Introducing Computer Systems

Introducing Computer Systems Computer is an electronic device that stores, retrieves, and processes data, and can be programmed with instructions. A Computer is composed of Hardware and Software, and can exist in a variety of sizes and configuration

What is Computer? The machine you think of as a “computer” is more precisely called a “general-purpose digital electronic computer.” It is general-purpose because it can be programmed to perform a wide variety of applications (making it different from a special-purpose computer designed to perform only one function). Digital means that computer handles all data internally in the form of numbers (all of the numeric data, all of the text data, and even sounds and pictures are stored as numbers). The word digit originally meant “finger” or “toe” and since people started counting on their fingers, the word digit also came to be applied to numbers. A different type of computer that represents values as voltage levels is called an analog computer, but you are unlikely to ever run into such a thing. Modern computers are all electronic because they manipulate data using electronic switching circuits (some older computing machines, or ideas for computers, were mechanical, using wheels, levers, etc. to perform calculations. A computer is a device that performs four functions: it inputs data (getting information into the machine); it stores data (holding the information before and after processing); it processes data (performing prescribed mathematical and logical operations on the information at high speed); and it outputs data (sending the results out to the user via some display method). A computer system consists of both hardware and software. The hardware is the physical equipment: the computer itself and the peripherals connected to it. The peripherals are any devices attached to the computer for purposes of input, output, and storage of data (such as a keyboard, monitor display, or external hard disk). The software consists of the programs and associated data (information) stored in the computer. A program is a set of instructions that the Computer follows to manipulate data. Being able to run different programs is the source of a computer’s versatility. Without programs, a computer is just a lot of high-tech hardware that doesn’t do anything. But with the detailed, step-by-step instructions of the program (painstakingly written by humans) the computer can be 1

used for tasks ranging from word processing a letter to Aunt Mary, to simulating global weather patterns. The computer appears to be so amazing simply because it can execute these sets of instruction very very fast; but it’s just following the program steps one by one in a very simple-minded manner. As a user, you will interact with the programs running on your computer through the input devices connected to it, such as a mouse and a keyboard. You use these devices to provide input (such as the text of a report you are working on) and also to give commands to the program (such as specifying what text is to appear with bold formatting). The computer program will provide output (the data resulting from the manipulations within the computer) via various output devices for presenting the information (such as a monitor, a printer, or a sound output system that beeps if the program needs your attention). These input and output devices are discussed in separate sections of this tutorial. Personal computers are used in a very interactive manner, with the user continuously inputting data and commands (to choose various program functions), and monitoring the output displaying the results of the commanded operations. This is very different from the way older large computers were used (where the user provided input in one operation, and received the output back later, in what is called batch processing).

Generation of Computer The history of computer development is often referred to in reference to the different generations of computing devices. Each generation of computer is characterized by a major technological development that fundamentally changed the way computers operate, resulting in increasingly smaller, cheaper, and more powerful and more efficient and reliable devices. Read about each generation and the developments that led to the current devices that we use today.

First Generation (1940-1956) Vacuum Tubes The first computers used vacuum tubes for circuitry and magnetic drums for memory, and were often enormous, taking up entire rooms. They were very expensive to operate and in addition to using a great deal of electricity, generated a lot of heat, which was often the cause of malfunctions.

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First generation computers relied on machine language, the lowest-level programming language understood by computers, to perform operations, and they could only solve one problem at a time. Input was based on punched cards and paper tape, and output was displayed on printouts. The UNIVAC and ENIAC computers are examples of first-generation computing devices. The UNIVAC was the first commercial computer delivered to a business client, the U.S. Census Bureau in 1951.

Second Generation (1956-1963) Transistors Transistors replaced vacuum tubes and ushered in the second generation of computers. The transistor was invented in 1947 but did not see widespread use in computers until the late 1950s. The transistor was far superior to the vacuum tube, allowing computers to become smaller, faster, cheaper, more energy-efficient and more reliable than their first-generation predecessors. Though the transistor still generated a great deal of heat that subjected the computer to damage, it was a vast improvement over the vacuum tube. Second-generation computers still relied on punched cards for input and printouts for output. Second-generation computers moved from cryptic binary machine language to symbolic, or assembly, languages, which allowed programmers to specify instructions in words. High-level programming languages were also being developed at this time, such as early versions of COBOL and FORTRAN. These were also the first computers that stored their instructions in their memory, which moved from a magnetic drum to magnetic core technology. The first computers of this generation were developed for the atomic energy industry.

Third Generation (1964-1971) Integrated Circuits The development of the integrated circuit was the hallmark of the third generation of computers. Transistors were miniaturized and placed on silicon chips, called semiconductors, which drastically increased the speed and efficiency of computers. Instead of punched cards and printouts, users interacted with third generation computers through keyboards and monitors and interfaced with an operating system, which allowed the device to run many different applications at one time with a central program that monitored the memory. Computers for the first time became accessible to a mass audience because they were smaller and cheaper than their predecessors.

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Fourth Generation (1971-Present) Microprocessors The microprocessor brought the fourth generation of computers, as thousands of integrated circuits were built onto a single silicon chip. What in the first generation filled an entire room could now fit in the palm of the hand. The Intel 4004 chip, developed in 1971, located all the components of the computer— from the central processing unit and memory to input/output controls—on a single chip. In 1981 IBM introduced its first computer for the home user, and in 1984 Apple introduced the Macintosh. Microprocessors also moved out of the realm of desktop computers and into many areas of life as more and more everyday products began to use microprocessors. As these small computers became more powerful, they could be linked together to form networks, which eventually led to the development of the Internet. Fourth generation computers also saw the development of GUIs, the mouse and handheld devices.

Fifth Generation (Present and Beyond) Artificial Intelligence Fifth generation computing devices, based on artificial intelligence, are still in development, though there are some applications, such as voice recognition, that are being used today. The use of parallel processing and superconductors is helping to make artificial intelligence a reality. Quantum computation and molecular and nanotechnology will radically change the face of computers in years to come. The goal of fifth-generation computing is to develop devices that respond to natural language input and are capable of learning and selforganization.

History of Computers 1) Mechanical beginnings

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Some mechanical control and computing devices preceded the development of the modern computer. In the early 1800’s, a French inventor named Joseph-Marie Jacquard produced a loom that could weave complex patterns into cloth. The loom was controlled automatically by reading instructions punched as holes in cards. An American named Herman Hollerith invented machines using punch cards that were used to tabulate statistics for the 1890 US census. Hollerith eventually sold out to a company named CTR (Computer Tabulating Recording), which later became IBM (International Business Machines. Punched cards (also marked cards) are still in limited use gathering input data for computers. You may recall the debacle in Florida during the 2000 presidential election where improperly-punched voting cards caused problems with the vote counting. More interesting from a theoretical standpoint was the work in the 1800’s by Englishman Charles Babbage. Babbage designed a mechanical computing device named the Analytical Engine in 1834. Although he was never able to build the device (he couldn't get funding to develop the precisely-machined gears, wheels, and lever systems of the machine) his ideas included many concepts that were later incorporated in modern computers.

2) The First Digital Electronic Computer

Atanasoff Berry Computer The first digital electronic computer was built by John Vincent Atanasoff and his assistant Clifford Berry at Iowa State University between 1937-1942. The 5

Atanasoff Berry Computer (ABC) used punched cards for input and output, vacuum tube electronics to process data in binary format, and rotating drums of capacitors to store data. The ABC, however, only performed one task: it was built to solve large systems of simultaneous equations (up to 29 equations with 29 unknowns), an onerous computing task commonly found in science and engineering. So, the ABC was not a general-purpose computer. Similarly, another special-purpose electronic computer named Colossus was built in England starting in 1943 for the purpose of breaking German codes. The project was worked on by Alan Turing and Max Newman. The existence of this computer was kept secret until the 1970’s.

3) ENIAC: The first General-Purpose Electronic Computer

ENIAC computer The first general-purpose digital electronic computer, one that could be programmed to perform a variety of calculation tasks, was the ENIAC (Electronic Numerical Integrator and Calculator). It was designed and built in the fall of 1945 by John Mauchly and J. Presber Eckert. ENIAC was originally built to calculate ballistic tables for the US military to aim their big guns. ENIAC was a monster of a machine, filling a large room and weighing 30 tons. It included 18,000 vacuum tubes and used 200 kilowatts of electrical power (the lights dimmed in its Philadelphia neighborhood when it was first turned on). ENIAC was the first general-purpose computer because it could be programmed (given different sets of instructions to follow) by the cumbersome procedure of reconnecting cables and flipping switches. Later computers were much more flexible because they incorporated the idea of stored programs, conceived in 1945 by mathematician John Von Neumann (who worked on the Manhattan Project in Los Alamos). In this scheme, both the data being manipulated and the program of instructions for the computer are stored in memory. Modern computers use this same method. Mauchly and Eckert later went on to work for the Univac division of Remington 6

Rand Corporation. The Univac I (Universal Automatic Computer) was the first commercial computer, coming out in 1951.Most of these early mainframes were purchased (or rented) by government bureaus, the military, research labs (such as Los Alamos National Lab), large corporations, and universities. IBM (International Business Machines) entered the computer market in 1953 with its 701 computer. By 1960, IBM was the dominant force in the market of large mainframe computers. Smaller players in the mainframe market included Burroughs, Control Data, General Electric, Honeywell, NCR, RCA, and Univac.

4) Transistors and Integrated Circuits

Integrated Circuit Vacuum tubes consume lots of electrical power and are prone to burning out, which caused problems for early computers that used thousands of them. By 1960, the transistor replaced the vacuum tube as the electrical switching device in computers. The transistor (developed at Bell Labs by William Shockley and others in the 1950’s) is a solid-state semiconductor device typically made of silicon or germanium. It is much smaller, much more reliable, and consumes much less energy than a vacuum tube. A vacuum tube computer that previous filled a sizable portion of a room could be replaced by a transistorized computer system that filled a few cabinets. A good example of an early computer using transistors is the IBM 360, which dominated the mainframe computer market in the mid to late 1960’s. The early 1960’s also saw the development of the microchip, or integrated circuit (IC), invented by Jack Kirby and Robert Noyce. An integrated circuit incorporates many transistors and other electrical components, all formed into a miniature circuit onto a single chip of silicon. 7

The invention of the integrated circuit allowed computers to become even smaller, with the whole central processing unit (CPU) of the computer fitting onto one circuit board. These minicomputers were cheaper and smaller than a mainframe (the computer was roughly the size of a drawer in a large filing cabinet). A minicomputer might cost $100,000 instead of the $1,000,000 a mainframe cost, allowing many more businesses and universities to afford their own computer systems. The most successful minicomputers were the PDP and Vax series made by Digital Equipment Corporation (DEC). Minicomputers were multiuser systems, in much the same way as mainframe computers, but on a smaller scale.Minicomputers are now a mainly obsolete class of computer, having been largely replaced by high-end microprocessor workstations.

Microprocessors As IC technology progressed, chip manufacturers could fit more and more circuitry onto the tiny silicon chips. By 1971, a company named Intel developed the first microprocessor (also called an MPU) that fit a whole CPU onto one microchip. The Intel 4004 processor contained 2300 transistors on a chip of silicon 1/8" x 1/16" in size. By 1974, Intel introduced their 8080 chip, a general purpose microprocessor offering ten times the performance of the earlier MPU. It was not too long before electronics hobbyists began building small computer systems based on the rapidly improving microprocessor chips.

The First Microcomputers The first commercially available microcomputer of note was the Altair 8800 computer sold by MITS (Micro Instrumentation & Telemetry Systems), a company founded by Dr. Ed Roberts that was based in Albuquerque, New Mexico. The computer was featured on the cover of the January 1975 issue of Popular Electronics, and was sold as a kit for $397 or assembled for $439. It used a 2 MHz Intel 8080 processor and had 256 bytes of RAM. Remember that a computer can’t do anything without software, and some companies sprang into existence to fill this need. One small company of note was formed in Albuquerque by a Harvard dropout to provide software (a BASIC language) for the Altair computer. The founder’s name was Bill Gates, and the company he forms (along with his partner Paul Allen) was Microsoft. Dozens of companies (most of which have long since vanished) began offering microcomputers for sale, most of them based on the Intel 8080 processor and running the CP/M operating system. Other companies had proprietary operating systems, such as Radio Shack, Atari, and Commodore. Of 8

particular note is a company named Apple founded by Steve Jobs and Steve Wozniak on April 1, 1976. Their Apple II computer was a hit, especially in the home and education markets.Two things caused the microcomputer market to really take off in the late 1970’s and early 1980’s: spreadsheet software, and the IBM PC. Spreadsheet software (the first was VisiCalc for the Apple II, written by Dan Bricklin) finally convinced business people that there was a serious use for microcomputers. The IBM PC, released by in 1981, gave a legitimacy to the microcomputer by virtue of the IBM name (remember, IBM was the maker of big mainframe computers; and it was said “Nobody ever gets fired for buying IBM”). It used a 4.77 MHz Intel 8088 processor. Microsoft went to IBM about an operating system (OS) for their new PC. Bill Gates told them, “Wwe have an OS that will run on this new machine you are planning,” and made a deal. Microsoft did not, in fact, have such an OS, but they quickly bought one from a third party and converted it into PCDOS. But what Gates did that was really clever was to make a deal with IBM that allowed Microsoft to also sell the OS to other companies as MS-DOS...and Microsoft’s future was set.IBM PC sales skyrocketed and IBM dominated the market within two years, releasing the PC XT (1983) and PC AT (1984) using the Intel 80286 processor. But, almost as quickly, IBM lost it dominance in the PC marketplace when other companies (such as Compaq) began to release “PC compatible” computers (also called “PC clones”). By 1986 the clones owned most of the market, and IBM never regained its dominance. Microsoft, on the other hand, supplied their operating systems to all PCs, becoming a huge corporation.

Graphical User Interface Computers were traditionally very difficult to use, requiring the user to memorize and type in the necessary commands (this is called a Command Line Interface). To make computers more accessible, the Graphical User Interface (GUI) was developed. In a GUI, the user interacts with a graphical display on the screen containing icons and windows and controls. Commands are chosen from menus rather than typed in. The GUI was developed at the Xerox Palo Alto Research Center, but the management at Xerox failed to see the usefulness of it. When Steve Jobs of Apple saw the GUI, however, he recognized its value. Apple licensed the concepts from Xerox, developed them further, and released the first successful GUI computer, the Macintosh, in 1984. Macintosh computers used the Motorola 68000 series of microprocessors (and later the PowerPC series of microprocessors.Microsoft was also quick to realize the worth of a GUI, but its 9

graphical user interface, Windows, was slow in displacing DOS on PCs (the first versions of Windows left much to be desired.More details about computer hardware and software can be f0und in other parts of this tutorial.

Types of Computers Computers come in a variety of types designed for different purposes, with different capabilities and costs.

Microcomputers A microcomputer is a computer that has a microprocessor chip as its CPU. They are often called personal computers because they are designed to be used by one person at a time. Personal computers are typically used at home, at school, or at a business. Popular uses for microcomputers include word processing, surfing the web, sending and receiving e-mail, spreadsheet calculations, database management, editing photographs, creating graphics, and playing music or games. Personal computers come in two major varieties, desktop computers and laptop computers:

Desktop personal computer Desktop computers are larger and not meant to be portable. They usually sit in one place on a desk or table and are plugged into a wall outlet for power. The case of the computer holds the motherboard, drives, power supply, and expansion cards. This case may lay flat on the desk, or it may be a tower that stands vertically (on the desk or under it). The computer usually has a separate monitor (either a CRT or LCD) although some designs have a display built into the case. A separate keyboard and mouse allow the user to input data and commands.

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Laptop personal computer Laptop or notebook computers are small and lightweight enough to be carried around with the user. They run on battery power, but can also be plugged into a wall outlet. They typically have a built-in LCD display that folds down to protect the display when the computer is carried around. They also feature a built-in keyboard and some kind of built-in pointing device (such as a touch pad). While some laptops are less powerful than typical desktop machines, this is not true in all cases. Laptops, however, cost more than desktop units of equivalent processing power because the smaller components needed to build laptops are more expensive.

PDAs and Palmtop Computers

Personal Digital Assistant A Personal Digital Assistant (PDA) is a handheld microcomputer that trades off power for small size and greater portability. They typically use a touch-sensitive LCD screen for both output and input (the user draws characters and presses icons on the screen with a stylus). PDAs communicate with desktop computers and with each other either by cable connection, infrared (IR) beam, or radio waves. PDAs are normally used to keep track of appointment calendars, to-do lists, address books, and for taking notes

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Palmtop computer . A palmtop or handheld PC is a very small microcomputer that also sacrifices power for small size and portability. These devices typically look more like a tiny laptop than a PDA, with a flip-up screen and small keyboard. They may use Windows CE or similar operating system for handheld devices.Some PDAs and palmtops contain wireless networking or cell phone devices so that users can check e-mail or surf the web on the move.

Workstations/Servers

Workstation computer A workstation is a powerful, high-end microcomputer. They contain one or more microprocessor CPUs. They may be used by a single-user for applications requiring more power than a typical PC (rendering complex graphics, or performing intensive scientific calculations). Alternately, workstation-class microcomputers may be used as server computers that supply files to client computers over a network. This class of powerful microcomputers can also be used to handle the processing for many users simultaneously who are connected via terminals; in this respect, high-end workstations have essentially supplanted the role of minicomputers (see below). Note! The term “workstation” also has an alternate meaning: In networking, any client computer connected to the network that accesses server resources may be

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called a workstation. Such a network client workstation could be a personal computer or even a “workstation” as defined at the top of this section. Note: Dumb terminals are not considered to be network workstations (client workstations on the network are capable of running programs independently of the server, but a terminal is not capable of independent processing.There are classes of computers that are not microcomputers. These include supercomputers, mainframes, and minicomputers.

Minicomputers A minicomputer is a multi-user computer that is less powerful than a mainframe. This class of computers became available in the 1960’s when large scale integrated circuits made it possible to build a computer much cheaper than the then existing mainframes (minicomputers cost around $100,000 instead of the $1,000,000 cost of a mainframe).The niche previously filled by the minicomputer has been largely taken over by high-end microcomputer workstations serving multiple users (see above).

Mainframes

Mainframe computer (this IBM z-series computer is about 6 feet tall)

A mainframe computer is a large, powerful computer that handles the processing for many users simultaneously (up to several hundred users). The name mainframe originated after minicomputers appeared in the 1960’s to distinguish the larger systems from the smaller minicomputers. Users connect to the mainframe using terminals and submit their tasks for processing by the 13

mainframe. A terminal is a device that has a screen and keyboard for input and output, but it does not do its own processing (they are also called dumb terminals since they can’t process data on their own). The processing power of the mainframe is time-shared between all of the users. (Note that a personal computer may be used to “emulate” a dumb terminal to connect to a mainframe or minicomputer; you run a program on the PC that pretends to be a dumb terminal). Mainframes typically cost several hundred thousand dollars. They are used in situations where a company wants the processing power and information storage in a centralized location. Mainframes are also now being used as high-capacity server computers for networks with many client workstations.

Supercomputers

Supercomputer (this one is a Cray-2 from the1980’s) A supercomputer is mainframe computer that has been optimized for speed and processing power. The most famous series of supercomputers were designed by the company founded and named after Seymour Cray. The Cray-1 was built in the 1976 and installed at Los Alamos National Laboratory. Supercomputers are used for extremely calculation-intensive tasks such simulating nuclear bomb detonations, aerodynamic flows, and global weather patterns. A supercomputer typically costs several million dollars.Recently, some supercomputers have been constructed by connecting together large numbers of individual processing units (in some cases, these processing units are standard microcomputer hardware).

Microprocessors Everywhere Computers are, in fact, all around you. Microprocessor chips are found in many electronic devices (in your iPod, in your DVD player, in your

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microwave, in your car, in your phone). These are special-purpose computers that run programs to control the equipment and optimize its performance.

What is a Microcomputer? A microcomputer is a digital electronic computer designed for use by a single person. These were the first computers to have their CPUs on a single micro chip (hence the name). They are also called personal computers because of their intended use for typical personal activities such as writing letters, browsing the web, playing games, balancing a checkbook, etc. Note that the term PC comes from “Personal Computer,” but the term “PC” is commonly used to refer specifically to microcomputers that use a system architecture descended from the IBM Personal Computer. Therefore, even though an Apple Macintosh microcomputer is also a personal computer, most people would not refer to it as a “PC.”You may have noticed that I used several terms in the first paragraph to define “microcomputer” which only beg additional questions such as “What is a digital electronic computer?” and “What is a CPU?” Read on, and we will define these terms later in this tutorial!

Input and Output Devices Before a computer can process your data, you need some method to input the data into the machine. The device you use will depend on what form this data takes (be it text, sound, artwork, etc.). Similarly, after the computer has processed your data, you often need to produce output of the results. This output could be a display on the computer screen, hardcopy on printed pages, or even the audio playback of music you composed on the computer. The terms “input” and “output” are used both as verbs to describe the process of entering or displaying the data, and as nouns referring to the data itself entered into or displayed by the computer.Below we discuss the variety of peripheral devices used for computer input and output.

Input Devices

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Keyboard The computer keyboard is used to enter text information into the computer, as when you type the contents of a report. The keyboard can also be used to type commands directing the computer to perform certain actions. Commands are typically chosen from an on-screen menu using a mouse, but there are often keyboard shortcuts for giving these same commands. In addition to the keys of the main keyboard (used for typing text), keyboards usually also have a numeric keypad (for entering numerical data efficiently), a bank of editing keys (used in text editing operations), and a row of function keys along the top (to easily invoke certain program functions). Laptop computers, which don’t have room for large keyboards, often include an “fn” key so that other keys can perform double duty (such as having a numeric keypad function embedded within the main keyboard keys. Improper use or positioning of a keyboard can lead to repetitive-stress injuries. Some ergonomic keyboards are designed with angled arrangements of keys and with built-in wrist rests that can minimize your risk of RSIs. Most keyboards attach to the PC via a PS/2 connector or USB port (newer). Older Macintosh computers used an ABD connector, but for several years now all Mac keyboards have connected using USB.

Pointing Devices The graphical user interfaces (GUIs) in use today require some kind of device for positioning the on-screen cursor. Typical pointing devices are: mouse, trackball, touch pad, track point, graphics tablet, joystick, and touch screen.Pointing devices, such as a mouse, connected to the PC via a serial ports (old), PS/2 mouse port (newer), or USB port (newest). Older Macs used ADB to connect their mice, but all recent Macs use USB (usually to a USB port right on the USB keyboard).

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The mouse pointing device sits on your work surface and is moved with your hand. In older mice, a ball in the bottom of the mouse rolls on the surface as you move the mouse and internal rollers sense the ball movement and transmit the information to the computer via the cord of the mouse. The newer optical mouse does not use a rolling ball, but instead uses a light and a small optical sensor to detect the motion of the mouse by tracking a tiny image of the desk surface. Optical mice avoid the problem of a dirty mouse ball, which causes regular mice to roll unsmoothly if the mouse ball and internal rollers are not cleaned frequently. A cordless or wireless mouse communicates with the computer via radio waves (often using Bluetooth hardware and protocol) so that a cord is not needed (but such mice need internal batteries). A mouse also includes one or more buttons (and possibly a scroll wheel) to allow users to interact with the GUI. The traditional PC mouse has two buttons, while the traditional Macintosh mouse has one button. On either type of computer you can also use mice with three or more buttons and a small scroll wheel (which can also usually be clicked like a button).

Touch pad

Wireless Macintosh mouse

Two-button mouse with scroll wheel

Most laptop computers today have a touch pad pointing device. You move the onscreen cursor by sliding your finger along the surface of the touch pad. The buttons are located below the pad, but most touch pads allow you to perform “mouse clicks” by tapping on the pad itself. Touch pads have the advantage over mice that they take up much less room to use. They have the advantage over trackballs 17

(which were used on early laptops) that there are no moving parts to get dirty and result in jumpy cursor control.

Track point

Touch pad of a PC laptop Some sub-notebook computers (such as the IBM ThinkPad), which lack room for even a touch pad, incorporate a track point, a small rubber projection embedded between the keys of the keyboard. The track point acts like a little joystick that can be used to control the position of the on-screen cursor.

Trackball

Track point The trackball is sort of like an upside-down mouse, with the ball located on top. You use your fingers to roll the trackball, and internal rollers (similar to what’s inside a mouse) sense the motion which is transmitted to the computer. Trackballs have the advantage over mice in that the body of the trackball remains stationary on your desk, so you don’t need as much room to use the trackball. Early laptop computers often used trackballs (before superior touch pads came along). Trackballs have traditionally had the same problem as mice: dirty rollers can make their cursor control jumpy and unsmooth. But there are modern optical trackballs that don’t have this problem because their designs eliminate the rollers.

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Joysticks

Trackball Joysticks and other game controllers can also be connected to a computer as pointing devices. They are generally used for playing games, and not for controlling the on-screen cursor in productivity software.

Touch screen Some computers, especially small hand-held PDAs, have touch sensitive display screens. The user can make choices and press button images on the screen. You often use a stylus, which you hold like a pen, to “write” on the surface of a small touch screen.

Graphics tablet A graphics tablet consists of an electronic writing area and a special “pen” that works with it. Graphics tablets allow artists to create graphical images with motions and actions similar to using more traditional drawing tools. The pen of the graphics tablet is pressure sensitive, so pressing harder or softer can result in brush strokes of different width (in an appropriate graphics program).

Scanners A scanner is a device that images a printed page or graphic by digitizing it, producing an image made of tiny pixels of different brightness and color values which are represented numerically and sent to the computer. Scanners scan graphics, but they can also scan pages of text which are then run through OCR (Optical Character Recognition) software that identifies the individual letter shapes and creates a text file of the page's contents

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Microphone A microphone can be attached to a computer to record sound (usually through a sound card input or circuitry built into the motherboard). The sound is digitized—turned into numbers that represent the original analog sound waves—and stored in the computer to later processing and playback

MIDI Devices MIDI (Musical Instrument Digital Interface) is a system designed to transmit information between electronic musical instruments. A MIDI musical keyboard can be attached to a computer and allow a performer to play music that is captured by the computer system as a sequence of notes with the associated timing (instead of recording digitized sound waves).

Graphics tablet.

Output Devices CRT Monitor The traditional output device of a person computer has been the CRT (Cathode Ray Tube) monitor. Just like a television set (an older one, anyway) the CRT monitor contains a large cathode ray tube that uses an electron beam of varying strength to “paint” a picture onto the color phosphorescent dots on the inside of the screen. CRT monitors are heavy and use more electrical power than flat panel displays, but they are preferred by some graphic artists for their accurate color rendition, and preferred by some gamers for faster response to rapidly changing graphics.

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Monitor screen size is measured diagonally across the screen, in inches. Not all of the screen area may be usable for image display, so the viewable area is also specified. The resolution of the monitor is the maximum number of pixels it can display horizontally and vertically (such as 800 x 600, or 1024 x 768, or 1600 x 1200). Most monitors can display several resolutions below its maximum setting. Pixels (short for picture elements) are the small dots that make of the image displayed on the screen. The spacing of the screen’s tiny phosphor dots is called the dot pitch (DP), typically .28 or .26 (measured in millimeters). A screen with a smaller dot pitch produces sharper images. Your computer must produce a video signal that a monitor can display. This may be handled by circuitry on the motherboard, but is usually handled by a video card in one of the computer’s expansion slots; often the slot is a special one dedicated to video use, such as an AGP slot (Accelerated Graphics Port). Video cards are also called video display adapters, and graphics cards. Many video cards contain separate processors and dedicated video memory for generating complex graphics quickly without burdening the CPU. These accelerated graphics cards are loved by gamers.

Flat Panel Monitor

CRT monitor

A flat panel display usually uses an LCD (Liquid Crystal Display) screen to display output from the computer. The LCD consists of several thin layers that polarize the light passing through them. The polarization of one layer, containing long thin molecules called liquid crystals, can be controlled electronically at each pixel, blocking varying amounts of the light to make a pixel lighter or darker. Other types of flat panel technology exist (such as plasma displays) but LCDs are most commonly used in computers, especially laptops. 21

Older LCDs had slow response times and low contrast, but active matrix LCD screens have a transparent thin film transistor (TFT) controlling each pixel, so response, contrast, and viewing angle are much improved. Flat panel displays are much lighter and less bulky than CRT monitors, and they consume much less power. They have been more expensive than CRTs in the past, but the price gap is narrowing. You will see many more flat panels in the future.As with CRTs, the display size of a flat panel is expressed in inches, and the resolution is the number of pixels horizontally and vertically on the display.

Ink Jet Printer

Flat panel display (LCD) For hardcopy (printed) output, you need some kind of printer attached to your computer (or available over a network). The most common type of printer for home systems is the color ink jet printer. These printers form the image on the page by spraying tiny droplets of ink from the print head. The printer needs several colors of ink (cyan, yellow, magenta, and black) to make color images. Some photo-quality ink jet printers have more colors of ink. Ink jet printers are inexpensive, but the cost of consumables (ink cartridges and special paper) makes them costly to operate in the long run for many purposes.

Laser Printer

Inkjet Printer

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A laser printer produces good quality images by the same technology that photocopiers use. A drum coated with photosensitive material is charged, and then an image is written onto it by a laser (or LEDs) which makes those areas lose the charge. The drum then rolls through toner (tiny plastic particles of pigment) that is attracted to the charged areas of the drum. The toner is then deposited onto the paper, and then fused into the paper with heat. Most laser printers are monochrome (one color only, usually black), but more expensive laser printers with multiple color toner cartridges can produce color output.Laser printers are faster than ink jet printers. Their speed is rated in pages per minute (ppm). Laser printers are more expensive than ink jets, but they are cheaper to run in the long term if you just need good quality black & white pages.

Other Printers

Laser Printer Multi-function printers are available that not only operate as a computer printer, but also include the hardware needed to be a scanner, photocopier, and FAX machine as wellDot matrix printers use small electromagnetically activated pins in the print head, and an inked ribbon, to produce images by impact. These printers are slow and noisy, and are not commonly used for personal computers anymore (but they can print multi-layer forms, which neither ink jet nor laser printers can).

Sound Output Computers also produce sound output, ranging from simple beeps alerting the user, to impressive game sound effects, to concert quality music. The circuitry to produce sound may be included on the motherboard, but high quality audio output from a PC usually requires a sound card in one of the expansion slots, connected to a set of good quality external speakers or headphones.

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Multimedia is a term describing computer output that includes sound, text, graphics, movies, and animation. A sound card is an example of a multimedia output device (as is a monitor that can display graphics).

Binary Representation Your personal computer is a type of digital electronic computer. It is called digital because all of the information inside it is represented and manipulated as numbers (the original meaning of “digit” is “finger,” and since people often count using their fingers, the term digit also came to to be applied to numbers). All of the numbers in a spreadsheet, all of the text characters in a Word document, all of the pictures and sounds stored in a computer, are ALL represented as numbers. The number system that you use is base 10 (since people have 10 fingers, this works out well for them). When you write the number 1853, for example it means:

Each digit (0-9) within a base 10 number is multiplied by the power of ten corresponding to its position. Notice that each digit place has 10 times the value of the digit place to the right of it. But you knew all this, of course.

Binary Numbers But what of the poor computer, which has no fingers to count on? Base 10 is not convenient for a fingerless computer to use. What computers DO have, are electrical circuits, which are either on or off. Just two states to work with. So the natural number system for use in an electronic computer is base 2 (called the binary number system). Unlike you who have ten digits to calculate with (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the computer has only two digits (0 and 1) with which it must do everything. So, in a computer’s memory, a tiny transistor that is 24

on (conducting a current) might represent a 1, while a transistor that is off would represent a 0 (zero). The binary number 11100111101, for example, means:

Ah! So they are the same number! 1853 (base 10) = 11100111101 (base 2) Notice that each binary digit position in the base 2 number has 2 times the value of the binary digit position to the right of it (since this is base 2; remember how base 10 worked). It gets cumbersome saying “binary digit” all the time, so the shorter term “bit” was invented. A bit is one binary digit. A bit can hold either a 1 or a 0 (zero). A string of bits can hold larger numbers (just as you use strings of base 10 digits to represent numbers larger than 9).

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Binary Representation of Numbers Base 10

Base 2

0

00000000

1

00000001

2

00000010

3

00000011

4

00000100

5

00000101

...

...

65

01000001

66

01000010

67

01000011

...

...

254

11111110

255

11111111

A particularly handy size chunk of computer memory happens to be 8 bits long. This size chunk of memory can be used to represent any number from zero (00000000) to 255 (11111111). Why does 11111111 (base 2) equal 255 (base 10)? Because it means: 1 x 128 + 1 x 64 + 1 x 32 + 1 x 16 + 1 x 8 + 1 x 4 + 1 x 2 + 1 x 1 = 255 And why is this handy size chunk of memory? Because if we want to represent all of the characters of the English alphabet, 8 digits is the first power of 2 that gives you enough possibilities to do this (a 4-bit long chunk can only hold numbers from zero to 7...not enough). We have a special name for a chunk of memory that is 8 bits long: it is called a byte. This is the basic unit we use to measure computer memory size. (A chunk of

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memory 4 bits long is called a “nibble,” but you don't need to know that for the test.) Text characters are represented in computer memory as numbers. How? You need a scheme for equating letters to numbers. The system used is called the ASCII code (American Standard Code for Information Interchange). The capital letter A is represented by the number 65 in the ASCII code (65 is 01000001 in binary). The first 65 ASCII codes (0 through 64) are used for an assortment of Control characters and special characters, so capital A ended up at 65. Capital B is 66 (01000010) and so on. ASCII Representation of Characters (just a sample) Character

Base 10

Base 2

(return)

13

00001101

(space)

32

00100000

!

33

00100001

1

49

00110001

2

50

00110010

@

64

01000000

A

65

01000001

B

66

01000010

C

67

01000011

a

97

01100001

b

98

01100010

c

99

01100011

(delete)

127

01111111

How does the computer know whether the 01000001 in a byte of memory is the number 65 or the letter A? Because an application program keeps track of what it put where in memory, so MS Word knows that a given byte where it has stored text contains numbers that represent letters.For foreign alphabets that contain many more letters than English (such as Japanese Kanji) a newer extension of the the ASCII scheme called Unicode is now used (it uses two bytes to hold each letter; two bytes give 65,535 different values to represent characters).

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Pictures are also represented as numbers in the computer. If you look closely at your display screen, you can see that the image on it is made up of lots of little spots, called picture elements (which are more commonly shortened to pixel). Each pixel in a screen image might be represented by three bytes in the computer; the numbers in the bytes tell the display how much red, blue, and green light should be mixed together to make the color of the pixel (three bytes can represent millions of possible colors for each pixel). The programs that a computer executes are also stored as numbers. Each number in this case represents an instruction for the microprocessor (each operation the processor can perform, such as “fetch a number into a register” and “add the contents of two resisters together,” are represented by unique binary codes).

Kilobytes, Megabytes, Gigabytes, etc. Memory capacity and data storage capacity for computers are measured in bytes. File sizes are also measure in bytes (one byte is 8 bits, remember). However, a byte is small (it can hold only one character) so we use larger units:A kilobyte (KB) is approximately 1,000 bytes. But it is NOT exactly 1,000 bytes; it is 1,024 bytes. Why a strange number like 1,024? Because 1,024 is exactly 10000000000 in binary; a nice multiple of two is very handy for the computer. So remember: When the computer tells you that your file takes up 40 kilobytes, it is actually using 40,960 bytes (not 40,000). But you can think of a kilobyte as “roughly 1,000 bytes,” which is how it got its name. This web page file is approximately 20 KB in size. Similarly, you can think of a megabyte (MB) as approximately a million bytes, but it is precisely 1,048,576 bytes (1,024 x 1,024). The MS Word application takes up about 13 MB on the computer’s hard disk (depending on version). A typical personal computer may have 512 MB of memory.A gigabyte (GB) is approximately one billion bytes (1,073,741,824 exactly). The root word for the giga is the same one our word giant comes from, so gigabyte should technically be pronounced with a soft g—but the pronunciation with either hard or soft g is acceptable. The storage capacity of a typical hard disk is measures in the tens or hundreds of GB. In case you are wondering, a trillion bytes is a terabyte, but PC capacities haven’t gotten into that range yet.

Hardware The term hardware refers to the physical components of the computer system (as opposed to the software). Your computer hardware will consist of the devices 28

within the case of the computer itself, and any peripheral devices that are connected to the computer (such as the mouse and keyboard). The primary component of the computer is the motherboard (also called the main circuit board, main logic board, main board, or system board). The motherboard is a large printed circuit board with microchips, connectors, and other components mounted on it, and with copper circuitry traces that connect the components together.

A motherboard typically holds the following items: • • • • • • •

CPU (Central Processing Unit) where the actual processing of data takes place. System clock circuitry (that keeps all of the digital chips in lockstep) Other controller chips that act as traffic cops directing data flow along the system busses (the circuitry connecting the chips to the CPU) and I/O ports. RAM (the main memory, plus additional slots for adding more memory) ROM (containing the BIOS) “CMOS memory” Expansion slots (for adding expansion cards such as Video cards and Sound cards)

Additional information about the parts listed above can be found in the other sections of this tutorial. Along with the motherboard, the case of your computer typically contains a power supply (to convert the AC line current from the wall outlet to the low-voltage DC current used by the computer) and several storage devices located in the expansion bays of the case (such as: hard drives, floppy drives, Zip drives, and CD drives, and DVD drives). The main circuit board of a microcomputer. The motherboard contains the connectors for attaching additional boards. Typically, the motherboard contains the CPU, BIOS, memory, mass storage interfaces, serial and parallel ports, expansion slots, and all the controllers required to control

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standard peripheral devices, such as the display screen, keyboard, and disk drive. Collectively, all these chips that reside on the motherboard are known as the motherboard's chipset. On most PCs, it is possible to add memory chips directly to the motherboard. You may also be able to upgrade to a faster PC by replacing the CPU chip. To add additional core features, you may need to replace the motherboard entirely. Motherboard is sometimes abbreviated as mobo.

Configurations and Specifications When you go to purchase a computer (either online or at a compute store) you will have several system configurations to choose from. Each configuration includes a particular set of parts or components (both hardware and software) in a specific arrangement. A similar term, architecture, also describes the layout and interactions of the components of a computer system. Each system configuration will have a specification that lists the details about the components included in that particular system. Below you will see a typical computer system specification; look it over carefully. By the time you finish with all the parts of this tutorial, you should be able to explain all of the terminology in this specification:

Typical Specification for a Desktop Computer System

1. 2. 3. 4. 5. 6. 7.

Processor: Intel® Pentium® 4 processor, 3 GHz, with 800 MHz front side bus, 512KB Level 2 Cache. Microsoft® Windows® XP Professional. Memory: 512MB DDR SDRAM at 400 MHz (expandable to 4 GB). Hard drive: 250 GB Serial ATA, 7 ms seek time, 7200 RPM, 512KB cache. Floppy drive: 3.5" 1.44 MB. Optical drive: 12x DVD-ROM / 48x CD-RW combo drive. Expansion slots: 1 AGP and 5 PCI.

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External ports: Six USB 2.0 (two on front panel), one Parallel, one Serial, two PS/2, and one IEEE 1394. 9. Modem: 56K PCI FAX/modem. 10. Video card: 256MB RADEON™ 9800 AGP graphics card. 11. Monitor: 17" CRT (16" viewable), 1,024 x 768, .27 dp. 12. Sound card: Sound Blaster® Audigy™2 card w/Dolby 5.1 stereo. 13. Speakers: Bose® B775 surrounds sound speaker system with subwoofer. 14. Networking: Ethernet 10/100 15.Keyboard: 101-key multi-function keyboard 16. Mouse: Logitech® MX™ 500 optical mouse with scroll wheel. 17. Case: Tower case with 6 expansion bays (two for internal-only drives). 18. Application Software: Microsoft® Office Professional 2003. 8.

When! What does all that mean? Read the other parts of this tutorial (processors, memory, input/output, storage, ports, and net/telecom) to find out. (The terms in italics in the specifications list above are brand names that you don’t need to know).

Components of a PC System Processors

Microprocessor chip close-up The microprocessor is the component of the personal computer that does the actual processing of data. A microprocessor is a central processing unit (CPU) that fits on one microchip. It is the “brain” of the computer, but that is a rather pretentious terms since it it really just a very complex switching circuit that executes simple instructions very rapidly. The microprocessor integrated circuit package holds a silicon chip that contains millions of transistors and other components fabricated into the silicon. Because the transistors on the chip are very tiny, even a small zap of high voltage current (such as from static electricity) can destroy a chip. This is why all large-scale integrated circuits must be handled in ways that minimize the 31

possibility of static electric discharge.Because of the large amount of circuitry packed into such a tiny area, microchips produce a lot of heat and they require cooling systems to keep the chip from overheating. On computer motherboards the CPU chip is covered by a large metal heat sink with “fins” to allow airflow from cooling fans to carry the heat away.

Clock Speed The digital chips on a motherboard are keep in sync with each other by the clock signal (a stream of pulses) of the motherboard. You can think of it like a “heartbeat” of the computer. The faster the clock pulses, the faster the computer runs; but, the clock can’t run faster than the speed rating of the chips, or they will “glitch” and drop data. As chip technology has improved, the speed that chips can run at has gotten faster. The CPU runs faster than the rest of the motherboard (which is clocked at a fraction of the rate of the CPU). Clock speed is measured in units of cycles per second, which is called a Hertz (Hz). Computer boards and CPUs run at rates of millions and billions of Hertz, megahertz (MHz) and gigahertz (GHz). A good speed for a PC microprocessor in 2004 was 4 GHz. You will want a fast processor, of course, but so does everyone else—and only a fraction of the chips produced in a batch are the fastest (they are all tested and rated after fabrication)—so faster CPUs cost more. You may recognize the terms megahertz and gigahertz from radio broadcasts. FM radio and TV broadcast in the MHz range, and some mobile phones broadcast in the GHz range. So, computer circuitry produces radio frequency interference that could cause problems for nearby devices. To prevent this, a computer contains thin metal shielding inside its case (if the case is not metal).

CPU Speed

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IBM PowerPC processor (G5) showing top and bottom. Clock speed is only one aspect that contributes to the overall processing speed of a microprocessor. The architecture of the chip also is a factor. This includes such considerations as the word size of the chip, which is how many bits it can input/output and process at a time. Early microprocessors used 8-bit word size; the newest microprocessors use 64-bit word size. The design of the chip may also include high speed cache memory that the processor can use to hold recently used instructions or data in case it needs them again, so that it doesn’t need to go back to the much slower main RAM memory to get them. Computers spend a lot of time in loops, repeating the same sequence of instructions, so this can greatly improve performance. (Depending on whether the cache memory is located on the CPU chip itself, or on nearby chips with a high-speed link, it is called Level 1 or Level 2 cache). Other aspects of chip architecture that affect speed include the ability of some CPUs to work on multiple instructions at the same time. Also, some CPUs are CISC (Complex Instruction Set Computing), while others are RISC (Reduced Instruction Set Computing). RISC chips have a smaller set of simpler instructions; they need multiple instructions to perform an action that a CISC chip does with one instruction, but the RISC chip is faster overall at completing the operation. The result is that you can’t simply compare different processors by looking at their clock speed ratings. A PowerPC chip with half the clock speed of a Pentium has roughly equivalent processing speed.

Types of Microprocessors

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The most commonly used CPU in PCs are made by Intel. Since IBM chose the Intel 8088 chip for the original IBM PC, most PC clones have used one of the Intel series of CPUs: 8088 - used in IBM PC 80286 - used in IBM PC AT 80386 - used in first PC clone from Compaq 80486 - you heard phrases like “I have a 486 PC” Pentium - Intel couldn’t trademark a number, such as 80586 Pentium II - (Hexium or sexium just wouldn’t sound right) Pentium III Pentium 4 - Most desktop PCs in 2004 used the P4 chip. Another manufacturer of microprocessors for the PC is AMD (Advanced Micro Devices, Inc.). Their line of Athlon processors has been successful in taking a substantial fraction of the PC CPU market away from Intel.The Macintosh series of computers from Apple originally used the Motorola 68000 series of microprocessors. The Motorola CPUs use a different instruction set than Intel CPUs, which is why you couldn’t easily run PC software on a Mac and vice versa (but transferring data files is no problem). Apple later used the RISC PowerPC CPU (developed jointly by Apple, Motorola, and IBM). New Macs in 2004 used either PowerPC G4 chips from Motorola, or the newer PowerPC G5 from IBM. As of 2006, Apple switched to using Intel processors in their new Macs (which then made it possible to run Windows software directly on the Mac).

Data Bus The data bus is the multi-lane electrical highway of connections that link the CPU to the other chips on the motherboard, such as the RAM memory and I/O controllers. It is also called the front side bus (FSB). The word size of the data bus determines how many bits can be moved simultaneously along it. The clock speed of the other chips on the data bus (of the motherboard as a whole), is slower than the clock speed of the CPU (typically in the hundreds of MHz).

Memory The memory of a microcomputer is where programs and data are stored when they are currently in active use. We will cover different kinds of memory your microcomputer contains: •

RAM 34

• •

ROM CMOS

RAM RAM is the main memory space of your computer. The term RAM means Random Access Memory, and it comes from the early days of computers when mainframes had two types of memory: Random access, in which any bit of memory could be addressed at any moment; and Sequential memory (such as data stored on tape) where bits could only be accessed in a certain order. All of the memory in your computer is random access, so don’t worry about sequential memory. The RAM is the workspace of your computer. If your computer has more RAM, it can open more and larger programs and documents simultaneously. It’s like you having a large worktable to spread work papers out on instead of a tiny desk. The documents you are currently editing (and the programs your computer is using to let you do it) are stored in the RAM.RAM consists of banks of microchip transistors that are either on or off (representing a 1 or a zero). RAM chips need constant power to remember what is stored in them; a power interruption of even a fraction of a second (perhaps caused by nearby lightning) can cause the RAM to lose its contents. For this reason, RAM is said to be volatile (from “easily evaporated”) and this is why it is important to save your work often to a more permanent storage such as a hard disk. The specific type of RAM used by your computer could be SDRAM (synchronous dynamic RAM), or RDRAM (Rambus dynamic RAM), or DDR SDRAM (Double-data-rate SDRAM)—but don’t worry about the details. Just be sure to get the proper kind for your system when you purchase more RAM. RAM is usually installed into sockets on the motherboard as DIMMs (Dual Inline Memory Module), small circuit boards that hold the RAM chips. You can even install more RAM into your computer, but you must take precautions not to allow static electricity to damage the RAM or motherboard. The architecture of a given motherboard will limit the amount of extra RAM you can add.RAM size is measured in bytes, kilobytes, megabytes, etc., as discussed in the section on binary numbers. A typical computer in 2004 might have 512 MB of RAM installed.

Virtual Memory

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The amount of RAM limits how large and how many programs and data files you can have open at once. You normally couldn’t simultaneously open two programs that each requires 70 MB of RAM on a computer that has only 128 MB of RAM. However, modern operating systems use virtual memory to get around this roadblock. If everything won’t fit into RAM at once, the OS can automatically swap out currently unused data to the hard disk, and swap in whatever data are needed. But this comes at a price! Hard disks (mechanical devices) are much slower than RAM (electronic storage), so your computer will run much slower if using virtual memory. If you run into this, it’s best to just buy more RAM.

ROM Microcomputers also have some ROM (Read Only Memory) on the motherboard. ROM does not need power to remember its contents, so this is where a computer stores the programs that are needed to start up (boot up) the computer system. (The instructions can’t be stored in RAM, since RAM loses its contents when the computer is off; and they can’t be kept on the hard disk, since just reading data from a hard disk requires programs.When the computer is first turned on, the program stored in the ROM is feed to the processor. This initial program checks to see that everything is in order and looks for storage devices on which it can locate a copy of the operating system; it then loads the first part of the OS into RAM, then hands control over to that program to finish the boot process. The startup instructions stored on ROM in a PC are part of the BIOS (Basic Input Output System). The BIOS also contains the low-level interface code needed to access the drives, keyboard, and produce simple display output. Note that what we call “ROM” is in most cases nowadays stored on an EEPROM chip (Electrically Erasable Programmable ROM). The motherboard includes special circuitry that allows the “permanent” contents of the chip to be updated if needed, but this is rarely done (EEPROM can only be re-written a limited number of times—but that limit may be 10,000 times).

CMOS

36

The “CMOS memory” of a computer is a small amount of “semi-permanent” storage where changeable data can be stored that needs to remain available while the computer is turned off. A small battery on the motherboard keeps the CMOS ‘alive’ when power is off. The CMOS memory (called PRAM or “Parameter RAM on the Macintosh) can store such information such as what hard drive or copy of the OS you want to boot from, what are your default monitor settings, etc. The BIOS picks up this information and uses it during boot up. The CMOS memory can also hold the time and date so that your computer remembers this even when power its has been off. If your computer can’t remember the proper time, or can’t remember system settings when it’s off, the small battery may need to be replaced. Note that I used the term “CMOS memory” in quotes. CMOS (pronounced ‘SeeMoss’) is a type of transistor memory that requires very little power to store data, so this type of chip was used in the early days for storing semi-permanent data. However, almost ALL of the chips in your computer nowadays (such as the RAM) uses CMOS technology, and (irony) the “CMOS semi-permanent memory” in many modern PCs is NOT stored on CMOS chips, but may use flash memory (a kind of EEPROM). But the terms “CMOS memory” and “the CMOS” are still used to refer to the semi-permanent memory

Storage Devices Storage devices, such as disk drives, store your documents (data files) and programs (executable files) when they are not currently in use for processing. Unlike the contents of RAM, the data stored on these devices does not vanish when power is turned off. The major categories of storage devices are magnetic, solid state, and optical.

Hard Disk Drive

Hard Disk

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A hard disk drive contains disks made of metal and coated with a metal oxide that can be magnetized. A tiny electromagnetic read/write head on the end of a seek arm magnetizes tiny spots on the disk to store data. Magnetic spots magnetized in one direction represent a one; spots magnetized in the opposite direction represent a zero (OK, I simplified things a little, but you get the idea). The same electromagnetic head can later sense the magnetic fields of the spots as they pass underneath the head, allowing the data to be read back from the disk.Hard drives are rated by their storage capacity, typically tens or hundreds of gigabytes. They are also rated by how fast the disks spin (in rpm, rotations per minute), which is typically thousands of rpm. Another way to rate a hard disk is by average access time (measured in milliseconds, ms), which tells on average how long it would take the drive to retrieve any bit of data from the disk. Typical seek times are around 6 ms. The electronics that control the hard disk often incorporate some cache memory. The drive reads in several sectors of data instead of just one —that way, if the CPU happens to request those next sectors, the drive can send them immediately without having to wait for the disk to rotate back around again.The controller electronics for a hard drive may be IDE, or ATA, or SCSI, or something else. Don’t worry about this detail here, but you do need to get the right kind to go into your computer if you want to add additional drives. You can also plug additional hard drives externally into the USB or Fire wire ports of a computer, if desired.

Floppy Diskette

Floppy Diskette (1.44 MB) In a floppy diskette the disk is made of flexible Mylar plastic coated with metal oxide that can be magnetized. Floppy diskettes are 3.5" in size (older style floppy diskettes for early PCs were 5.25").A shutter protects the disk surface from dirt and fingerprints; the shutter slides out of the way when the disk is 38

inserted into the drive so that the read/write heads can reach the disk.A small plastic slider can be slid to unblock a hole in the corner of the diskette to writeprotect the disk (so data can’t be accidentally erased). High-density floppy diskettes hold 1.44 MB. The access time is much slower than for a hard disk, and they are somewhat unreliable. Many new computers don’t have a floppy drive, but you can purchase an external drive to plug in if you need to.

Zip Disk

Zip Disk (250 MB) A Zip disk is similar in size to a floppy diskette, but thicker. It is basically a “super floppy” but the higher construction tolerances and smaller read/write heads allow the Zip disk to hold more data than a floppy. The first Zip disks held 100 MB. Later Zip drives could read 250 MB Zips (in addition to the old 100 MB disks). An even newer model Zip drive uses 750 MB disks. Both Zip disk and floppy diskettes have the advantage of being removable media. Data stored on these disks can be removed and taken to other locations. Both Zips and floppies can be formatted for either the PC or the Macintosh (Macs can read both formats.We had Zip drives in our previous PCs and Macs at UNM-LA, but our newer computers don't use these, so you may never have to deal with them.

Flash Drive

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USB Flash Drive A USB Flash Drive is a portable solid state memory device that plugs into a USB port on your computer. They have many other names (such as key drive, pocket drive, thumb drive, pen drive). They have replaced floppy diskettes and Zip disks at UNM-LA as our preferred means to carry files around. They work on both Macs and PCs.These small drives store data on flash memory microchips (a kind of EEPROM). Flash memory can be erased and re-written a limited number of times (typically many thousands of times). Some units have a write-protect switch. The storage capacity varies, but anything from 16 MB to over a gigabyte is available.

Flash Memory Cards

The same kind of flash memory used in the USB flash drives above are is used in small memory cards (a Secure Data SD card and a Compact Flash card are shown on the right). These cards are are used by PDAs, digital cameras, MP3 music players, and other digital devices. You can attach a flash memory card reader to your computer to read and write data to these cards as well. These memory cards (and other types not shown here) come in a variety of storage capacities from tens of megabytes to over a gigabyte. Example Flash Memory cards (SD card left, Compact Flash card right)

CD-ROM

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CD-ROM disc A CD-ROM (Compact Disc Read Only Memory) is an optical storage medium that can hold about 670MB. “Optical” means that light is used to read the data from the disk (it is not a magnetic medium). CD-ROMs are very cheap to produce in large quantities, so most software is distributed on CD-ROMs.Data is stored on a CDROM as small pits in the plastic of an inner layer, which is then aluminized and over coated with another clear layer. A laser beam inside the CD-ROM drive is bounced off the disk and the sequences of pits and not-pits (the reflectivity is different) are converted into the ones and zeros of the data. CD-ROM drives are rated by speed, such as 32 xs, which means 32 times faster than the first CD-ROM drives.

CD-R and CD-RW CD-R (Compact Disc - Recordable) and CD-RW (Compact Disk - Rewritable) are CDs that can be written to (if your computer has a CD-RW drive).The CD-R discs have a layer of dye that is changed by a higher power laser in the drive to record data (the low power reading laser does not change the data). The CD-R can only have its data surface changed ONCE at each spot (although you can write multiple sessions to one disk until it is full). After that, it is readonly. CD-Rs can hold 700 MB of data. The CD-RW discs contain a phase-change material that different power laser beams can read, write, and erase, so these disks can be used many times (but must be erased before re-writing).

DVD DVD-ROM discs (DVD = Digital Versatile Disc) are optical storage media similar to CD-ROMs, but with a higher storage capacity. DVDs use smaller spots to record data, and the disks can be dual-layer and double-sided, with each layer holding 4.7 GB of data (so a dual-layer/double-sided DVD can hold 18 GB of data. Like CDs, DVDs also have recordable variants, although there are 41

still multiple formats (DVD-R and DVD+R) competing for dominance. A singlelayer DVD-R can hold 4.7 GB of data (Dual Layer discs can hold twice as much). DVD drive speeds are rated in terms of how many times faster that the original DVD drives they are (a 6x DVD drive is 6 times faster)

Ports and Peripherals Peripherals are devices that are attached to a computer system to enhance its capabilities. Peripherals include input devices, output devices, storage devices, and communications devices. All peripherals must have some way to access the data bus of the computer (the communications channel on the motherboard that connects the processor, RAM, and other components). To do this, peripherals are connected via some kind of port (also called an I/O port, for input/output) on the computer (and a cable with the proper connectors is needed). In some cases, the controller chips and circuitry for a port may be included on the motherboard itself (especially in laptop computers). In other cases, an expansion card in one of the expansion slots on the motherboard provides the needed port. This card is also called a controller card or an interface card. The software needed to handle the interface through the controller card is called device driver, a type of system software.A video card that allows output to be sent to a monitor is an example of an interface card. The part of the video controller card that protrudes through the case of the computer includes a port (or ports) that monitor cables can connect to. Let’s look at some of the kinds of ports used on personal computers.

Serial Port

Parallel and Serial ports on the back of a PC laptop

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A serial port transmits data one bit at a time. Typically on older PCs, a modem, mouse, or keyboard would be connected via serial ports. Serial cables are cheaper to make than parallel cables and easier to shield from interference.

Parallel Port The parallel port of older PCs could transmit 8 bits of data at a time, so it was faster than the old serial port (just as more traffic can move along a multi-lane highway than can move along a one-lane road). The parallel port was typically used to connect a printer to the computer.

USB Port

USB port on the side of a PowerBook. USB (Universal Serial Bus) is a newer type of serial connection that is much faster than the old serial ports. USB is also much smarter and more versatile since it allows the “daisy chaining” of up to 127 USB peripherals connected to one port. USB ports can support the connection of many kinds of devices (keyboard, mouse, printer, audio in/out, external floppy or Zip drives, scanner, flash drive, etc.). Newer PCs and Macs include several USB ports, some often located in handy spots on the front panel of the computer case or the side of the keyboard. USB connections are hot-swappable (they can be connected and disconnected while the devices are turned on; this is not always true for older connection methods). An updated version, called USB 2.0 has a speed of 480 Mbits/sec, which is 40 times faster than the older USB port’s high-speed mode (the connectors look the same).

FireWire (IEEE 1394)

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FireWire connectors

FireWire is a high speed interface that was developed by Apple computer. Sony uses a version called i.Link. It is often called IEEE 1394 when used on PCs (since Apple charges an extra fee for using the name “FireWire”). The first version of FireWire (FireWire 400) works at speeds of 400 Kbits/sec, and the newer FireWire 800 supports twice that speed. FireWire is hot-swappable and supports up to 63 daisy-chained peripherals per port.FireWire works especially well for digital video and audio (from a digital camcorder) as well for connecting external hard drives or other high-bandwidth peripherals. The FireWire connection can also supply 60 watts of power to the peripheral.

SCSI

Assorted SCSI connectors SCSI (Small Computer System Interface), pronounced “scuzzy,” is an older highspeed interface technology. Up to six devices can be daisy-chained to a SCSI port on your computer, but, unlike the plug-and-play nature of USB and FireWire, the user must manually set the SCSI ID number of each device and add connection terminators as needed. In other words, it was a much bigger pain than the newer

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interfaces.SCSI was used as the fast port on older Macintosh computers and some PC laptops. SCSI is also used as an interface bus for connecting internal hard disk drives in some machines.

PCMCIA

PCMCIA cards PCMCIA stands for (Personal Computer Memory Card International Association). It is a standard for extension cards for mobile computers. PCMCIA cards are about the size of a credit card and are typically inserted into a slot in the side of your laptop. The card may contain extra memory (which was it primary original use) or it may contain expansion peripherals such as a modem, a tiny hard disk drive, a networking adapter, etc. Type I, II, and III cards are different thicknesses. PCMCIA cards are also called PC Cards since most people can’t remember PCMCIA (the standard joke is that it stands for “People Can’t Memorize Computer Industry Acronyms”).

Ethernet

Ethernet connectors Connecting your computer to a network requires a network adapter. This circuitry and port could be built into the motherboard (as is often the case in laptops and Macs), or your computer may have a network interface card (NIC) in one of its

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expansion slots. Your computer also needs the necessary networking software installed. The most commonly used networking technology is Ethernet (we use it to connect together the PCs, Macs, and server computers on the UNM-LA Local Area Network). The picture at the right shows a typical Ethernet port and Ethernet cable connector. Ethernet comes in different speed ratings, such as 10 megabits/sec, 100 megabits/sec, and gigabit/sec speeds.

PS/2 Ports

Assorted ports PS/2 ports are special ports for connecting the keyboard and mouse to some PC systems. This type of port was invented by IBM.

Audio Ports The three small connectors shown at the right are for connecting sound input (from a tape player, for example), sound out (to connect you PC’s sound output to your stereo system of external speakers), and a microphone input port.

PCI, ISA, ATA, IDE, SCSI Your computer may have interface bus standards built in that have no external connectors. The expansion slots in a PC or Mac are typically PCI (Peripheral Component Interconnect), which displaced the older ISA standard in PCs and NuBus slots in Macs. Often you will find a special slot for a video card, such as an AGP (Accelerated Graphics Port) slot. You will also run into interface standards for adding additional hard drives or optical drives inside your computer, such as IDE, EIDE, ATA, Serial ATA, and SCSI. You don’t need to remember that last bunch for this class.

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