for the computer's parallel output port(s).

Laser Printer, an electrophotographic printer that is based on the technology used by photocopiers. A focused laser beam and a rotating mirror are used to draw an image of the desired page on a photosensitive drum.







This image is converted on the drum into an electrostatic charge, which attracts and holds toner. A piece of electrostatically charged paper is rolled against the drum, which pulls the toner away from the drum and onto the paper. Heat is then applied to fuse the toner to the paper.


Finally, the electrical charge is removed from the drum and the excess toner is collected. By omitting this final step and repeating only the toner-application and paper-handling steps, the printer can make multiple copies.






The only serious drawback of a laser printer is that it offers less paper-handling flexibility than do dot-matrix printers. Both multipart forms and wide-carriage printing, for example, are better handled by dot-matrix or daisy-wheel printers.

Printer, a computerperipheral that puts text or a computer-generated image on paper or on another medium, such as a transparency. Printers can be categorized in any of several ways. The most common distinction is impact vs. nonimpact. Impact printers physically strike the paper and are exemplified by pin dot-matrix printers and daisy-wheel printers; nonimpact printers include every other type of print mechanism, including laser, ink-jet, and thermal printers. Other possible methods of categorizing printers include (but are not limited to) the following:








Print technology: Chief among these, with microcomputers, are pin dot-matrix, ink-jet, laser, thermal, and (although somewhat outdated) daisy-wheel or thimble printers. Pin dot-matrix printers can be further classified by the number of pins in the print head: 9, 18, 24, and so on.

Character formation: Fully formed characters made of continuous lines (for example, those produced by a daisy-wheel printer) vs. dot-matrix characters composed of patterns of dots (such as those produced by standard dot-matrix, ink-jet, and thermal printers). Laser printers, while technically dot-matrix, are generally considered to produce fully formed characters because their output is very clear and the dots are extremely small and closely spaced.







Method of transmission: parallel (byte-by-byte transmission) vs. serial (bit-by-bit transmission). These categories refer to the means by which output is sent to the printer rather than to any mechanical distinctions. Many printers are available in either serial or parallel versions, and still other printers offer both choices, yielding greater flexibility in installation options.

Method of printing: Character by character, line by line, or page by page. Character printers include standard dot-matrix, ink-jet, thermal, and daisy-wheel printers. Line printers include the band, chain, and drum printers that are commonly associated with large computer installations or networks. Page printers include the electrophotographic printers, such as laser printers.


Print capability: Text-only vs. text-and-graphics. Text-only printers, including most daisy-wheel and thimble printers and some dot-matrix and laser printers, can reproduce only characters for which they have matching patterns, such as embossed type, or internal character maps. Text-and-graphics printers—dot-matrix, ink-jet, laser, and others—can reproduce all manner of images by “drawing” each as a pattern of dots.

Information from an input device or from the computer’s memory is communicated via the bus to the central processing unit (CPU), which is the part of the computer that translates commands and runs programs. The CPU is a microprocessor chip—that is, a single piece of silicon containing millions of tiny, microscopically wired electrical components. Information is stored in a CPU memory location called a register. Registers can be thought of as the CPU’s tiny scratchpad,



temporarily storing instructions or data. When a program is running, one special register called the program counter keeps track of which program instruction comes next by maintaining the memory location of the next program instruction to be executed. The CPU’s control unit coordinates and times the CPU’s functions, and it uses the program counter to locate and retrieve the next instruction from memory.

In a typical sequence, the CPU locates the next instruction in the appropriate memory device. The instruction then travels along the bus from the computer’s memory to the CPU, where it is stored in a special instruction register. Meanwhile, the program counter changes—usually increasing a small amount—so that it contains the location of the instruction that will be executed next. The current instruction is analyzed by a decoder, which determines what the instruction will do. Any data the instruction needs are retrieved via the bus and placed in the CPU’s registers. The CPU executes the instruction, and the results are stored in another register or copied to specific memory locations via a bus. This entire sequence of steps is called an instruction cycle. Frequently, several instructions may be in process simultaneously, each at a different stage in its instruction cycle. This is called pipeline processing.

Monitor, in computer science, device connected to a computer that displays information on a screen. Modern computer monitors can display a wide variety of information, including text, icons (pictures representing commands), photographs, computer rendered graphics, video, and animation.




Most computer monitors use a cathode-ray tube (CRT) as the display device. A CRT is a glass tube that is narrow at one end and opens to a flat screen at the other end. The CRTs used for monitors have rectangular screens, but other types of CRTs may have circular or square screens. The narrow end of the CRT contains a single electron gun for a monochrome, or single-color monitor, and three electron guns for a color monitor—one electron gun for each of the three primary colors: red, green, and yellow. The display screen is covered with tiny phosphor dots that emit light when struck by electrons from an electron gun.







Monochrome monitors have only one type of phosphor dot while color monitors have three types of phosphor dots, each emitting either red, green, or blue light. One red, one green, and one blue phosphor dot are grouped together into a single unit called a picture element, or pixel. A pixel is the smallest unit that can be displayed on the screen. Pixels are arranged together in rows and columns and are small enough that they appear connected and continuous to the eye.

Electronic circuitry within the monitor controls an electromagnet that scans and focuses electron beams onto the display screen, illuminating the pixels. Image intensity is controlled by the number of electrons that hit a particular pixel. The more electrons that hit a pixel, the more light the pixel emits. The pixels, illuminated by each pass of the beams, create images on the screen. Variety of color and shading in an image is produced by carefully controlling the intensity of the electron beams hitting each of the dots that make up the pixels. The speed at which the electron beams repeat a single scan over the pixels is known as the refresh rate. Refresh rates are usually about 60 times a second.

Monochrome monitors display one color for text and pictures, such as white, green, or amber, against a dark color, such as black, for the background. Gray-scale monitors are a type of monochrome monitor that can display between 16 and 256 different shades of gray.

Manufacturers describe the quality of a monitor’s display by dot pitch, which is the amount of space between the centers of adjacent pixels. Smaller dot pitches mean the pixels are more closely spaced and the monitor will yield sharper images. Most monitors have dot pitches that range from 0.22 mm (0.008 in) to 0.39 mm (0.015 in).

The screen size of monitors is measured by the distance from one corner of the display to the diagonally opposite corner. A typical size is 38 cm (15 in), with most monitors ranging in size from 22.9 cm (9 in) to 53 cm (21 in). Standard monitors are wider than they are tall and are called landscape monitors. Monitors that have greater height than width are called portrait monitors.

The amount of detail, or resolution, that a monitor can display depends on the size of the screen, the dot pitch, and on the type of display adapter used. The display adapter is a circuit board that receives formatted information from the computer and then draws an image on the monitor, displaying the information to the user. Display adapters follow various standards governing the amount of resolution they can obtain. Most color monitors are compatible with Video Graphics Array (VGA) standards, which are 640 by 480 pixels (640 pixels on each of 480 rows), or about 300,000 pixels. VGA yields 16 colors, but most modern monitors display far more colors and are considered high resolution in comparison. Super VGA (SVGA) monitors have 1024 by 768 pixels (about 800,000) and are capable of displaying more than 60,000 different colors. Some SVGA monitors can display more than 16 million different colors.

A monitor is one type of computer display, defined by its CRT screen. Other types of displays include flat, laptop computer screens that often use liquid-crystal displays (LCDs). Other thin, flat-screen monitors that do not employ CRTs are currently being developed.

Microphone, device used to transform sound energy into electrical energy (see Sound Recording and Reproduction). Microphones are important in many kinds of communications systems and in instruments that measure sound and noise. The American inventor Alexander Graham Bell built the first microphone in 1876 when he constructed his telephone transmitter.





The simplest type of modern microphone is the carbon microphone, used in telephones. This microphone consists of a metallic cup filled with carbon granules; a movable metallic diaphragm mounted in contact with the granules covers the open end of the cup. Wires attached to the cup and diaphragm are connected to an electrical circuit so that a current flows through the carbon granules. Sound waves vibrate the diaphragm, varying the pressure on the carbon granules. The electrical resistance of the carbon granules changes with the varying pressure, causing the current in the circuit to change according to the vibrations of the diaphragm. The varying current may either actuate a nearby telephone receiver or may be amplified and transmitted to a distant receiver. If the current variation is suitably amplified, it may also be used to modulate a radio transmitter.

Another common type, the crystal microphone, utilizes piezoelectric crystals, in which a voltage develops between two faces of the crystal when pressure is applied to the crystal (see Piezoelectric Effect). In this microphone sound waves vibrate a diaphragm, which in turn varies the pressure on a piezoelectric crystal. This generates a small voltage, which is then amplified.




Examples of dynamic microphones include ribbon microphones and moving-coil microphones. In ribbon microphones, a thin metallic ribbon is attached to the diaphragm and placed in a magnetic field. When sound waves strike the diaphragm and vibrate the ribbon, a small voltage is generated in the ribbon by electromagnetic induction. A moving-coil microphone operates on essentially the same principle but has a coil of light wire, instead of a ribbon, attached to the diaphragm. Some modern microphones, designed to pick up sound from one direction only, combine both ribbon and coil elements.
Another type of microphone is the condenser microphone. The condenser microphone has two thin metallic plates placed close to each other that serve as a capacitor. The back plate of the capacitor is fixed, and the front plate serves as the diaphragm. Sound waves alter the spacing between the plates, changing the electrical capacitance between them. By placing such a microphone in a suitable circuit, these variations may be amplified, producing an electrical signal. Condenser microphones can be very small. A common type of condenser microphone, the electret condenser microphone, is used in hearing aids.

Among the important characteristics of microphones are their frequency response, directionality, sensitivity, and immunity to outside disturbances such as shock or vibration.

Mouse (computer), a common pointing device, popularized by




its inclusion as standard equipment with the Apple Macintosh. With the rise in popularity of graphical user interfaces (Graphical User Interface) in MS-DOS; UNIX, and OS/2, use of mice is growing throughout the personal computer and workstation worlds.


The basic features of a mouse are a casing with a flat bottom, designed to be gripped by one hand; one or more buttons on the top; a multidirectional detection device (usually a ball) on the bottom; and a cable connecting the mouse to the computer.


See the illustration.

By moving the mouse on a surface (such as a desk), the user typically controls an on-screen cursor. A mouse is a relative pointing device because there are no defined limits to the mouse's movement and because its placement on a surface does not map directly to a specific screen location.


To select items or choose commands on the screen, the user presses one of the mouse's buttons, producing a “mouse click.”

An analog modem converts the digital signals of the sending computer to analog signals




that can be transmitted through telephone lines. When the signal reaches its destination, another modem reconstructs the original digital signal, which is processed by the receiving computer. A standard analog modem has a maximum speed of 33.6 Kbps.

The word modem is an acronym formed from the two basic functions of an analog modem: modulation and demodulation. To convert a digital signal to an analog one, the modem generates a carrier wave and modulates, or adjusts, it according to the digital signal. The kind of modulation used depends on the application and the speed of operation for which the modem is designed. For example, many high-speed modems use a combination of amplitude modulation, in which the amplitude (see Wave Motion) of the carrier wave is changed to encode the digital information, and phase modulation, in which the phase of the carrier wave is changed to encode the digital information. The process of receiving the analog signal and converting it back to a digital signal is called demodulation.

Cable modems permit the transmission of data over community antenna television (CATV) networks—that is, the network of cables used to distribute cable television. A cable modem transmits data from the network at about 3 Mbps and transmits data to the network at between 500 Kbps and 2.5 Mbps.

Like a standard analog modem, a cable modem converts between a digital signal and an analog signal.




Cable modems are much more complex than standard analog modems. They also incorporate a tuner that separates the digital data from the rest of the broadcast television signal. Because users in multiple locations share the same cable, the modem also includes hardware that permits multiple connections and an encryption/decryption device that prevents data from being intercepted by another user or being sent to the wrong place.

Random access memory is also called main memory because it is the primary memory that the CPU uses when processing information.






The electronic circuits used to construct this main internal RAM can be classified as dynamic RAM (DRAM), synchronized dynamic RAM (SDRAM), or static RAM (SRAM). DRAM, SDRAM, and SRAM all involve different ways of using transistors and capacitors to store data. In DRAM or SDRAM, the circuit for each bit consists of a transistor, which acts as a switch, and a capacitor, a device that can store a charge.



To store the binary value 1 in a bit, DRAM places an electric charge on the capacitor.To store the binary value 0, DRAM removes all electric charge from the capacitor.The transistor is used to switch the charge onto the capacitor.


When it is turned on, the transistor acts like a closed switch that allows electric current to flow into the capacitor and build up a charge.The transistor is then turned off, meaning that it acts like an open switch, leaving the charge on the capacitor.To store a 0, the charge is drained from the capacitor while the transistor is on, and then the transistor is turned off, leaving the capacitor uncharged.To read a value in a DRAM bit location, a detector circuit determines whether a charge is present or absent on the relevant capacitor.DRAM is called dynamic because it is continually refreshed.
The memory chips themselves cannot hold values over long periods of time. Because capacitors are imperfect, the charge slowly leaks out of them, which results in loss of the stored data. Thus, a DRAM memory system contains additional circuitry that periodically reads and rewrites each data value. This replaces the charge on the capacitors, a process known as refreshing memory. The major difference between SDRAM and DRAM arises from the way in which refresh circuitry is created. DRAM contains separate, independent circuitry to refresh memory. The refresh circuitry in SDRAM is synchronized to use the same hardware clock as the CPU. The hardware clock sends a constant stream of pulses through the CPU’s circuitry. Synchronizing the refresh circuitry with the hardware clock results in less duplication of electronics and better access coordination between the CPU and the refresh circuits.

In SRAM, the circuit for a bit consists of multiple transistors that hold the stored value without the need for refresh. The chief advantage of SRAM lies in its speed. A computer can access data in SRAM more quickly than it can access data in DRAM or SDRAM. However, the SRAM circuitry draws more power and generates more heat than DRAM or SDRAM. The circuitry for a SRAM bit is also larger, which means that a SRAM memory chip holds fewer bits than a DRAM chip of the same size. Therefore, SRAM is used when access speed is more important than large memory capacity or low power consumption.







The time it takes the CPU to transfer data to or from memory is particularly important because it determines the overall performance of the computer. The time required to read or write one bit is known as the memory access time. Current DRAM and SDRAM access times are between 30 and 80 nanoseconds (billionths of a second). SRAM access times are typically four times faster than DRAM.

The internal RAM on a computer is divided into locations, each of which has a unique numerical address associated with it. In some computers a memory address refers directly to a single byte in memory, while in others, an address specifies a group of four bytes called a word. Computers also exist in which a word consists of two or eight bytes, or in which a byte consists of six or ten bits.

When a computer performs an arithmetic operation, such as addition or multiplication, the numbers used in the operation can be found in memory. The instruction code that tells the computer which operation to perform also specifies which memory address or addresses to access. An address is sent from the CPU to the main memory (RAM) over a set of wires called an address bus. Control circuits in the memory use the address to select the bits at the specified location in RAM and send a copy of the data back to the CPU over another set of wires called a data bus.

Inside the CPU, the data passes through circuits called the data path to theircuits that perform the arithmetic operation. The exact details depend on the model of the CPU. For example, some CPUs use an intermediate step in which the data is first loaded into a high-speed memory device within the CPU called a register.

Modem, device that enables computers, facsimile machines, and other equipment to communicate with each other across telephone lines or over cable television network cables. In the strictest sense, a modem is a device that converts between analog signals, such as sound waves, and digital signals, which are used by computers. However, the term has also come to include devices that permit the transmission of entirely digital signals.
Modems transmit data at different speeds, measured by the number of bits of data,




they send per second (bps). A 28.8 Kbps modem sends data at 28,800 bits per second. A 56 Kbps modem is twice as fast, sending and receiving data at a rate of 56,000 bits per second.

Optical Scanner, a computer input device (see Input/Output Device) that uses light-sensing equipment





to scan paper or another medium, translating the pattern of light and dark (or color) into a digital signal that can be manipulated by either optical character recognition software or graphics software. A frequently encountered type of scanner is “flatbed,” meaning that the scanning device moves across or reads across a stationary document.

On a flatbed scanner such as the common office copier, such objects are placed face down on a flat piece of glass and scanned by a mechanism that passes under them. Another type of flatbed scanner uses a scanning element placed in a stationary housing above the document.


Other scanners work by pulling in sheets of paper, which are scanned as they pass over a stationary scanning mechanism, as in the common office fax machine. Some specialized scanners work with a standard video camera, translating the video signal into a digital signal for processing by computer software.A very popular type of scanner is the hand-held scanner, so called because the user holds the scanner in his or her hand and moves it over the document to be scanned. Hand-held scanners have the advantage of relatively low cost; however, they are somewhat limited by their inability to scan areas more than a few inches wide.

Keyboard, in computer science, a keypad device with buttons or keys that a user presses to enter data characters and commands into a computer. Keyboards emerged from the combination of typewriter and computer-terminal technology. They are one of the fundamental pieces of personal computer (PC) hardware, along with the central processing unit (CPU), the monitor or screen, and the mouse or other cursor device.
The most common English-language key pattern for typewriters and keyboards is called QWERTY, after the layout of the first six letters in the top row of its keys (from left to right).



In the late 1860s, American inventor and printer Christopher Sholes invented the modern form of the typewriter. Sholes created the QWERTY keyboard layout by separating commonly used letters so that typists would type slower and not jam their mechanical typewriters. Subsequent generations of typists have learned to type using QWERTY keyboards, prompting manufacturers to maintain this key orientation on typewriters.

Computer keyboards copied the QWERTY key layout and have followed the precedent set by typewriter manufacturers of keeping this convention. Modern keyboards connect with the computer CPU by cable or by infrared transmitter. When a key on the keyboard is pressed, a numeric code is sent to the keyboard’s driver software and to the computer’s operating system software. The driver translates this data into a specialized command that the computer’s CPU and application programs understand. In this way, users may enter text, commands, numbers, or other data. The term character is generally reserved for letters, numbers, and punctuation, but may also include control codes, graphical symbols, mathematical symbols, and graphic images.Almost all standard English-language keyboards have keys for each character of the American Standard Code for Information Interchange (ASCII) character set, as well as various function keys. Most computers and applications today use seven or eight data bits for each character. Other character sets include ISO Latin 1, Kanji, and Unicode. Each character is represented by a unique number understood by the computer. For example, ASCII code 65 is equal to the letter A. The function keys generate short, fixed sequences of character codes that instruct application programs running on the computer to perform certain actions. Often, keyboards also have directional buttons for moving the screen cursor, separate numeric pads for entering numeric and arithmetic data, and a switch for turning the computer on and off. Some keyboards, including most for laptop computers, also incorporate a trackball, mouse pad, or other cursor-directing device. No standard exists for positioning the function, numeric, and other buttons on a keyboard relative to the QWERTY and other typewriting keys. Thus layouts vary on keyboards.
An alternative keyboard design not yet widely used but broadly acknowledged for its speed advantages is the Dvorak keyboard. In the 1930s, American educators August Dvorak and William Dealy designed this key set so that the letters that make up most words in the English language are in the middle row of keys and are easily reachable by a typist’s fingers. Common letter combinations are also positioned so that they can be typed quickly. Most keyboards are arranged in rectangles, left to right around the QWERTY layout. Newer, innovative keyboard designs are more ergonomic in shape. These keyboards have separated banks of keys and are less likely to cause carpal tunnel syndrome, a disorder often caused by excessive typing on less ergonomic keyboards.

Dot-Matrix Printer, in computer science, any printer that produces characters made up of dots using a wire-pin print head.












The quality of output from a dot-matrix printer depends largely on the number of dots in the matrix, which might be low enough to show individual dots or might be high enough to approach the look of fully formed characters. Dot-matrix printers are often categorized by the number of pins in the print head—typically 9, 18, or 24.

Joystick, in computer science, a popular pointing device, used mostly for playing computer games but used for other tasks as well. A joystick usually has a square or rectangular plastic base to which is attached a vertical stem. Control buttons are located on the base and sometimes on top of the stem. The stem can be moved omnidirectionally to control the movement of an object on the screen.





The buttons activate various software features, generally producing on-screen events. A joystick is usually a relative pointing device, moving an object on the screen when the stem is moved from the center and stopping the movement when the stem is released. In industrial control applications, the joystick can also be an absolute pointing device, with each position of the stem mapped to a specific on-screen location.

Light Pen, a pointing device in which the user holds a wand, which is attached to the computer, up to the screen and selects items or chooses commands on the screen (the equivalent of a mouse click) either by pressing a clip on the side of the light pen or by pressing the light pen against the surface of the screen.





The wand contains light sensors and sends a signal to the computer whenever it records a light, as during close contact with the screen when the non-black pixels beneath the wand's tip are refreshed by the display's electron beam. The computer's screen is not all lit at once—the electron beam that lights pixels on the screen traces across the screen row by row, all in the space of 1/60 of a second.



By noting exactly when the light pen detected the electron beam passing its tip, the computer can determine the light pen's location on the screen. The light pen doesn't require a special screen or screen coating, as does a touch screen, but its disadvantage is that holding the pen up for an extended length of time is tiring to the user.

Digital cameras are now available for both professional photographers and amateur enthusiasts. The more expensive professional cameras function as sophisticated 35-millimeter cameras but record the picture information as pixels, or digital dots of color (see Computer Graphics).




There can be several million pixels in a high-resolution, full-color digital photograph. Some digital cameras are able to transfer their large picture files directly into a computer for storage. Others accept a disc or similar portable storage unit to achieve the same purpose. The original high-resolution image can later be reproduced in ink (in a magazine, for example) or as a conventional silver halide print.

Digital cameras aimed at the amateur photography market function much as point-and-shoot cameras do, with automatic focus, automatic exposure, and built-in electronic flash. Pictures from these cameras contain fewer pixels than those from a more expensive camera and are therefore not as sharp. After taking pictures, the user can connect the camera directly to a television set or video cassette recorder, so the whole family can look at snapshots together. Alternatively, image files can be transferred to a home computer, stored on disks, or sent to friends via electronic mail.

From 1954 to 1958 American computer scientist John Backus of International Business Machines, Inc. (IBM) developed Fortran, an acronym for Formula Translation. It became a standard programming language because it could process mathematical formulas. Fortran and its variations are still in use today, especially in physics.

It is an interface between user and the computer machine.

Operating System (OS), in computer science, the basic software that controls a computer. The operating system has three major functions: It coordinates and manipulates computer hardware, such as computer memory, printers, disks, keyboard, mouse, and monitor;

it organizes files on a variety of storage media, such as floppy disk, hard drive, compact disc, digital video disc, and tape; and it manages hardware errors and the loss of data.

When a computer is turned on it searches for instructions in its memory. These instructions tell the computer how to start up. Usually, one of the first sets of these instructions is a special program called the operating system, which is the software that makes the computer work. It prompts the user (or other machines) for input and commands, reports the results of these commands and other operations, stores and manages data, and controls the sequence of the software and hardware actions. When the user requests that a program run, the operating system loads the program in the computer’s memory and runs the program. Popular operating systems, such as Microsoft Windows and the Macintosh system (Mac OS), have graphical user interfaces (GUIs)—that use tiny pictures, or icons, to represent various files and commands. To access these files or commands, the user clicks the mouse on the icon or presses a combination of keys on the keyboard. Some operating systems allow the user to carry out these tasks via voice, touch, or other input methods.

Software, on the other hand, is the set of instructions a computer uses to manipulate data, such as a word-processing program or a video game. These programs are usually stored and transferred via the computer's hardware to and from the CPU. Software also governs how the hardware is utilized; for example, how information is retrieved from a storage device. The interaction between the input and output hardware is controlled by software called the Basic Input Output System software (BIOS).