Device Management

An operating system is a resource manager which is responsible for the allocation of various resources. Resources such as CPU and main memory are considered pre-emptible. A user program that occupies a particular range of location in main memory may be removed or pre-empted by another program. CPU is the most pre-emptible resource on a computer system. Thus, the control of CPU can be transferred from one process to another.

Certain resources are not pre-emptible. They cannot be removed from the processes to which they are assigned. Tape drives for example, are normally assigned to a particular process for several minutes or hours and as long as it is assigned to one process, it cannot be taken from that process and given to others.

Share and Non-sharable Resources.

Some resources may be shared among several processes while others are dedicated to a single process. Main memory and CPU are shared among many processes and hough CPU can belong o one process at a time, the muliplexing of CPU among many processes creates the illusion of simultaneous sharing.

While disk drives are sometimes dedicated to a single process( are non-sharable) devices such as printers and keyboard are non-sharable resources.

Buffering

A buffer is an area of memory used to hold daa during input/ouput transfer to or form a disk or tape and from the keyboard to the printer.

Spooling

Spool stands for Simultaneous Periperals Operations On Line. Spooling is a technique used to reduce the speed mismatch between high-speed CPU and slow-speed devices such as printers. For example, if a program that sends lines to the printer waits for each line to be printed before it can transmit the next line, the program will be executed slowly. to speed up program execution, output lines are routed to a faster device such as the disk drive, where they are temporarily stored until they are printed. This procedure is called spooling.

Difference between Spooling and Buffering

Spooling is usually associated with printing jobs whereas Buffering allocates memory for any kind of job. Buffering also allows the use of main memory for performing some other functions temporarily.

Secondary Storage Devices

Primary memory is volatile, costly and of less capacity. A second type of memory which one can store for a long time and which can also be modified, is needed. This would perform like an audiocassette of a tape-recorder, where an already-recorded cassette can be played any number of times. Secondary Storage memory devices can therefore R/W (read and write) data a number of times. Such devices are discussed below:

Magnetic Tape

A magnetic tape is one of the most popular storage mediums for storing a large volume of data that is to be accessed and processed sequentially. It is a plastic ribbon usually half-an-inch wide, coated on one side with ironoxide which can be magnetized. the tape ribbon itself is stored in reels of 50 to 2,400 feet in small cartridges or cassettes. It is similar to the tape used in tape-recorders. Below are a few important terms related to the magnetic tape.

IBG: there is a gap between two consecutive blocks (group) of records stored on a magnetic tape. this gap is known as IBG (Inner Block Gap).

Blocking Factor: The total number of records in a single block is the blocking factor.

IRG: If the system stores only one record per block, the storage is known as unblocked and IBG is termed as IRG (Inter Record Gap).

The following formulae can be used to determine tape-processing speeds.

     1. Transfer rate = tape speed x recording density

     2. Length of a block = block size x tape density

     3. Time spent per block of data = block size / transfer rate

     4. time spent in IBG = IBG size / tape speed

     5. Maximum number of  =                Length of tape

               blocks on a tape     ---------------------------------

                                            Length of Block + Length of IBG       

Limitations of magnetic tapes:

     1. Lack of direct access to records: Records must be accessed sequentially

     2. Environment problem : The tape unit is susceptible to dust, humidity and high temperature.

 

Magnetic Disk ( Direct access storage device)

This is also a secondary storage device which provides large storage capabilities. A magnetic disk is circular disk on which a coating of y-ferrite on both sides has been applied. This coating has the property of being magnetized locally ie., it allows the recording of data in the form of magnetised spots. Both sides of the disk are coated for independent recordings.

Data is stored on the disks in a number of concentric circles called tracks. A disk can have 40 to 400 tracks per inch of surface. These tracks begin at the outer edge of the disk and continue towards the centre. Each track has a designated number. Tracks are divided into sectors and the number of sectors per track varies from computer to computer. A motor rotates the disk at a very fast but constant speed. Data are recorded on and read from the tracks of the spinning disk surface in the form of tiny magnetic spots by the disk drives read/write head.

The tracks can be divided into sectors by a software-controlled formatting operation.

the process is known as soft sectoring. If the sectors are permanently marked on the tracks by the manufacturers of the disk then the process is known as hard sectoring. the disks are accordingly known as soft-sectored disks (floppies) and hard-sectored disks (hard disks).

Floppy Disks

The floppy Disk is a medium used to store valuable data and programs. The floppy disk rotates inside the drive and the drive's read/write head moves onto that to perform read and write operations. Floppy disks are made up of mylar or vinyl plastic material with magnetic coating on one or both sides. These plastic disks coated with magnetic material are further encapsulated permanently in a plastic jacket to protect them against dust and scratches. Floppy disks come in 2 sizes:

      5.25 inches with a storage capacity of 1.2 MB

      3.5 inches with a storage capacity of 1.44 MB

Each floppy contains a write-protected notch / slide, which can be slided to protect it against being written on or to enable it to write.

Hard Disk

A hard disk is a device used for mass storage of data that can be accessed directly. A hard disk is capable of storing a large quantity of data. It is rigid, stable and hermetically sealed in a dust-free environment. a hard disk conntains more than one platter.

A platter is a round disk coated with magnetic recording material. these platters are of different sizes in different disks. Data can be read from and written onto both sides of the platter with the help of R/W heads. A platter can have more than one R/W head to read and write the data. The platter is hard; therefore the R/W head never touches the surface. all these platters are connected onto a single spindle which rotates at a very fast pace and moves back and forth to access the data needed to be read or to be written onto.

Winchester disks:

These are the hard disks which IBM introduced first in their personal computers. Before that, hard disks were used only in mainframes and mini-computers. They were generally known as disk-drives. Old disk drives are very large in size.

Output Device : Plotters

Plotters are used to produce precise and good quality graphics and drawings. they use an ink pen or inkjet, a single coloured or multicoloured pens. The pens are driven by a motor. Pen plotters are slow. Drawings can be prepared on paper vellum or mylar (polyester film). colour transparencies can also be prepared with the help of pen plotters. there are different types of pen plotters:

1. Drum plotters

A drum plotter contains a long cylinder and a pen cartridge. The drum rotates under the control of plotting instructions sent by the computer either in clockwise or anti-clockwise direction. the pen is mounted horizontally on the carriage and moves horizontally under the computer's control with the carriage left to right and right to left on the paper to produce a drawing. Pens with inks of different colours can be mounted on the carriage and multicoloured drawings can be produced.

2. Micro-grip Plotter:

This type of plotter do not use a drum. the paper or any other medium is held on both sides at the edges by pinch wheels which ensure back and forth movement of the paper.

3. Flatbed Plotter:

A flatbed plotter consists of a horizontal flat surface on which paper or any other medium is fixed. the pen is mounted on a carriage which can move along the horizontal and vertical axis.

4. Inkjet-Plotter:

These plotters are capable of producing large multicoloured drawings while closing inkjets in place of ink pens. the paper is placed on a drum and inkjets with different coloured inks are mounted on a carriage.

Output Device : Printers

A printer is a peripheral which produces a text and/or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable or, in most newer printers, a USB cable to a computer which serves as a document source.

Some printers, commonly known as network printers, have built-in network interfaces, typically wireless and/or Ethernet based, and can serve as a hard copy device for any user on the network. Individual printers are often designed to support both local and network connected users at the same time.

In addition, a few modern printers can directly interface to electronic media such as memory cards, or to image capture devices such as digital cameras, scanners; some printers are combined with a scanners and/or fax machines in a single unit, and can function as photocopiers. Printers that include non-printing features are sometimes called multifunction printers (MFP), multi-function devices (MFD), or all-in-one (AIO) printers. Most MFPs include printing, scanning, and copying among their many features.

Impact Printers:

These printers use the electromechanical mechanism, that is pressing a typeface against an inked ribbon, Which marks it on a paper and prints the characters. Egs: DMP (Dot Matrix Printer), Drum and Daisy-Wheel printer. Impact printers can further be divided into two types:

   a) Line Printers which print one line at a time eg. drum and chain printers whereas

   b) Character printers print one character at a time.

Drum Printer: It has a rapidly rotating cylindrical drum on which characters are embossed in a standard 132-column printer. There are 132 bands containing every possible character.

There are 132 hammers lifting the paper-and -ribbon drums. Logic circulating in the printer makes a hammer(which is electromagnetically operated) strike against the drum as the required character to be printed passes through. Likewise, the sequence continues and a line is printed on the paper.

Chain Printers: The mechanism of printing is slightly different as in a drum printer. The drum rotates vertically in circular motion and there is a set of characters for every hammer. In a chain printer, the chain is the unit on which letters are embossed and it moves horizontally in a circular motion against the hammers. The mechanism of hammers striking against the chain is almost similar. A chain consists of five or six sets of characters. whenever a character which is required to be printed passes in front of the hammer position, the hammer strikes and the character impression gets printed on the paper. the paper and ribbon both move in between the chain and hammer.

Character Printers:

They print a character at a time. Examples are the dot matrix printer and the Daisy Wheel printer.

Dot-Matrix Printer:

Unlike the drum and chain printer which have letters embossed with the hammer striking against it, dot-matrix printers generally have seven or nine impact pins. these pins fire or strike under the control of a printer logic through an electromagnetic motion. the complete unit comprises of impact pins and coils - called the printer head.

The paper on which the printing is done moves up and down depending upon the model of the printer. The printer head moves left to right or vice-versa, permitting a 5x7 - matrix character for a 7-pin head and a 7x9 matrix character for a 9-pin head. The speed of the dot matrix printers is noted in CPS (characters per second). These printers come in widths of 80 columns and 132 columns.

Daisy-wheel Printers:

This is a character with a daisy wheel on which characters are embossed. There is a hammer against the wheel. the paper movement is similar to the dot-matrix printer. The presence of one hammer ensures that one character is printed at a time in Daisy-Wheel printer.

Non-Impact Printers

These printers do not use an electro-mechanical printing head to strike against ribbon and paper. They use thermal, chemical, electrostatic, laser beam or inkjet technology for printing. Examples are thermal, inkjet and laser printers. these printers are faster than the impact printers.

Inkjet Printers:

They print characters by spraying small drops of ink onto paper. A special ink with high iron content is used. the droplets of ink are charged electrically after being lead onto a nozzle. The droplets are then guided to thin proper positions on paper by electrically-charged deflection plates. These are high-quality printers and are therefore better than impact printers.

Laser Printers

Laser printers use light beams to form images on paper using a toner ink as a medium. A beam of light strikes the exposed parts of the drum surface. The light beam gets electrically charged and exposed areas attract the toner ink particles. The toner particles are then deposited on and permanently fixed to the paper using heat or pressure. Laser printers are quiet workers. they produce very high-quality output both textually and graphically.

Thermal Printers

These printers use heat to make a mark on heat-sensitive paper. the print head contains needles which are pressed against the paper. On applying heat to the selected pins, the paper changes colour to form a pattern of dots.

Output Devices : VDU (Visual Display Unit)

The job of the output device is to bring the result of computation to the outside world. The output devices accept data in binary form from the computer, converts the coded data into human readable form, and then the converted result is displayed as output.

All computers are connected to a television like screen, called the monitor. Monitor includes the graphic card or the video adapter, which is an expansion card which sends electric signals to the monitor. The monitor is connected by a cable to the video card. A device driver uses the operating system to control the video card, to make it send the right signals to the monitor.

Cathode Ray Tube (CRT)

The first computer monitors used Cathode ray tubes (CRTs), which was the dominant technology until they were replaced by LCD monitors in the 21st Century.

The first computer monitors used cathode ray tubes (CRT). Until the early 1980s, they were known as video display terminals and were physically attached to the computer and keyboard. The monitors were monochrome, flickered and the image quality was poor. In 1981, IBM invented the Color Graphics Adapter, which could display four colors with a resolution of 320 by 200 pixels. They introduced the Enhanced Graphics Adapter in 1984, which was capable of producing 16 colors and had a resolution of 640 by 350.

CRT remained the standard for computer monitors through the 1990s. CRT technology remained dominant in the PC monitor market into the new millennium partly because it was cheaper to produce and offered viewing angles close to 180 degrees.

Liquid Crystal diaplay (LCD) Screens :

LCD screens are flat and soft. An example is the TFT screen. It contains no cathode ray tube. It produces a sharp and high resolution image.

There are multiple technologies that have been used to implement Liquid Crystal Displays (LCDs). Throughout the 1990s the primary use of LCD technology as computer monitors was in laptops where the lower power consumption, lighter weight, and smaller physical size of LCDs justified the higher price verses a CRT. Commonly, the same laptop would be offered with an assortment of display options at increasing price points (active or passive monochrome, passive color, active matrix color (TFT). As volume and manufacturing capability have improved the monochrome and passive color technologies were dropped from most product lines.

TFT is a variant of liquid crystal display (LCD) which is now the dominant technology used for computer monitors.

Input Device: Mouse

In computing, a mouse is a pointing device that functions by detecting two-dimensional motion relative to its supporting surface. Physically, a mouse consists of an object held under one of the user's hands, with one or more buttons. It sometimes features other elements, such as "wheels", which allow the user to perform various system-dependent operations, or extra buttons or features that can add more control or dimensional input. The mouse's motion typically translates into the motion of a cursor on a display, which allows for fine control of a graphical user interface.

1. Mechanical mouse : It is a hand held pointing device. A mechanical mouse has a rotating ball on its base. It is rolled over a flat surface or on a mouse pad. The cursor on the screen moves in the direction of mouse's movement. Two rotating wheels placed at right angles to each other inside the mouse detect the direction of movement. Each wheel is connected to a shaft encoder,which limits electrical pulses for every incremental movement of wheel. The pulse transmitted by the mouse determines the distance moved.

There may be two or three buttons on a mouse. The button on the left is for selecting items on the screen and the button on the right is normally used for displaying and selecting pop-up menus. 

2. Optical Mouse : The optical mouse has a light beam instead of a rotating ball to direct movement across a specially-patterned mouse pad. the optical mouse use LEDs and photo detectors to trap its movements. Optical mice make use of one or more light-emitting diodes (LEDs) and an imaging array of photodiodes to detect movement relative to the underlying surface, rather than internal moving parts as does a mechanical mouse. A Laser mouse is an optical mouse that uses coherent (Laser) light.

3. Inertial and gyroscopic mouse : Often called "air mice" since they do not require a surface to operate, inertial mice use a tuning fork or other accelerometer to detect rotary movement for every axis supported. The most common models (manufactured by Logitech and Gyration) work using 2 degrees of rotational freedom and are insensitive to spatial translation. The user requires only small wrist rotations to move the cursor, reducing user fatigue or "gorilla arm". Usually cordless, they often have a switch to deactivate the movement circuitry between use, allowing the user freedom of movement without affecting the cursor position.

4. 3D mouse : Also known as bats, flying mice, or wands, these devices generally function through ultrasound and provide at least three degrees of freedom. Probably the best known example would be 3DConnexion/Logitech's SpaceMouse from the early 1990s.

In the late 1990s Kantek introduced the 3D RingMouse. This wireless mouse was worn on a ring around a finger, which enabled the thumb to access three buttons. The mouse was tracked in three dimensions by a base station. Despite a certain appeal, it was finally discontinued because it did not provide sufficient resolution.

Input Device : Scanner

In computing, an image scanner—often abbreviated to just scanner— is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. Common examples found in offices are variations of the desktop (or flatbed) scanner where the document is placed on a glass window for scanning. Hand-held scanners, where the device is moved by hand, have evolved from text scanning "wands" to 3D scanners used for industrial design, reverse engineering, test and measurement, gaming and other applications. Mechanically driven scanners that move the document are typically used for large-format documents, where a flatbed design would be impractical.

Modern scanners typically use a charge-coupled device (CCD) or a Contact Image Sensor (CIS) as the image sensor, whereas older drum scanners use a photomultiplier tube as the image sensor. A rotary scanner, used for high-speed document scanning, is another type of drum scanner, using a CCD array instead of a photomultiplier. Other types of scanners are planetary scanners, which take photographs of books and documents, and 3D scanners, for producing three-dimensional models of objects.

Another category of scanner is digital camera scanners, which are based on the concept of reprographic cameras. Due to increasing resolution and new features such as anti-shake, digital cameras have become an attractive alternative to regular scanners. While still having disadvantages compared to traditional scanners (such as distortion, reflections, shadows, low contrast), digital cameras offer advantages such as speed, portability and gentle digitizing of thick documents without damaging the book spine. New scanning technologies are combining 3D scanners with digital cameras to create full-color, photo-realistic 3D models of objects.

In the biomedical research area, detection devices for DNA microarrays are called scanners as well. These scanners are high-resolution systems (up to 1 µm/ pixel), similar to microscopes. The detection is done via CCD or a photomultiplier tube (PMT).

Scanners are input devices which are capable of recognizing marks or characters. They are capable of entering information directly from the computer without the user keying it in. They are fast and accurate. the two major types of scanners are OCR (Optical character Reader) and MICR (Magnetic Ink character Reader)

OCR (Optical Character Reader)

These scanners are capable of detecting alphabetic and numeric characters. If the characters are handwritten, they should be of standard size and there should be no stylish loops in the letters and lines. The characters should be properly connected if hand-written or in characters of a special font called OCR, if typed. OCR devices examine each character as if they were made up of a collection of minute spots.

OMR (Optical Mark Reader)

These scanners are capable of recognizing a pen or pencil mark made on a paper. An OMR scanner can sense the presence and absence of a mark. When you appear in objective-type tests or examinations in future, where you have to mark your answers by filling up a square box or a circular shape with a pencil, out of all the square/circular shapes provided, to indicate your choice of answer - you will know that these types of answer sheets are to be fed directly in to the computer to be scanned by an OMR.

MICR (Magnetic Ink character Reader)

MICR was developed to assist the banking industry to process a large number of cheques everyday. The bank identification code and the customer's account number are pre-printed with a special ink on every cheque. the ink contains magnetizable particles of iron oxide. a magnetic ink character reader reads these characters by examining their shapes with the help of a matrix, and sends this information to the system.

Input Device: Digitizer

A graphics tablet (or digitizer, digitizing tablet, graphics pad, drawing tablet) is a computer input device that allows one to hand-draw images and graphics, similar to the way one draws images with a pencil and paper. These tablets may also be used to capture data or handwritten signatures. It can also be used to trace an image from a piece of paper which is taped or otherwise secured to the surface. Capturing data in this way, either by tracing or entering the corners of linear poly-lines or shapes is called digitizing.

A graphics tablet (also called pen pad or digitizer) consists of a flat surface upon which the user may "draw" or trace an image using an attached stylus, a pen-like drawing apparatus. The image generally does not appear on the tablet itself but, rather, is displayed on the computer monitor. Some tablets, however, come as a functioning secondary computer screen that you can interact with images directly by using the stylus.

A digitizer or tablet is a surface over which a stylus (similar to a pencil) or hand cursor is moved. The location of the hand cursor or stylus is available to the computer system. The size of the tablet, a square block, varies from 10 inches to 5 sq. ft. depending upon application. The stylus senses a position through a transducer (pressure-sensitive switch) so that the movement of the stylus over the tablet causes a corresponding line on the CRT screen.

Input Device : Joystick

The joystick is an input device used for playing games. It is a stick which can be moved lefr, right, forward and backward. The movements are sensed by a potentiometer. As the stick moves, the movements are translated into binary instructions with the help of electrical contacts in its base.

A popular variation of the joystick used on modern video game consoles is the analog stick.

The joystick has been the principal flight control in the cockpit of many aircraft, particularly military fast jets, either as a center stick or side-stick.

Joysticks are also used for controlling machines such as cranes, trucks, underwater unmanned vehicles, wheelchairs, surveillance cameras and zero turning radius lawn mowers. Miniature finger-operated joysticks have been adopted as input devices for smaller electronic equipment such as mobile phones.

The name "joystick" is thought to originate with early 20th century French pilot Robert Esnault-Pelterie. There are also competing claims on behalf of fellow pilots Robert Loraine, James Henry Joyce and A. E. George. Loraine is credited with entering the term "joystick" in his diary in 1909 when he went to Pau to learn to fly at Bleriot's school. George was a pioneer aviator who with his colleague Jobling built and flew a biplane at Newcastle in England in 1910. He is alleged to have invented the "George Stick" which became more popularly known as the joystick.

Input Device:Light Pen

A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's CRT TV set or monitor. It allows the user to point to displayed objects, or draw on the screen, in a similar way to a touch screen but with greater positional accuracy.

A light pen is fairly simple to implement. Just like a light gun, a light pen works by sensing the sudden small change in brightness of a point on the screen when the electron gun refreshes that spot. By noting exactly where the scanning has reached at that moment, the X,Y position of the pen can be resolved. This is usually achieved by the light pen causing an interrupt, at which point the scan position can be read from a special register, or computed from a counter or timer. The pen position is updated on every refresh of the screen.

The light pen is a photosensitive pen. It is also a poinnting device, capable of sensing a position on the screen. When a light pen's tip is moved over the screen surface, its photocell sensing element detects light coming from the screen. Corresponding signals are sent to the processor that identifies the point (pixel) on the screen. A light pen is used to draw images onscreen.

Input Device : Keyboard

The following functions are carried out by input devices:

1. They accept data from the outside world.

2. They convert this data into binary form acceptable to the machine.

3. They send data in binary form to the computers for further processing.

A keyboard looks like a typewriter. It enables one to enter data into a computer. Computer keyboards are similar to electronic-typewriter keyboards but contain additional keys. The keys on computer keyboards are often classified as follows:

  1. Alphanumeric keys - Letters and numbers

  2. Punctuation keys - comma, period, semicolon and so on.

  3. Special keys - function keys, control keys, arrow keys, caps lock key, etc.

In normal usage, the keyboard is used to type text and numbers into a word processor, text editor or other program. In a modern computer, the interpretation of key presses is generally left to the software. A computer keyboard distinguishes each physical key from every other and reports all key presses to the controlling software.

Keyboards are also used for computer gaming, either with regular keyboards or by using keyboards with special gaming features, which can expedite frequently used keystroke combinations. A keyboard is also used to give commands to the operating system of a computer, such as Windows' Control-Alt-Delete combination, which brings up a task window or shuts down the machine. It is the only way to enter commands on a command-line interface.