User:Niteen007

What is a computer? A computer is an electronic device that executes the instructions in a program. A computer has four functions: a. accepts data 	Input The Information Processing Cycle b. processes data	Processing c. produces output 	Output d. stores results	Storage Some Beginning Terms Hardware 	The physical parts of the computer. Software 	the programs (instructions) that tell the computer what to do Data 	individual facts like first name, price, quantity ordered Information 	data which has been massaged into a useful form, like a complete mailing address Default 	the original settings; what will happen if you don't change anything

What makes a computer powerful? Speed 	A computer can do billions of actions per second. Reliability 	Failures are usually due to human error, one way or another. (Blush for us all!) Storage	A computer can keep huge amounts of data.

Generations 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, 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. First generation computers relied on machine language 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 50s. 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. 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 self-organization Classification of Computers : Personal or micro Computers for personal use come in all shapes and sizes, from tiny PDAs (personal digital assistant) to hefty PC (personal computer) towers. More specialized models are announced each week - trip planners, expense account pads, language translators... Descriptions of Personal Computers When talking about PC computers, most people probably think of the desktop type, which are designed to sit on your desk. (Bet you figured that one out!) The tower and the smaller mini-tower style cases have become popular as people started needing more room for extra drives inside. Repairmen certainly appreciate the roominess inside for all the cables and circuit boards ... and their knuckles.

A workstation is part of a computer network and generally would be expected to have more than a regular desktop PC of most everything, like memory, storage space, and speed.

The market for the smallest PCs is expanding rapidly. Software is becoming available for the small types of PC like the palmtop (PPC) and handheld (HPC). This new software is based on new operating systems like Windows CE (for Consumer Electronics). You may find simplified versions of the major applications you use. One big advantage for the newer programs is the ability to link the small computers to your home or work computer and coordinate the data. So you can carry a tiny computer like a PalmPilot around to enter new phone numbers and appointments and those great ideas you just had. Then later you can move this information to your main computer.

With a Tablet PC you use an electronic stylus to write on the screen, just like with a pen and paper, only your words are in digital ink. The Tablet PC saves your work just like you wrote it (as a picture), or you can let the Hand Recognition (HR) software turn your chicken-scratches into regular text. Main Frame The main frame is the workhorse of the business world. A main frame is the heart of a network of computers or terminals which allows hundreds of people to work at the same time on the same data. It requires a special environment - cold and dry.

Supercomputers The supercomputer is the top of the heap in power and expense. These are used for jobs that take massive amounts of calculating, like weather forecasting, engineering design and testing, serious decryption, economic forecasting, etc. A list of the top 500 supercomputers -who made them, where they are installed and what they are used for. (www.top500.org)

Distributed or Grid Computing The power needed for some calculations is more than even a single supercomputer can manage. In distributed computing, using a PC grid, many computers of all sizes can work on parts of the problem and their results are pooled. A number of current projects rely on volunteers with computers connected to the Internet. The computers do the work when they are not busy otherwise. Other Important Terms Server The term server actually refers to a computer's function rather than to a specific kind of computer. A server runs a network of computers. It handles the sharing of equipment like printers and the communication between computers on the network. For such tasks a computer would need to be somewhat more capable than a desktop computer. It would need:  more power  larger memory  larger storage capacity  high speed communications

Input

Everything we tell the computer is Input

Types of Input Data is the raw facts given to the computer. Programs are the sets of instructions that direct the computer. Commands are special codes or key words that the user inputs to perform a task, like RUN "ACCOUNTS". These can be selected from a menu of commands like "Open" on the File menu. They may also be chosen by clicking on a command button. User response is the user's answer to the computer's question, such as choosing OK, YES, or NO or by typing in text, for example the name of a file. Keyboard Pointing Device

Mouse Trackball Joystick Glidepad Game Device Stylus Touch Screen Digitizers and Graphics Tablet

Multimedia

Sound Input – Mike, Voice (sound waves to Digital)

Video – Digital Camera, Video Camera, Web cam, Scanner

The CPU, or Central Processing Unit, is the part of the computer where work gets done. In most computers, there is one processing chip. Main Memory stores the commands that the CPU executes and the results.

Control Unit This is the part of the computer that controls the Machine Cycle. It takes numerous cycles to do even a simple addition of two numbers. ALU stands for Arithmetic/Logic Unit This is the part that executes the computer's commands. A command must be either a basic arithmetic operation: +  -  *  / or one of the logical comparisons: > <  =  not =. Everything else has to be broken down into these few operations. Only one operation is done in each Machine Cycle. The ALU can only do one thing at a time but can work very, very fast.

Applications These are the various programs that are currently running on the computer. By taking turns with the Machine Cycle, modern computers can have several different programs running at once. This is called multi-tasking. Each open application has to have some data stored in Main Memory, even if the application is on rest break and is just sitting there. Some programs (graphics programs are notorious for this) require a lot of the Main Memory space, and may not give it up even if they are shut down! Rather rude, actually!! Control Unit This is the part of the computer that controls the Machine Cycle. It takes numerous cycles to do even a simple addition of two numbers. Fetch -	get an instruction from Main Memory Decode -	translate it into computer commands Execute -	actually process the command Store -	write the result to Main Memory

The Machine Cycle

Input/Output Storage When you enter new data, the keystrokes must be stored until the computer can do something with the new data. When you want data printed out or displayed, it must be stored somewhere handy first. Main Memory This is where the computer stores the data and commands that are currently being used. When the computer is turned off, all data in Main Memory vanishes. A data storage method of this type is called volatile since the data "evaporates." Note on the left the various kinds of data that are stored. The CPU can fetch one piece of data in one machine cycle Operating System This is the instructions that the computer uses to tell itself how it "operates". It's the answer to "Who am I and what can I do?" Some common operating systems are DOS, various versions of Windows, OS/2, UNIX, LINUX, System 7. These all behave in very different ways and have different hardware requirements. So they won't all run on all machines. Unused Storage One hopes that there is always some storage space that is not in use. If space runs out in Main Memory, the computer will crash, that is, stop working. There are programs that sense when space is getting short and warn the user. The user could then close some of the open applications to free up more space in Main Memory. Sometimes the warning is too late to prevent the crash. Remember that all the data in Main Memory vanishes when the power goes off. Thus a crash can mean a lot of lost work. Working Storage The numbers and characters that are the intermediate results of computer operations must be stored until the final values are calculated. These values "in progress" are kept in temporary locations. For example, if the computer is adding up the numbers 3, 5, and 6, it would first add 3 to 5 which yields a value of 8. The 8 is stored in working storage. Then the 8 and 6 are added and the new value 14 is stored. The value of 14 is now available to be displayed on the screen or to be printed or to be used in another calculation. The computer can only do one thing at a time. Each action must be broken down into the most basic steps. One round of steps from getting an instruction back to getting the next instruction is called the Machine Cycle.

For example, to add the numbers 5 and 6 and show the answer on the screen requires the following steps: 1. 	Fetch instruction:	"Get number at address 123456" 2.	Decode instruction. 3.	Execute:	ALU finds the number. (which happens to be 5) 4.	Store:	The number 5 is stored in a temporary spot in Main Memory. 5 - 8 Repeat steps for another number (= 6) 9.	Fetch instruction:	"Add those two numbers" 10.	Decode instruction. 11.	Execute:	ALU adds the numbers. 12.	Store:	The answer is stored in a temporary spot. 13.	Fetch instruction:	"Display answer on screen." 14.	Decode instruction. 15.	Execute:	Display answer on screen. Speed The immense speed of the computer enables it to do millions of such steps in a second. In fact, MIPS, standing for millions of instructions per second, is one way to measure computer speeds.

We need a method of naming the places where Main Memory stores data. Each location needs a unique name, just like houses in a town need a unique street address Rather than a street name and house number, memory addresses are just numbers. A memory address holds 1 byte of data where 1 bit =	0 or 1, on or off 1 byte = 	8 bits 1 kilobyte (K or KB) =	1024 bytes 1 megabyte (MB) =	1024 kilobytes You might wonder why 1024 instead of 1000 bytes per kilobyte. That is because computers don't count by tens like people. Computers count by twos and powers of 2. 1024 is 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2, that is 2 times itself ten times. It's a rather convenient size number (for computers!). Update: Things are changing faster than I can type! The explanation above is no longer entirely true (July 2000). Different scientific and technical areas are using the words differently. For data storage devices and telecommunications a megabyte is 1 000 000 bytes. For data transmission in LANs a megabyte is 1 048 576 bytes as described above. But for data storage on a floppy disk a megabyte is 1 024 000 bytes! A new set of words has been created to make it clear what size is really being used. See http://physics.nist.gov/cuu/Units/binary.html   for a further explanation. We all are impatient and want our computer to work as fast as possible, and certainly faster than the guy's at the next desk! Many different factors determine how fast your computer gets things done. Processor speed is one factor. But what determines the processor's speed? System clock rate = rate of an electronic pulse used to synchronize processing (Only one action can take place between pulses.) Measured in megahertz (MHz) where 1 MHz = 1 million cycles per second or gigahertz (GHz) where 1 GHz = 1 billion cycles per second. This is what they are talking about if they say a computer is a 2.4 GHz machine. It's clock rate is 2.4 billion cycles per second. Bigger number = faster processing Bus width = the amount of data the CPU can transmit at a time to main memory and to input and output devices. (Any path bits travel is a bus.) An 8-bit bus moves 8 bits of data at a time. Bus width can be 8, 16, 32, 64, or 128 so far. Think of it as "How many passengers (bits) can fit on the bus at once to go from one part of the computer to another." Bigger number = faster transfer of data Word size = a word is the amount of data the CPU can process at one time. An 8-bit processor can manipulate 8 bits at a time. Processors can be 8-, 16-, 32-, or 64-bit so far. Bigger the number = faster processing Output

Hardcopy – Prrinted on paper Printers Plotters

Softcopy – displayed on Screen

Categories of Output Text Graphics Mumltimedia

The speed of a printer is measured in:

cps	= characters per second lpm	= lines per minute ppm	= pages per minute

The faster the printing, the more expensive the printer.

LQ 	Letter Quality = 	as good as best typewriter output NLQ	Near Letter Quality = 	nearly as good as best typewriter output Draft	used internally or for a test print

The better the quality, the slower the printing.

A more numerical measure of print quality is printer resolution. Measured in dots per inch (dpi), this determines how smooth a diagonal line the printer can produce. A resolution of 300 dpi will produce text that shows jagged edges only under a magnifying glass. A lower resolution than this will produce text with stair-step edges, especially at large sizes. Even higher resolutions are needed to get smooth photo reproduction. Professionals in graphics use 1200 to 2400 dpi printers. Draft quality on such a printer would be 600 dpi. What kind of cable connection? Serial cable  	Sends data only 1 bit at a time Printer can be up to 1000 feet away from the computer. Maximum data transfer speed = 115 kilobits/s (.115Mbits/s) Parallel cable  	Sends data 8 bits at a time Printer must be within 50 feet of the computer. Maximum data transfer speed: 115 kilobytes/s (.115MBYTES/s). This is 8 times faster than the maximum serial speed. Newer printers may need bi-directional cable so that the printer can talk back to the computer. Such a cable is required if the printer can give helpful error messages. It's startling, but nice, the first time your computer politely says "Ink is getting low" or "Please place paper in the AutoSheet feeder." Oddly, Windows XP does not support spooling for a parallel connection to a printer. Spooling is what allows you to do other things on the computer while the printer is processing and printing the document. WinXP does spool when the printer uses a USB connection. USB cable	Printer must be within 5 meters (16.5 feet) of the computer, when connecting straight to the computer. [You can hook up several 5 m. cables and USB hubs in a chain - up to 25 meters.] Maximum data transfer speed: 12 megabits/s (1.5 MBYTES/s) Lots faster! Best choice: The new USB (Universal Serial Bus) connection is likely your best choice, if your printer can use it. It is faster and a USB connector can be unplugged and re-plugged without turning off the system. USB ports are rapidly replacing parallel ports. The printer cannot handle the data as fast as the USB port can send it. The real limit on how fast a printer works is in how fast printer can get the characters onto the paper. Serial cable may have to be used if a printer is shared in a fairly large office, due to the length of cable needed.

Monitors:

The device which displays computer output to us has various names: Screen	from "computer screen" or "display screen" Monitor  	from its use as a way to "monitor" the progress of a program VDT	= video display terminal from early network terminals CRT	= cathode ray tube  from the physical mechanism used for the screen. VDU	= visual display unit to cover all the mechanisms from desktop CRTs to LCD flat screens on laptops to LED screen on palmtops

CRT screen: A standard monitor screen is a CRT (cathode ray tube). The screen is coated on the inside surface with dots of chemicals called phosphors. When a beam of electrons hits a dot, the dot will glow. On a color monitor these phosphor dots are in groups of three: Red, Green, and Blue. This RGB system can create all the other colors by combining what dots are aglow. There are 3 signals that control the 3 electron beams in the monitor, one for each RGB color. Each beam only touches the dots that the signal tells it to light. All the glowing dots together make the picture that you see. The human eye blends the dots to "see" all the different colors. A shadow mask blocks the path of the beams in a way that lets each beam only light its assigned color dots. (Very cool trick!) LCD screen LCD (Liquid Crystal Display) screens use an entirely different technique. The screen is still made of dots but is quite flat. LCD displays are made of two layers of a polarizing material with a liquid crystal solution in between. An electrical signal makes the crystals line up in a way that keeps light from going through entirely or just partly. A black screen has all the crystals lined up so that no light gets through. A color LCD screen uses groups of 3 color cells instead of 3 phosphor dots. The signal for a picture cleverly lets just the right spots show their colors. Your eye does the rest. Scan Pattern There are two patterns used by different monitors to cover the whole screen. Both scan across the screen, in a row 1 pixel high, from left to right, drop down and scan back left. The non-interlaced pattern scans each row of pixels in turn, from top to bottom. This type is more prone to flicker if the scan has not started over by the time the phosphor dots have quit glowing from the last scan. This can make your eyes hurt or even make you nauseous. The interlaced pattern scans every other row of pixels. So the odd rows are done, then the even rows, in the same left to right to left way. But since the rows of pixels are very close together, the human eye doesn't notice as easily if a row has gone dim before it is rescanned. Much friendlier to your eyes and stomach. Screen Features Size	Desktop screens are usually 14 - 19 in. by diagonal measurement. (This is how TV screens are measured, too.) Larger sizes are available, at a significantly higher cost. Prices are dropping, however. Resolution 	Determines how clear and detailed the image is. Pictures on a screen are made up of tiny dots. 1 dot on screen = 1 pixel (from "picture element") The more pixels per inch, the clearer and more detailed the picture. One measure of this is the dot pitch, the distance between the dots that make up the picture on the screen. However, different manufacturers measure differently. Most measure from dot center to the center of the nearest same color dot. Some measure from the center of a dot to an imaginary vertical line through the center of the nearest dot of the same color, giving a smaller number for the same dots as the previous method. Some monitors use skinny rectangles instead of dots and so must use a different method altogether. So, dot pitch has become less useful as a measure of monitor quality. A dot pitch of .28 is very common and .26 should be good for nearly all purposes, however it is measured. Refresh Rate	How often the picture is redrawn on the monitor. If the rate is low, the picture will appear to flicker. Flicker is not only annoying but also causes eye strain and nausea. So, a high refresh rate is desirable. 60 times per second is tolerable at low resolutions for most people. 75 times per second or more is better and is necessary for high resolutions. Type	Old types = CGA, EGA, VGA Current type = super VGA Determines what resolutions are available and how many colors can be displayed. Type Stands for Resolution(s)

CGA Color Graphics Adapter 320 x 200

EGA Extended Graphics Adapter 640 x 350

VGA Video Graphics Adapter 640 x 480

SVGA Super VGA 800 x 600, 1024 x 768, or 1280 x 1024 etc.

New systems now come with super VGA with a picture size of 800 x 600 pixels (as a minimum) and 16 million colors Color	The number of colors displayed can vary from 16 to 256 to 64 thousand to 16.7 million. The more colors, the smoother graphics appear, especially photos. The number of colors available actually depends more on the video card used and on how much memory is devoted to the display. It takes 8 bits to describe 1 pixel when using 256 colors. It takes 24 bits per pixel when using 16 million colors. So a LOT of memory is needed to get those millions of colors. Video cards now come with extra memory chips on them to help handle the load. Reverse video 	example: Cursor/ Pointer	The symbol showing where you are working on the screen, like:    and In the olden days of just DOS, there were few choices for the cursor. The invention of the blinking cursor was a tremendous event. Under Windows there are a huge number of basic to fantasy cursors to choose from. Scrolling	Moving the lines displayed on the screen up or down one line at a time Type of Screens Monochrome	one color text on single color background, i.e. white letters on blue, or green characters on black Color	various colors can be displayed. (This one is easy!) CRT	The most common type of monitor, which uses a cathode ray tube. Liquid Crystal Display (LCD)	Used in laptops esp. Large flat monitors are becoming affordable, especially if you do not have desk space for a large CRT monitor. Plasma Screens	Used for very large screens and some laptops. Flat, good color, but much more expensive.

Storage refers to the media and methods used to keep information available for later use. Some things will be needed right away while other won't be needed for extended periods of time. So different methods are appropriate for different uses. Earlier when learning about processing, we saw all the kinds of things that are stored in Main Memory. Main Memory = Primary Storage Main memory keeps track of what is currently being processed. It's volatile, meaning that turning the power off erases all of the data. For Main Memory, computers use RAM, or Random Access Memory. These memory chips are the fastest, but most expensive, type of storage.

Auxiliary Storage = Secondary Storage Auxiliary storage holds what is not currently being processed. This is the stuff that is "filed away", but is ready to be pulled out when needed. It is nonvolatile, meaning that turning the power off does not erase it. Auxiliary Storage is used for:  Input - data and programs  Output - saving the results of processing So, Auxiliary Storage is where you put last year's tax info, addresses for old customers, programs you may or may not ever use, data you entered yesterday - everything that is not being used right now All magnetic disks are similarly formatted, or divided into areas, called Tracks sectors and cylinders The formatting process sets up a method of assigning addresses to the different areas. It also sets up an area for keeping the list of addresses. Without formatting there would be no way to know what data went with what. It would be like a library where the pages were not in books, but were scattered around on the shelves and tables and floors. You'd have a hard time getting a book together. A formatting method allows you to efficiently use the space while still being able to find things. Tracks A track is a circular ring on one side of the disk. Each track has a number. The diagram shows 3 tracks. Sectors A disk sector is a wedge-shape piece of the disk, shown in yellow. Each sector is numbered. On a 5¼" disk there are 40 tracks with 9 sectors each. On a 3½" disk there are 80 tracks with 9 sectors each. So a 3½" disk has twice as many named places on it as a 5¼" disk. A track sector is the area of intersection of a track and a sector, shown in yellow.

Clusters A cluster is a set of track sectors, ranging from 2 to 32 or more, depending on the formatting scheme in use. The most common formatting scheme for PCs sets the number of track sectors in a cluster based on the capacity of the disk. A 1.2 gig hard drive will have clusters twice as large as a 500 MB hard drive. 1 cluster is the minimum space used by any read or write. So there is often a lot of slack space, unused space, in the cluster beyond the data stored there. There are some new schemes out that reduce this problem, but it will never go away entirely. The only way to reduce the amount of slack space is to reduce the size of a cluster by changing the method of formatting. You could have more tracks on the disk, or else more sectors on a track, or you could reduce the number of track sectors in a cluster. Cylinders A cylinder is a set of matched tracks. On a double-sided floppy, a track from the top surface and the same # track from the bottom surface of the disk make up a cylinder. The concept is not particularly useful for floppies.

On a hard disk, a cylinder is made of all the tracks of the same # from all the metal disks that make up the "hard disk". If you put these all together on top of each other, you'd have something that looks like a tin can with no top or bottom - a cylinder. The computer keeps track of what it has put where on a disk by remembering the addresses of all the sectors used, which would mean remembering some combination of the cylinder, track, and sector. Thank goodness we don't have to remember all these numbers! Where the difference between addressing methods shows up is in the time it takes for the read/write head to get into the right position. The cylinder method writes data down the disks on the same cylinder. This works faster because each metal platter has a read/write head for each side and they all move together. So for one position of the read/write heads, the computer can put some data on all the platters before having to move the heads to a new position. What happens when a disk is formatted? 1. 	All data is erased. Don't forget this!! 2. 	Surfaces are checked for physical and magnetic defects. 3. 	A root directory is created to list where things are on the disk.

An entirely different method of recording data is used for optical disks. These include the various kinds of CD and DVD discs. You may guess from the word "optical" that it has to do with light. You'd be exactly right! Laser light, in fact. Optical disks come in several varieties which are made in somewhat different ways for different purposes. How optical disks are similar  Formed of layers  Data in a spiral groove on starting from the center of the disk  Digital data (1's and 0's)  1's and 0's are formed by how the disk absorbs or reflects light from a tiny laser. The different types of optical disks use different materials and methods to absorb and reflect the light. How It Works (a simple version) An optical disc is made mainly of polycarbonate (a plastic). The data is stored on a layer inside the polycarbonate. A metal layer reflects the laser light back to a sensor. To read the data on a disk, laser light shines through the polycarbonate and hits the data layer. How the laser light is reflected or absorbed is read as a 1 or a 0 by the computer. In a CD the data layer is near the top of the disc, the label side. In a DVD the data layer is in the middle of the disc. A DVD can actually have data in two layers. It can access the data from 1 side or from both sides. This is how a double-sided, double-layered DVD can hold 4 times the data that a single-sided, single-layered DVD can Advantages of Optical Disks 1. Physical: An optical disk is much sturdier than tape or a floppy disk. It is physically harder to break or melt or warp. 2. Delicacy: It is not sensitive to being touched, though it can get too dirty or scratched to be read. It can be cleaned! 3. Magnetic: It is entirely unaffected by magnetic fields. 4. Capacity: Optical disks hold much more data than floppy disks. Plus, the non-data side of the disk can have a pretty label! For software providers, an optical disk is a great way to store the software and data that they want to distribute or sell. Materials The materials used for the data (recording) and metal (reflecting) layers are different for different kinds of optical disks. CD-	DVD-	Type	Data Layer	Metal Layer CD-ROM (Audio/video PC software)	DVD-ROM (Video/audio, PC use)	Read Only	Molded	Aluminum (Also silicon, silver, or gold in double-layered DVDs) CD-R	DVD-R DVD+R	Recordable (once!)	Organic dye	Silver, gold, silver alloy CD-RW	DVD-RW DVD+RW DVD+RAM	Rewritable (write, erase, write again)	Phase-changing metal alloy film	Aluminum

Read Only: The most common type of optical disk is the CD-ROM, which stands for Compact Disc - Read Only Memory. It looks just like an audio CD but the recording format is quite different. CD-ROM discs are used for computer software. DVD used to stand for Digital Video Device or Digital Versatile Device, but now it doesn't really stand for anything at all! DVDs are used for recording movies. The CDs and DVDs that are commercially produced are of the Write Once Read Many (WORM) variety. They can't be changed once they are created. The data layer is physically molded into the polycarbonate. Pits (depressions) and lands (surfaces) form the digital data. A metal coating (usually aluminum) reflects the laser light back to the sensor. Oxygen can seep into the disk, especially in high temperatures and high humidity. This corrodes the aluminum, making it too dull to reflect the laser correctly. CD-ROM and DVD-ROM disks should be readable for many, many years (100? 200?), but only if you treat them with respect. Write Once: The optical disks that you can record on your own computer are CD-R, DVD-R, and DVD+R discs, called writable or recordable disks. The metal and data layers are separate. The metal layer can be gold, silver, or a silver alloy. Go for the Gold: Gold layers are best because gold does not corrode. Naturally, the best is more expensive. Sulfur dioxide can seep in and corrode silver over time. The data layer is an organic dye that the writing laser changes. Once the laser modifies the dye, it cannot be changed again. Write Once! Ultraviolet light and heat can degrade the organic dye. Manufacturers say that these disks have a shelf-life of 5 - 10 years before they are used for recording. There is no testing yet about how long the data will last after you record it. Humph! A writable disk is useful as a backup medium when you need long-term storage of your data. It is less efficient for data that changes often since you must make a new recording each time you save your data. Pricing of the disks will be important to your decision to use writable disks. Rewrite: An option for backup storage of changing data is rewritable disks, CD-RW, DVD-RW, DVD+RW, DVD+RAM. The data layer for these disks uses a phase-changing metal alloy film. This film can be melted by the laser's heat to level out the marks made by the laser and then lasered again to record new data. In theory you can erase and write on these disks as many as 1000 times, for CD-RW, and even 100,000 times for the DVD-RW types. System software

is a catch-all term for the programs that handle the running of your computer's hardware. The two main categories are:

Operating Sysytem

Utility Programs

Operating Systems Between the hardware and the application software lies the operating system. The operating system is a program that conducts the communication between the various pieces of hardware like the video card, sound card, printer, the motherboard and the applications.

Digital Data:

Modern computers are digital, that is, all info is stored as a string of zeros or ones - off or on. All the thinking in the computer is done by manipulating these digits. The concept is simple, but working it all out gets complicated

1 bit = one on or off position 1 byte = 8 bits So 1 byte can be one of 256 possible combinations of 0 and 1. Numbers written with just 0 and 1, are called binary numbers. Each 1 is a power of 2 so that the digits in the figure represent the number: = 2 7 + 0 + 2 5 + 0 + 2 3 + 2 2 + 0 +0 = 128 +0 +32 + 0 + 8 + 4 + 0 + 0 = 172 Every command and every input is converted into digital data, a string of 0's and 1's. Number System

Decimal Base 10

4567 (4 x 103)+ (5 x 102)+ (6 x 101)+ (7 x 100 ) Each digit is multiplied by a power of 10 to get the complete number. Binary Base 2 Computers don't have ten fingers to count with. All they have is on and off. Everything inside a computer must be represented with some combination of on and off. We humans use the digits 1 for on and 0 for off. We call this base 2 since there are only 2 symbols used. A sequence of on off off on off on on off on is written for the benefit of humans as 100101101. This is only a little bit better but it takes less energy to write down. Such base 2 numbers are called binary numbers. Now the number 100101101 in base 2 uses the same symbols as 100,101,101 in base ten. But the base 10 number is equal to one hundred million one hundred and one thousand one hundred and one. This is a much larger number than the base 2 number. The 1s and 0s must be multiplied by powers of 2 to see how many cows this number represents. 100101101 in base 2	= in base 10 1 x 28 =	1 x 256	256 0 x 27 =	0 x 128	0 0 x 26 =	0 x 64	0 1 x 25 =	1 x 32	32 0 x 24 =	0 x 16	0 1 x 23 =	1 x 8	8 1 x 22 =	1 x 4	4 0 x 21 =	0 x 2	0 1 x 20 =	1 x 1	1 Total =	301 in base 10 The rules for adding base 2 numbers are simple. 0 + 0 = 0 0 + 1 = 1 1 + 1 = 10 1 + 1 + 1= 11 For example, adding the two numbers below: (don't forget to carry a one over if the column is 1 + 1 = 10) 10101101  +  1011110    100001011 Hexadecimal (Base 16) Because people have a really hard time keeping straight numbers like those above, computer numbers are often written in yet another base - base 16. Such numbers are called hexadecimal. Hexadecimal numbers look really odd because they have more symbols than we are used to. Letters are used for the numbers from ten through fifteen: A = 10; B = 11; C = 12; D = 13; E = 14; F = 15. They can be either upper case or lower case. The numbers we added above if written as base 16 numbers look like: 3F2 + B37 F29 To interpret such numbers as base 10 numbers, ( for those of us who just can't quit counting on our fingers!) you need to know the powers of 16. 163 = 4096 162 = 256 161 = 16 160 = 1 So the number F29 in base 16 is equal to: (F x 162)+ (2 x 161) + (9 x 160 ) = 15 x 256 = 3840 2 x 16  =   32 9 x 1   =    9 3881 in base 10 This is certainly not all that easy to do. Few people can multiply 15 by powers of 16 easily. But the really advantage for base 16 is in how easy it is to change from base 2 to base 16 and back. Every hexadecimal digit is broken down into a 4 digit binary number. These digits are just written down in the same order as the hexadecimal number and you've got the equivalent binary (base 2) number. For our number F29: F is equal to 15 which is 8 + 4 + 2 + 1. (These are the powers of 2). That means that F = (1 x 23) + (1 x 22) + (1 x 21) + (1 x 20). (We'll use little numbers at the bottom, called subscripts, to show what base the number is using) So F16 = 11112 Since 216 is (1 x 21), then 216 = 00102 Since 916 is (1 x 23 ) + (1 x 20), then 916 = 10012 Putting this together makes F2916 = 1111001010012. Once you learned how to write the numbers 0 to 15 in base 2, you could whip back and forth between base 2 and base 16 rapidly. Some folks who work with computers have to do just that. Thank goodness most of us don't have to do this all the time! One number system is enough to deal with!! But now you know a little about the numbers computers think with and how people write them down.