Presentation on theme: "Compiled by : S. Agarwal, Lecturer & Systems Incharge St. Xaviers Computer Centre, St. Xaviers College Kolkata. March-2003."— Presentation transcript:
Compiled by : S. Agarwal, Lecturer & Systems Incharge St. Xaviers Computer Centre, St. Xaviers College Kolkata. March-2003
BASIC STRUCTURE The computer receives input, processes it and delivers output. To perform these tasks it has different units and each unit is responsible for a specific task. The units are INPUT, MEMORY, CONTROL UNIT (CU), ARITHMETIC & LOGIC UNIT (ALU) AND OUTPUT. The CU and ALU together are called CENTRAL PROCESSING UNIT (CPU).
INPUT DEVICE It is used for transferring data from the users end to the computer. OUTPUT DEVICE It is used to transfer processed information from the computer to the user in a way required by the user.
MEMORY UNIT It stores instruction and data and provides them to the various other units as and when required. It is basically the working memory of the computer system. This memory unit is volatile, i.e. it is temporary memory and nothing can be stored here permanently. The information is stored in the main memory as long the computer is switched on or as long as it is required by the computer.
CONTROL UNIT Controls the various operations within a computer. It basically manages all the other units and devices of the computer system. It does so by transmitting timing and control signals to the various devices and units.
ARITHMETIC & LOGIC UNIT It performs the various arithmetic and logical operations on the data stored in memory, as dictated by the instruction. There are various basic circuits to perform these operations.
SECONDARY STORAGE It stores the various data, information and programs permanently for future retrieval. The information is organised in such a way to retrieve it in minimum time whenever required. The stored information remains as long the user wants it.
BUS These are a set of connecting wires used for setting interconnection between the various devices in the system. Each set of bus has a specific function to perform like carrying data, carrying control signals and addresses.
BLOCK DIAGRAM MEMORY UNIT INPUT DEVICE CONTROL UNIT ARITHMETIC & LOGIC UNIT OUTPUT DEVICE SECONDARY STORAGE CONTROL FLOW DATA FLOW CPU
KEYBOARD A keyboard on a computer is almost identical to a keyboard on a typewriter. Computer keyboards will typically have extra keys, however. Some of these keys (common examples include Control, Alt, and Meta) are meant to be used in conjunction with other keys just like shift on a regular typewriter. Other keys (common examples include Insert, Delete, Home, End, Help, function keys,etc.) are meant to be used independently and often perform editing tasks. Keyboards on different platforms will often look slightly different and have somewhat different collections of keys. Some keyboards even have independent shift lock and caps lock keys. Smaller keyboards with only math-related keys are typically called "keypads".
Although the typing portion of the computer keyboard is identical to a standard typewriter, computers have several additional keys that perform different functions.
Pointing Device A pointing device is any hardware component that allows a user to input spatial data to a computer. CAD systems and Graphical User Interfaces (GUI) allow the user to control and provide data to the computer using physical "gestures" - point, click, and drag - typically by moving a hand-held mouse across the surface of the physical desktop and activating switches on the mouse. Movements of the pointing device are echoed on the screen by movements of the mouse pointer and other visual changes. While the most common pointing device by far is the mouse, other kinds include trackball, touchpad, pointing stick, lightpen, various kinds of digitizing tablets which use a stylus, and even a special "data glove" that translates the user's movements to computer gestures.
MOUSE A mouse is a small device that a computer user pushes across a desk surface in order to point to a place on a display screen and to select one or more actions to take from that position. It is a handheld pointing device for computers, involving a small object fitted with one or more buttons and shaped to sit naturally under the hand. The underside of the mouse houses a device that detects the mouse's motion relative to the flat surface on which it sits. The mouse's 2D motion is typically translated into the motion of a cursor on the display.
Common mouse actions include: clicking the mouse button to select an object or to place the cursor at a certain point within a document; double-clicking the mouse button to start a program or open a folder; and dragging (holding down) the mouse button and moving the mouse to highlight a menu command or a selected bit of text.
Touch pad A touch pad is a device for pointing (controlling input positioning) on a computer display screen. It is an alternative to the mouse. Originally incorporated in laptop computers, touch pads are also being made for use with desktop computers. A touch pad works by sensing the user's finger movement and downward pressure.
Graphics Tablet A graphics tablet (or digitizing tablet) is a computer peripheral device that allows for a relatively simple method of inputing hand-drawn graphics or art into a computer in real time. They typically consist of a large flat surface for drawing on, and an attached "stylus" for drawing on the surface, originally as a part of the electronics, but later simply to provide an accurate but smooth "point". This device also allows the computer user to control the mouse cursor by tracking the stylus pen across the tablet. To most users, the cordless, stylus pen is much easier and faster to use than a mouse and has less strain on your hand.
TRACKBALL A trackball is a computer cursor control device used in many notebook and laptop computers. The trackball is usually located in front of the keyboard toward the user. Essentially, the trackball is an upside-down mouse that rotates in place within a socket. The user rolls the ball to direct the cursor to the desired place on the screen and can click one of two buttons (identical to mouse buttons) near the trackball to select desktop objects or position the cursor for text entry.
TOUCHSCREEN A touchscreen is an input device that allows users to operate a PC by simply touching the display screen. Touch input is suitable for a wide variety of computing applications. A touchscreen can be used with most PC systems as easily as other input devices such as track balls or touch pads. Browse the links below to learn more about touch input technology and how it can work for you. Touchscreen systems are being used in a variety of applications, including point-of-sale systems, public information displays, industrial control systems, and more. Follow this link for additional examples of how touch technology is being used today.
Light pen A lightpen is a device similar to a touch screen, but is facilitated by use of a special light sensitive pen instead of the finger. The advantage of using a pen is more accurate screen input other that of a touch screen.
JOYSTICK A lever that moves in all directions and controls the movement of a pointer or some other display symbol. A joystick is similar to a mouse, except that with a mouse the cursor stops moving as soon as you stop moving the mouse. With a joystick, the pointer continues moving in the direction the joystick is pointing. To stop the pointer, you must return the joystick to its upright position. Most joysticks include two buttons called triggers. Joysticks are used mostly for computer games, but they are also used occasionally for CAD/CAM systems and other applications.
SCANNER Scanner is an image acquisition device connected to the computer, which captures either an image of a text document or a picture and transfers it into bits of information, which a computer can understand and manipulate. Scanning an image is like a copier copying an image. The major difference is that the output of scanning is an electronic file which can be edited by a software and stored in a disk.
DIGITAL CAMERA A digital camera records and stores photographic images in digital form that can be fed to a computer as the impressions are recorded or stored in the camera for later loading into a computer or printer. The big advantage of digital cameras is that making photos is both inexpensive and fast because there is no film processing.
A digital camera stores images digitally rather than recording them on film. Once a picture has been taken, it can be downloaded to a computer system, and then manipulated with a graphics program and printed. Unlike film photographs, which have an almost infinite resolution, digital photos are limited by the amount of memory in the camera, the optical resolution of the digitizing mechanism, and, finally, by the resolution of the final output device. Even the best digital cameras connected to the best printers cannot produce film-quality photos. However, if the final output device is a laser printer, it doesn't really matter whether you take a real photo and then scan it, or take a digital photo. In both cases, the image must eventually be reduced to the resolution of the printer.
MICR MICR, an acronym that stands for Magnetic Ink Character Recognition, is the special set of characters and symbols that appear at the bottom of checks and other financial documents. MICR technology was developed in the mid-1950s to more efficiently process checks which were increasing in volume and quickly overwhelming the manual processing system. Today, many types of financial documents utilize MICR technology, not only traditional bank deposit slips and loan payment stubs, but also gift certificates and stock certificates have adopted this standard. However, checks still account for the largest volume of MICR documents.
MICR characters are either printed in special inks using offset printing presses or printed with MICR toners using a laser printer. The MICR font characters are magnetized and read electronically by special reader/sorter equipment by financial institutions. The MICR line contains critical account information and instructions on how the payment is to be automatically routed and processed. Processing is done initially at the bank of deposit and progresses through the Federal Reserve System in the U.S. and/or other banking systems
OCR Next to keypunching, Optical Character Recognition is the oldest data entry technique in existence. Long before the first key-to-disk system of CRT was used, Optical Character Readers were entering data in commercial and government EDP installations. Data Entry through OCR is faster, more accurate, and generally more efficient than keystroke data entry. Desktop OCR scanners can read typewritten data into a computer at rates up to 2400 words per minute!
Reasons for Using OCR There are a number of reasons for choosing OCR scanning over other methods of data entry. Some of the more significant include: To reduce Data Entry Errors To Consolidate Data Entry To Handle Peak Loads Human Readable Can Be Used with Many Printing Techniques Scanning Corrections
Bar Code Bar code, composed of bars and spaces of varying width, provides a means of expression for human-readable characters in a form (bars and spaces) readable by machines. And in order to read the bar code, there are a wide variety of readers available, each designed for a specific purpose. The advantages are that the bar code provides timely, error free information that can be used to validate receipt, movement or counting of products. It reduces key entry time, transcribing time and almost all errors.
Black and white stripes, such as those shown below, are often seen on packages of snacks, foodstuff, and sundries stacked on supermarket shelves or convenience stores - these are known as "Bar Code". The bar code can also be found on industrial products, or on delivery request forms for home and office deliveries - their uses are wide ranging.
OMR OMR (Optical Mark Reading) is the process to detect the presence of intended marked responses. A mark or response position as it is often called, registers significantly less light than the surrounding paper. In order to be detected by the a mark has to: Be positioned correctly on the paper Be significantly darker than the surrounding paper OMR ususally refer to a technique that uses special hardware equiped with light sensors that capture the reflection or absence of reflection on paper.
Advantages of OMR readers : OMR has a better recognition rate because fewer mistakes are made by machines to read marks than by reading handwritten characters. Large volumes of data can be collected quickly and easily without the need for specially trained staff. The cost of inputting data and the chance of data input errors can be reduced because it is not necessary to type the details for data entry. Disadvantages of OMR readers: Documents for optical mark recognition are complicated to design. The OMR reader needs to be reprogrammed for each new document design. OMR readers are relatively slow. The person putting marks on the documents must follow the instructions precisely. Any folding or dirt on a form may prevent the form from being read correctly.
Input DeviceDescriptionUsage KeyboardData can be entered by pressing the corresponding keys on the keyboard. Text input Mouse/ Trackball It uses the movement of a small ball to control the pointer and uses buttons to perform activities. Moving the pointer, selecting options and graphics input Touch padIt uses the movement of a finger to control the pointer and uses buttons to perform activities. Moving the pointer, selecting options and graphics input JoystickIt uses the movement of a vertical stem to control the pointer and uses buttons to perform actions. Playing video games
Input Device DescriptionUsage Touch screen Data can be entered by touching the screen. Information seeking DigitizerIt converts points, lines and curves from a drawing or a photograph into digital signals. Graphics input Graphic tablet It allows a user to input graphic designs into a computer. Graphics input Bar code reader It identifies a bar code by the reflected light pattern from the bar code lines. Reading bar codes
Input Device DescriptionUsage Optical character reader It recognizes characters with special typefaces. Reading retail prices, text input Image scanner It captures images or text into the computer. Graphics input Magnetic card reader It reads data stored in the magnetic strip on a magnetic card. Reading magnetic cards
Input DeviceDescriptionUsage Smart card reader It reads data stored in the chip on a smart card. Reading smart cards Speech recognition device Data or commands can be inputted to the computer with spoken words. Security purposes, handicapped people Digital cameraIt records images in the form of digital data. Image input Digital video camera It allows a user to input video images into the computer in digital form. Video image input
Output devices used to produce information to users in the form of hardcopies (printed output) or screen display.
VDU or MONITOR A monitor, also called the Visual Display Unit (VDU) or a display screen, is the physical unit where the CRT, or other image-projection means and other related parts are placed. It is a separate unit connected to the computer via cables. One distinguishable part of a monitor, the screen, is a glass surface on which images produced by the computer s video adapter are displayed. The size of a screen is measured from one corner to the diagonally opposite corner. Common screen sizes are 14, 17, 19 and 21 inches. Monitors can be classified by color capability into monochrome (two- color display), gray-scale (displays different shades of gray), and colored (many-color display, RGB monitor). Together with the development on the color capabilities of monitors, are the improvements on the display modes (sometimes called video or graphics standards).
The technology behind monitors and televisions is The cathode-ray tube, or CRT, A CRT is an sealed glass bottle with no air inside. It begins with a slim neck and tapers outward until it forms a large base. The base is the monitor s screen and is coated on the inside with a matrix of thousands of tiny phosphor dots. Phosphors are chemicals which emit light when excited by a stream of electrons: different phosphors emit different coloured light. Each dot consists of three blobs of coloured phosphor: one red, one green, one blue. These groups of three phosphors make up what is known as a single pixel
Display modes Originally, a PC could only display text, and in monochrome only. There are now a variety of display modes, but display quality still depends on the PC s video card or adapter. For the full benefit of higher resolution and more colors, the following display modes have been developed over the years. CGA : Color Graphics Adapter (CGA) was introduced by IBM in It was capable of providing four colors and displaying images at a maximum resolution of 320x200 pixels. EGA : EGA (Extended Graphics Adapter) was introduced in It provides up to 16 colors and an improved resolution of 640x350 pixels. EGA made text-reading easier than CGA, but it is still did not provide enough resolution for sophisticated graphics designs or desktop publishing.
VGA : Introduced in 1987, Video Graphics Array(VGA) provides a 640x480 resolution at 16 colors, or a 320 X 200 resolution at 256 colors. VGA has become the accepted standard for the usually less expensive IBM PC-compatibles or clone computers. SVGA : Super VGA provides greater resolution than VGA. Typically, it can support a palette of up to 16,000,000 colors. The number of colors that can be displayed at the same time in a certain computer, however, may be limited by the amount of its video memory. Most PCs now support the SVGA display. Depending on its size, an SVGA monitor can provide one of the following resolutions: 800x600 pixels 1024x768 pixels 1280x1024 pixels 1600x1200 pixels XGA : Extended Graphics Display (XGA) was introduced in The newer version of XGA offers a good resolution of 800x600 pixels in 16,000,000 colors (true color) and 1024x768 pixels in 65,536 colors.
Printer In computers, a printer is a device that accepts text and graphic output from a computer and transfers the information to paper, usually to standard size sheets of paper. Printers are sometimes sold with computers, but more frequently are purchased separately. Printers vary in size, speed, sophistication, and cost. In general, more expensive printers are used for higher-resolution color printing.
Personal computer printers can be distinguished as impact or non-impact printers. Early impact printers worked something like an automatic typewriter, with a key striking an inked impression on paper for each printed character. The dot-matrix printer was a popular low-cost personal computer printer. It's an impact printer that strikes the paper a line at a time. The best-known non-impact printers are the inkjet printer, of which several makes of low-cost color printers are an example, and the laser printer. The inkjet sprays ink from an ink cartridge at very close range to the paper as it rolls by. The laser printer uses a laser beam reflected from a mirror to attract ink (called toner) to selected paper areas as a sheet rolls over a drum.
Dot Matrix printer : A Dot Matrix printer is an impact printer that forms characters as a series of dots. The print head contains a grid of pins which form a character. This heads stamps on to the paper through an inked ribbon to produce a character (for example, "H") that is made up of dots. The print heads can have from 9 to 24 pins in them. The more pins the print head has, the better the quality of the printed output. Printers with 24 pins produce letter quality output. The quality of the printed output is measured in dpi (dots per inch). The higher the dpi, the better the quality of the printed output.
Ink-Jet printer : An inkjet printer is any printer that places extremely small droplets of ink onto paper to create an image. If you ever look at a piece of paper that has come out of an inkjet printer, you know that: The dots are extremely small (usually between 50 and 60 microns in diameter), so small that they are tinier than the diameter of a human hair (70 microns)! The dots are positioned very precisely, with resolutions of up to 1440x720 dots per inch (dpi). The dots can have different colors combined together to create photo-quality images.
Laser Printer : A Laser printer uses a laser beam to print. The laser creates an image on a drum inside the printer. This picks up toner and prints the image on to the paper like a photocopier does. The main advantages of laser printers are speed, precision and economy. A laser can move very quickly, so it can "write" with much greater speed than an ink jet. And because the laser beam has an unvarying diameter, it can draw more precisely, without spilling any excess ink.
The four printer qualities of most interest to most users are: Color : Color is important for users who need to print pages for presentations or maps and other pages where color is part of the information. Color printers can also be set to print only in black-and-white. Color printers are more expensive to operate since they use two ink cartridges (one color and one black ink) that need to be replaced after a certain number of pages. Users who don't have a specific need for color and who print a lot of pages will find a black-and-white printer cheaper to operate. Resolution : Printer resolution (the sharpness of text and images on paper) is usually measured in dots per inch (dpi). Most inexpensive printers provide sufficient resolution for most purposes at 600 dpi.i Speed : If you do much printing, the speed of the printer becomes important. Inexpensive printers print only about 3 to 6 sheets per minute. Color printing is slower. More expensive printers are much faster. Memory : Most printers come with a small amount of memory (for example, one megabyte) that can be expanded by the user. Having more than the minimum amount of memory is helpful and faster when printing out pages with large images or tables with lines around them (which the printer treats as a large image). Large volume of information are printed with the help of a print spooler which uses the printer memory efficiently to complete the print job.
Plotter A plotter is a printer that interprets commands from a computer to make line drawings on paper with one or more automated pens. Unlike a regular printer, the plotter can draw continuous point-to- point lines directly from vector graphics files or commands. There are a number of different types of plotters: a drum plotter draws on paper wrapped around a drum which turns to produce one direction of the plot, while the pens move to provide the other direction; a flatbed plotter draws on paper placed on a flat surface; and an electrostatic plotter draws on negatively charged paper with positively charged toner.
Output DeviceDescriptionUsage Visual display unitIt displays images on screen.Image output Dot-matrix printerIt is a noisy, low speed and quality printer. Printing multiple hardcopies in one strike Inkjet printerIt is a quiet, quite high speed and quality printer. Printing documents, graphics Laser printerIt is quiet, high speed and best quality printer. Printing high quality documents and graphics PlotterIt produces high quality line drawings. Producing maps, charts, building plans and circuit diagrams. Sound/speech synthesizing devices It converts information into speech.Automatic telephone answering systems, voice dictionaries Comparisons between different output devices:
MAIN MEMORY & BACKING STORE (STORAGE DEVICES)
MEMORY MEMORY A computer requires a memory to store and retrieve instructions and data. There are a variety of storage devices including semiconductor memories and magnetic memories. Generally, the term memory refers to only the small integrated circuits called chips, which are used as a computer's internal memory.
Volatile or nonvolatile memory A memory in which the data stored is lost when the power is turned off is volatile memory. A volatile memory is a nonpermanent memory. RAM is a volatile memory while ROM is a nonvolatile memory the stored data are retained even when power is removed. RAM holds your information while you are working on it, but when the computer power is removed, RAM loses all of the information it contained.
How is data stored in a memory chip? A memory chip is made up of a large number of memory locations called cells. Each memory cell can store one bit of data either a 1 or 0. Each memory location has a unique address. Data is stored in a memory by a process called write and retrieved from the memory by a process called read. The process write replaces the previously stored data at an address by new data. The read process takes a copy of data and leaves the stored data intact.
Memory Capacity : The amount of information a computer can store in memory. Bytes are used to measure both computer memory (RAM) and the storage capacity of floppy disks, CD- ROM drives, and hard drives. BYTE : One character. A character can be a number, letter or symbol. Note: A byte consists of 8 bits. KILOBYTE (K) Approximately one thousand characters, or one page of double spaced text. MEGABYTE (MB) Approximately one million characters, or one novel. GIGABYTE (GB) Approximately one billion characters, or one thousand novels.
Primary memory The main internal memory of a computer system is called the primary memory. This memory is accessed directly by a computer's central processing unit (CPU). The primary memory is made of mainly semiconductor memories in form of chips. There are two types of memories: random access memory (RAM) and read only memory (ROM). RAM has both read and write capabilities, while ROM can only be read and there is no write operation. Computers main RAM and ROM are located on it's motherboard, besides some cards may have their own RAM and ROM.
Secondary memory A memory that is not to be directly accessed by CPU is known as secondary memory or auxiliary memory. The information or data from these memories is first transferred to RAM (main memory) and then can be accessed by CPU. A few examples of secondary memory are hard disk, magnetic tape, floppy disk, and CD-ROM.
Cache memory A cache memory is increasingly being used in computers. It is a small but fast memory that is logically positioned between CPU's own internal memory (registers) and the primary or main memory. The cache memory stores or catches some of the contents of main memory. These are the contents currently being used by the processor. A cache memory plays a big role in the speed and performance of a processor chiefly because it is much nearer to the CPU than the RAM. Cache memory stores anticipated instructions so that the processor can quickly and easily access them, instead of constantly returning to RAM. This makes the processors job easier and helps it to perform more quickly.
RAM The RAM is a random access memory that has both read and write capabilities. This memory comes in the form of memory chips. There are two types of RAMs: Dyanmic RAM (DRAM) and Static RAM (SRAM). The two common RAM chip modules are single-in-line memory modules (SIMMs) and dual-in-line memory modules (DIMMs). Most often when we talk about memory in a computer system, it is about RAM. It is the memory where a computer temporarily holds information currently being used so the data can move quickly between the central processing unit or CPU and the operating system. Essentially, RAM is where the CPU, the computers main processing chip, does its "thinking."
RAM is the vital workspace of computing; the more RAM a system contains, the more information it can handle and the more programs it can run at one time. It is the arrangement of information in RAM that determines how much data your PC can handle. The programs you use determine how much RAM you need. Graphical programs, for example, handle a lot of information and require much more RAM than word processors or spreadsheets. RAM is a volatile memory or nonpermanent memory. Though RAM holds your information while you are working on it, but when the computer power is removed, RAM loses all of the information it contained. That means a blink in the power can erase hours of work if you have not been continually saving your work to permanent storage on a hard drive or diskette.
ROM The ROM is a read only memory. It is a type of memory that only can be read. There is no write operation. A ROM is a nonvolatile memory. The stored data are retained even when power is removed. In ROM data is permanently or semi- permanently stored. In a ROM in which data are permanently stored, the specified data are either manufactured into the device or programmed into the device by user and can not be altered. In a ROM in which the data are semi-permanently stored, the data can be altered by special methods. Based on this, we have two types of ROMs the mask ROM and the PROM.
ROM contains many of the basic instructions in BIOS (Basic Input/Output System). The CPU communicates with diskettes, hard drive, keyboard and other components through the BIOS contained in ROM chips. The BIOS is always stored in ROM. The ROM and BIOS are so interconnected that you will frequently hear the terms ROM BIOS and BIOS ROM.
Without the BIOS instructions, a computer will not even know how to read important files such as the Config.sys and Autoexec.bat files that are integral parts of starting up DOS. The BIOS must be set up and told exactly what kind and number of disks, ports, etc., exist in order to successfully intermediate between these parts and the CPU. The BIOS traditionally was built into the ROM chip during the manufacturing process. Later technology let ROM be programmed after the manufacturing process but only once. Now an increasing number of computers have ROM that can be erased and reprogrammed and a user can customize the BIOS instructions to suit his needs.
RAM and cache work together to match CPU speed, thereby ensuring the efficiency of your computer. The amount of RAM in a system is in direct relationship to the speed at which an application will run. Generally, having more RAM means accessing the hard drive less frequently resulting in smoother, faster, more efficient performance.
PROM : The PROM or the programmable ROM is the type in which data are electrically stored by the user with the aid of specialized equipment. Some PROM can be written to only one time. EPROM : One of the most popular kinds of PROM is erasable PROM (EPROM, pronounced "e-prom"), which can be erased and reprogrammed when removed from the motherboard and exposed to ultraviolet light. EPROMs have a clear window covered with a label of some kind. Lifting the label exposes the semiconductor chip and lets it be reprogrammed. The correct dosage of ultraviolet light quickly erases the chips and manufacturers change EPROMs with a special hardware device called a PROM blaster.
Electrically erasable PROM (EEPROM) can be erased and reprogrammed with an electrical process in which a higher- than-normal voltage is applied to the chip. EEPROM is more expensive than EPROM, it is less popular and is rarely used on the motherboard of a PC. New computers store BIOS on flash ROM, which like EEPROM, can be electrically rewritten. It is cheaper than EPROM and easier to reprogram. Updating the BIOS instructions in a flash ROM is a much easier than other type of PROMs.
RAMROM The stored data vanish immediately when the computer is turned off (volatile). The stored data are retained when the computer is turned off (non-volatile). Data can be read from or written on the RAM Data can only be read form the ROM User programs and working data are temporarily stored Systems programs and data are stored permanently by the manufacturer. Comparisons between RAM and ROM:
MEMORY MANAGEMENT It is a collection of techniques for providing sufficient memory to one or more processes in a computer system, especially when the system does not have enough memory to satisfy all processes' requirements simultaneously. Techniques include swapping, paging and virtual memory. Memory management is usually performed mostly by a hardware memory management unit.
Real and Virtual Memory Real memory refers to the actual memory chips that are installed in the computer. All programs actually run in this physical memory. However, it is often useful to allow the computer to think that it has memory that isn't actually there, in order to permit the use of programs that are larger than will physically fit in memory, or to allow multitasking (multiple programs running at once). This concept is called virtual memory.
Working of Virtual Memory The way virtual memory works is relatively simple. Let's suppose the operating system needs 80 MB of memory to hold all the programs that are running, but there are only 32 MB of RAM chips installed in the computer. The operating system sets up 80 MB of virtual memory and employs a virtual memory manager, a program designed to control virtual memory, to manage the 80 MB. The virtual memory manager sets up a file on the hard disk that is 48 MB in size (80 minus 32). The operating system then proceeds to use 80 MB worth of memory addresses. To the operating system, it appears as if 80 MB of memory exists. It lets the virtual memory manager worry about how to handle the fact that we only have 32 MB of real memory.
Why virtual memory? Virtual memory was a very important invention in computing, as it allows multitasking as we know it today. Without virtual memory, you couldn't run a spreadsheet, word processor and database program at the same time unless you had enough memory to hold all of them at once, because you would constantly be running out of memory and having to shut down program "A" in order to open program "B". Most PCs, when running multitasking operating systems like Windows 95, are using virtual memory.
SWAPPING This is process to move a program from fast-access memory to a slow-access memory ("swap out"), or vice versa ("swap in"). The term often refers specifically to the use of a hard disk (or a swap file) as virtual memory or "swap space". When a program is to be executed, possibly as determined by a scheduler, it is swapped into core for processing; when it can no longer continue executing for some reason, or the scheduler decides its time slice has expired, it is swapped out again. This contrasts with "paging" systems in which only parts of a program's memory is transferred.
PAGING It is a technique for increasing the memory space available by moving infrequently-used parts of a program's working memory from RAM to a secondary storage medium, usually disk. The unit of transfer is called a page. Paging allows the total memory requirements of all running tasks (possibly just one) to exceed the amount of physical memory, whereas swapping simply allows multiple processes to run concurrently, so long as each process on its own fits within physical memory.
The paging process : A memory management unit (MMU) monitors accesses to memory and splits each address into a page number (the most significant bits) and an offset within that page (the lower bits). It then looks up the page number in its page table. The page may be marked as paged in or paged out. If it is paged in then the memory access can proceed after translating the virtual address to a physical address. If the requested page is paged out then space must be made for it by paging out some other page, i.e. copying it to disk. The requested page is then located on the area of the disk allocated for "swap space" and is read back into RAM. The page table is updated to indicate that the page is paged in and its physical address recorded. The MMU also records whether a page has been modified since it was last paged in. If it has not been modified then there is no need to copy it back to disk and the space can be reused immediately.
Backing store or Storage Device It is a secondary storage with large capacity used to store programs and data permanently for future use. Four common types of storage media are Floppy disk Hard disk Compact disc Tape
Floppy Disk : A floppy disk is a thin, circular, flexible plastic disk with a magnetic coating enclosed in a square- shaped plastic shell. A floppy disk is also called a diskette. The most widely used floppy disk today is 3.5 inches wide. Before anything can be written on a new floppy disk, it must be formatted. Today, most floppy disk are high-density floppy disk. Floppy disks usually spin at about 350 revolutions per minute. The average access time for current floppy disk drives to locate an item on the disk is 84 milliseconds, or approximately 1/12 second. To care for a floppy disk, it should not be exposed to heat, cold, magnetic fields, and contaminants such as dust, smoke, or salt air.
Advantages of floppy disks : Floppy disk is portable and inexpensive. Data on a floppy disk can be accessed randomly. Data on a floppy disk can be write-protected from being changed accidentally. Floppy disk can be used to transfer data from one computer to another. Disadvantages of floppy disks : Floppy disk is not durable (due to dust and dirt) and can be destroyed by magnetic field. Access time of floppy disk is slow (about 84 ms). Storage capacity of floppy disk is limited (only 1.44 MB).
Hard Disk : A hard disk usually consists of several inflexible, circular disks, called platters, on which items are stored electronically. The hard disk in most desktop and laptop computers is housed inside the system unit, and is considered fixed disks because being not portable. Hard disks are sealed tightly to keep out of contaminants (e.g., dust and smoke particles), which may result in head crash. Hard disks undergo two formatting steps: a low-level format and a high- level format. Some computers are able to improve the hard disk access time by using disk caching. The flow of data, instructions, and information to and from a hard disk is managed by a special purpose chip and its associated electronic circuits called the hard disk controller (HDC). Some manufacturers develop a type of hard disk system, called RAID, which connects several smaller disks into a single unit that acts like a single large hard disk.
Working of a hard disk : Hard drives are magnetic storage devices that contain several discs inside called "Platters" that are attached to a spindle motor. The number of platters varies depending on the capacity of the drive. Platters are coated with a film of magnetically sensitive substance that is primarily made of iron oxide. Another important ingredient is a thin layer of cobalt alloy. The read/write heads are responsible for reading and writing to the platters and are attached to the head actuator which is in charge of moving the heads around the platters.
There s a read/write head for each side of each platter, mounted on arms which can move them towards the central spindle or towards the edge. The arms are moved by the head actuator, which contains a voice-coil - an electromagnetic coil that can move a magnet very rapidly. Loudspeaker cones are vibrated using a similar mechanism. The heads are designed to touch the platters when the disk stops spinning - that is, when the drive is powered off. During the spin-down period, the airflow diminishes until it stops completely, when the head lands gently on the platter surface - to a dedicated spot called the landing zone (LZ). The LZ is dedicated to providing a parking spot for the read/write heads, and never contains data.
Advantages of hard disks over floppy disk : Hard disks provide far larger storage capacities and much faster access times than floppy disks. Hard disk is cheaper than floppy disk per Megabyte. Hard disk is usually more reliable than floppy disk (a better protection against dust and dirt). Disadvantages of hard disk when compared with floppy disk : Hard disk is not portable (except removable hard disk, which is usually more expensive). Data becomes less secure if left on the hard disk. Head crash may occur due to extreme shock or contaminants.
Compact Disk : A compact disc (CD) is a flat, round, portable, storage medium that is usually 4.75 inches in diameter and less than one- twentieth of an inch thick. Compact discs store items by using microscopic pits and land that are in the middle layer of the disc. A compact disc stores items in a single track, which is also divided into evenly sized sectors, that spirals from the center of the disc to the edge of the disc. The drive designation of a compact disc drive usually follows alphabetically after that of the hard disk (i.e., if the hard disk is drive C, then the compact disc is drive D). Variations of compact discs designed for use with computers are CD- ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, DVD-R, DVD+RW, PhotoCD
CD-ROM (Compact Disc Read-Only Memory) is a compact disc that uses the same laser technology as audio CDs for recording music. A CD-ROM, however, can contain text, graphics, video, and sound. The contents of standard CD-ROMs are written, or recorded, by the manufacturer, and cannot be modified by the user. A CD-ROM drive or CD-ROM player is required to read items on a CD-ROM. A CD-ROM can hold up to 650 MB of data, instructions, and information. CD-R (Compact Disc-Recordable) is a technology that allows a user to write on a compact disc using his or her own computer. CD-R is a write once, read many technology. A user can write on the disc in stages, writing on part of it one time and writing on another part at a later time. However, the user can write on each part only once and cannot erase it. CD-R software and a CD-R drive are required in order to write on a CD-R.
A CD-RW (Compact Disc-Rewritable) is an erasable compact disc that can be written on multiple times. CD-RW acts like a floppy disk or hard disk, allowing data, instructions, and information be written and rewritten onto it multiple times. CD-RW software and a CD-RW drive are required in order to write and rewrite on a CD-RW. One problem with CD-RW is that they cannot be read by all CD-ROM drives. A DVD-ROM (Digital Video Disc-ROM) is an extremely high capacity compact disc capable of storing from 4.7 GB to 17 GB. A DVD-ROM drive or DVD player is required to read a DVD-ROM. DVD+RW is the recordable and rewritable versions of DVD-ROM, which can be written and read (play) for many times. DVD+RW is similar to a CD-RW, except it has storage capacities up to 4.7 GB. PhotoCD is a type of CD that contains digital photographic images saved in the PhotoCD format developed Eastman Kodak.
Advantages of compact disc over hard disk include A compact disc is more portable than a hard disk. Advantages of compact disc over floppy disk include The storage capacity of a compact disc is very much larger than that of a floppy disk. The average access time of a compact disc is faster than that of a floppy disk. Disadvantages of compact disc include Some kinds of compact discs are read only (CD-ROM and DVD- ROM). The average access time of a compact disc is slower than that of a hard disk.
Magnetic tape Magnetic Tape is a magnetically coated ribbon of plastic capable of storing large amounts of data and information at a low cost. It is one of the first storage media used with mainframe computers. Tape storage requires sequential access, which refers to reading or writing data consecutively. Floppy disks, hard disks, and compact discs all use direct access, or random access, which means that a particular data item or file can be located immediately, without having to move consecutively through items stored in front of it. Tapes are used most often for long-term storage and backup.
Advantages of magnetic tapes include Magnetic tape is light and portable. The storage capacity of a magnetic tape is very much larger than a floppy disk or a compact disc. Disadvantages of magnetic tapes include Magnetic tape can be sequentially accessed only. Tape has a limited shelf life of about 2 years for reliable results.
Secondary Storage CostCapacityAccess Mode Access Speed Use Floppy disk Lowest Direct access SlowUsually store data files Hard disk HighestVery highDirect access FastestStore program and data Magnetic tape High Sequential access SlowestBackup programs and data Optical disk LowHigherDirect access FastStore reference materials and programs Comparisons among floppy disks, hard disks, magnetic tapes and optical disks: