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COMPUTER ARCHITECTURE
Before knowing the internal architecture of computer system, what is computer?
A computer is an electronic device which is capable of receiving the inputs (data from the user),
storing it for a desired period of time, manipulating it according to the set of instructions (called
program) and producing the output to the user in desired form. It performs a variety of
operations in accordance to the set of instructions.
In another word, A computer is a programmable device which responds to a specific set of instructions
in a well-defined manner and executes a prerecorded list of instructions. A computer is an electronic device which takes input from the user, processes it, stores it and gives output in desired form. The computer as a system is a combination of hardware and software components that jointly offer the necessary services to the user. Computer is derived from the Latin word ‘ Computare ’ which means ‘to calculate’. Every computer is constructed by combining various parts. All these parts work in an organized way to perform calculations, organize data, and communicate with other computer systems. The design, arrangement, construction or organization of the different parts of a computer system is called as “ computer architecture ”.
Computer system architecture (Anatomy) is concerned with the structure of computer. In other words, the style of construction & organization of many parts of the computer system is called as “computer architecture”. Computer System Architecture consists of the various components such as: Input Unit Central Processing Unit (CPU) Memory Unit (MU) Output Unit
Fig: Block diagram of computer system architecture
CPU
ALU
CU
Input Unit Output Unit
Primary Memory
Secondary Memory
Unit – 2 Digital Computers and its Peripherals
HARDWARE AND SOFTWARE
Hardware refers to all the physical parts that make up a computer. That is, all the electric, electronic, and mechanical devices of the computer are hardware. Examples of hardware are keyboard, mouse, monitor, and printer. So, hardware is those parts of the computer, which we can touch.
Software is a set of electronic instructions that make computer perform tasks. In other words, software tells the hardware what to do. Without software, hardware is useless. Hardware needs instructions to process data into information. Those instructions are supplied from software.
Software is also referred to as a set of program. Computer uses varieties of programs. Some programs are developed for computer‟s own use and internal maintenance. Other types of programs are used for calculating and processing data.
Relationship between Hardware and Software
- Both hardware and software are necessary for a computer to do useful job. Both are complementary for each other.
- The same hardware can be loaded with different software to make a computer system perform different types of jobs, just as different songs can be played using same cassette player.
- Except for upgrades (like increasing the main memory and hard disk capacities, or adding speakers, modems etc.), hardware is normally a one-time expense, whereas software is a continuing expense. Just as we buy new cassettes for the newly released songs, or songs whose cassette we do not have, similarly one buys new software to be run on the same hardware, as and when need arises or funds become available.
USERS AND DATA
Users
People or computer operators. Some computer systems are complete without person‟s involvement; no computer is autonomous. Even if a computer can do its job without person, people still design, build, program & repair computer system. Data
Individual facts or bits of info, which by themselves may not make much sense to person. Computer reads and stores data of all kinds whether words, numbers, images, or sounds in the form of numbers. Computerized data, digital, means it has been reduced to digits or nos. Following instructions from software and users, the computer manipulates data by performing calculations, doing comparisons, or arranging the bits of info so they make sense to user. Data are organized into files.
operation and logical operation. Arithmetic operation means calculations like addition, subtraction, multiplication, and division on data. Similarly, logical operation means comparison of data like equal to, greater than, less than, and not equal.
All the operations in the ALU are done under directions of the control unit. Once data enter into memory, the data necessary for processing are transferred into the ALU. When processing completes, the data is again transferred back into the memory unit. This transfer of data from the memory unit to the ALU and vice-versa is also done under the directions of the control unit.
Main functions of the ALU are given below.
Carries out arithmetic operations such as addition, subtraction, multiplication, and division Performs logical operations such as comparison among data (equal to, greater than, less than and not equal to). Works under the direction of the control unit
b. Control Unit (CU) Control unit is the most vital part of the CPU. It reads every instruction stored into the memory. There after, it issues control signals necessary to execute the instruction. The control unit, actually, ensures that every instruction is read, understood, and executed in sequence.
The CU coordinates and controls all the parts of the computer. It is the nerve center of the computer. It coordinates and controls the interpretation, flow and manipulation of all data and information. It also controls the flow of data from input devices to memory and from memory to output device.
Main functions of the Control Unit are as here under :
Receives inputs send by input device, and store them into memory unit Bring instruction from the memory unit, and decode the instruction (interpret what the instruction is saying) Controls movement of data in and out of the ALU. Directs the ALU to process data Control the movement of data and instruction in and out of the memory unit Controls movement of processed data (information) to output device Directs output device to produce information Directs and controls reading and writing of data on storage unit
3. OUTPUT UNIT
When the ALU completes processing, it returns back the data into memory unit. The memory unit, thereafter, transfers the processed data towards output unit. Finally, the output unit shows the processed data in suitable form. Thus, function of the output unit is to present processed data (information) in human -readable form to the user.
Computer uses different types of hardware to show information. All those hardware are called output devices. Every output device is capable of presenting information in user- understandable form. The most
common output device is the monitor. Other output devices used in computer are printer and speaker. There are two types of output devices.
Softcopy Output devices: Softcopy output refers to the output displayed on the screen. The output on the screen is lost when computer is turned off. The most common output device is monitor. Sound produced by voice output device (speaker) is also softcopy output.
Hardcopy Output devices: Hardcopy output refers to recording letters, graphics or pictures on a permanent medium such as paper. Such output can be read immediately or stored and read later. The most commonly used hardcopy output devices are printers and plotters.
Some main functions of the output unit are:
Receives information (in binary form) transferred from the memory unit Converts the information from binary form into human-readable form Use a hardware to show the information to user
4. MEMORY UNIT (MU)
Memory unit is the area, where the computer keeps programs, files, data, and instructions. All these things are only stored temporarily. Memory unit is built inside the computer, so it is also known as internal memory.
The CPU needs data and instructions to operate the computer. These things must be stored in an
area somewhere inside the computer. When data and programs are not in use, they must be kept
permanently in another safe area. In computer, all these areas are called memory. Therefore,
memory is the area in the computer, which holds data, instructions or programs.
As input unit sends data or instructions, the computer at first keeps them into the memory unit. When the computer completes processing of data, it again keeps them into memory unit. When a user launches a program, it is also loaded into and run from memory unit. Thus memory unit is a space, which is allocated for temporary storage of data and programs. Memory is a part of computer (device) that is used to store data as well as instructions (programs). Physically, memory consists of chips either on the motherboard or on a small circuit board attached to the motherboard.
There are mainly two types of memory: Primary Memory and Secondary Memory.
Primary Memory
Primary memory is also called main memory. Primary memories are made up of semiconductor materials and consist of large number of 8-bit registers, arranged in a sequence (lines). A register can store eight binary bits. Primary memory is of two types:
Read Only Memory (ROM)
Conventional/ Regular DRAM
This is the oldest and the slowest type of DRAM technology. This type of DRAM is now quite obsolete and is not used on any new systems, having been replaced by Fast Page Mode (FPM) DRAM and other new technologies. This type of DRAM has been used in PCs since original PC days. It is said to asynchronous meaning that it is not synchronized with system clock.
Fast Page Mode (FPM) DRAM
This type of DRAM is only slightly faster than conventional DRAM. Before the introduction of EDO DRAM it was the traditional DRAM used in PCs. It is pretty slow with access time of 120 ns however it is also found in 70 ns versions. It is not much used today due to its slow speed but it is almost universally supported. It is also asynchronous type.
Extended Data Out (EDO) DRAM
Economical and most common type of asynchronous DRAM is EDO DRAM. This DRAM is improvement of FPM DRAM and slightly faster than FPM DRAM. By switching from FPM to EDO, one can expect a performance improvement of 3 to 5 percent.
Synchronous DRAM (SDRAM)
This type of memory is relatively new and different kind of DRAM for PCs which is faster since it runs “in sync” with computer system clock. By running at the same speed, CPU and SDRAM move data much more efficiently than is possible with other DRAM. Rambus DRAM (RDRAM) and Double Data Rate SDRAM (DDR SDRAM) are examples of this type of DRAM which are now commonly used in the newer computer systems.
Video RAM (VRAM)
Another type of DRAM that is used to handle graphical data and displays images on screen, so it is also called display or graphic memory. VRAM: mounted on display card, is quite fast, as it needs to respond quickly to the CPU‟s instructions and constantly refresh and update the screen image so that there is no flicker.
Read Only Memory (ROM) The read only memory (ROM) is a memory unit that performs the read operation only; it does not have write capabilities. The information stored in a ROM is made permanent during the hardware production and cannot be altered. ROMs are non-volatile memory, i.e. information stored in ROM is not lost even if the power supply goes off. So, ROMs are used for storing the programs to boot the computer handling the operating system and monitor program controlling a machine. They are slower than RAM. Actually, the ROM is built and assembled in the motherboard.
Typical uses of ROM are:
- It allows storing permanently routines that are to be executed after the computer is turned on. These routines contain information for booting up the computer and performing computer‟s diagnostics such as whether h/w devices are connected properly, whether rest of memory is functioning properly and so on.
- It allows user to program critical and lengthy operations into micro programs that are fused or coded into ROM chip.
- I t also allows manufacturers to store their information on ROM chips permanently that never altered.
There are various types of ROM:
a. PROM (Programmable Read Only Memory)
b. EPROM (Erasable Programmable Read Only Memory)
c. EEPROM (Electrically Erasable Programmable Read Only Memory)
PROM (Programmable ROM)
A PROM is a memory chip on which data can be written only once. Once a program has been written onto a PROM, it remains there forever and cannot be changed. They are manufactured as blank memory. To write data onto a PROM chip, you need a special device called a PROM programmer or PROM burner. The process of programming a PROM is sometimes called burning the PROM.
EPROM (Erasable Programmable ROM)
EPROM is a special type of memory that retains its contents until it is exposed to ultraviolet light for 10 to 20 minutes. The ultra-violet light clears its contents, making it possible to reprogram the memory. For erasing purpose, the EPROM chip has to be removed from computer.
EEPROM (Electrically Erasable Programmable ROM)
It is a special type of PROM that can be erased by exposing it to an electrical charge. The time required to erase this type of PROM is very short (few seconds). Unlike EPROM chips, EEPROMS do not need to be removed from the computer to be modified.
CACHE MEMORY
In computer, data moves between RAM and the CPU frequently. In fact, RAM is very much slower than the CPU. Hence, moving data between RAM and the CPU is a time consuming process. It makes the CPU often idle and waiting for data. A solution to this problem is a special type of memory called cache memory (pronounced cash ).
The secondary memory is used to store data, information and programs permanently. So, they are often referred as “storage memory”. The capacity of secondary memory is larger than the main memory. The two main categories of storage technology used today are magnetic storage and optical storage. The common secondary or auxiliary memories used in computer are floppy disks, hard disks and compact disk.
Followings are the functions of the memory unit:
Stores data and instructions sent from input units Stores data and instructions before they undergo processing (or store raw data) Stores data and instructions during their processing Stores data and instructions after their processing (or store useful information) Loads and unloads programs run by users Loads files opened by users
CHARACTERISTICS OF COMPUTERS
1. Speed : The speed of a computer is incredibly faster than what man can possibly record or calculate normally.The speed of computer for performing a single operation can be measured in terms of Milliseconds, Microseconds, Nanoseconds and Picoseconds.
1/1000 or (10-3) sec……………………………..1 Millisecond (ms)
1/1000000 or (10-6) sec………………………....1 Microsecond (μs)
1/1000000000 or (10-9) sec …………………….1 Nanosecond (ns)
1/000000000000 or (10-12) sec…………………1 Picosecond (ps)
1/000000000000000 or (10-15) sec……………..1 Femtosecond (fm)
2. Storage : One of man's failing is perhaps his inability to remember and "store" large volumes of information in his brain. The computer is capable of overriding this deficiency as it can store. The storing capacity is measured in terms of Bytes, Kilobytes, Megabytes, Gigabytes and Terabytes.
4 bits………………………..................................1 Nibble
8 bits………………………..................................1 Byte
1024 Bytes ............................................................1 Kilobyte (KB)
1024 Kilobytes.......................................................1 Megabyte (MB)
1024 Megabytes.....................................................1 Gigabyte (GB)
1024 Gigabytes .....................................................1 Terabyte (TB)
3. Word Length: A word is simply a fixed-sized group of bits that are handled together by the machine. The number of bits in a word (the word size or word length) is an important characteristic of a computer system. The majority of the registers in the computer are usually word-sized. The amount of data transferred between the processing part of the computer and the memory system is most often a word.The size of the registers, which sometimes is called the word length (or word size); indicate the amount of data with which the computer can work at given time. The bigger the word size, the more quickly the computer can process the set of data. Occasionally, you will hear people refer to “32-bit processors” or “64-bit processors” or even “64-bit computers”. This terminology refers to the size of registers in the processor. If the entire factors are kept equal, a CPU with 32-bit registers can process data twice as fast as one with 16-bit registers. 4. Accuracy and Reliability : Inspite of high speed, the computers are quite accurate and reliable in their calculations. The accuracy of operation of a computer is always 100%. Computer is only a machine and does not make error on its own. It is thus reliable. Computer can perform all the calculations and comparisons accurately. Sometimes, errors may be produced by computers due to the fault in the machine or due to mistakes in the programs. If the input data are not correct, this may also lead to incorrect output. 5. Automatic: Once the process has been initiated, computer is quite capable of functioning automatically. It does not require a prompt from an operator at each stage of the process. Computer is an automatic machine. Everything that is given to computer are processed and done by computer automatically according to the instruction provided. 6. Versatility : Computer has a wide range of application areas i.e., computer can do many types of jobs. It can perform operations ranging from simple mathematical calculations to highly complex and logical evaluations for any extended period of time. Some of the application areas of computers are Education, Science and Technology, Business, Medicine, Astronomy etc. 7. Diligence : Man suffers from physical and mental fatigue, lack of concentration and laziness which do not permit him to carry on his task at the same level of speed and accuracy through the entire day. The computer, on the other hand is capable of operating at exactly the same level of speed and accuracy even if it has to carry out the most voluminous and complex operations for a long period of time. A computer can perform repetitive tasks without being bored, tired and losing concentration. It can continuously work for several hours without human intervention after the data and programs are fed to it. There is no ageing effect on computer i.e. efficiency does not decrease over the years of use.
LIMITATIONS OF COMPUTERS
1. Sometime the failure in devices and programs can produce unreliable information. 2. Computer is a dull machine. It doesn‟t have intelligence on it. 3. It cannot operate without human involvement. 4. Computers lack emotions.
Software for CAI can be prepared in a variety of modes, such as tutorial, discovery, problem solving, modeling, and drilling and practice modes.
6. Health and Medical Field: Computers are widely used in hospitals to help doctors in diagnosis, getting information on patients, diseases, treatment, drugs, keeping patient‟s record etc. Examples: Magnetic Resonance Imaging (MRI), CT scan machine, Eye testing machines, Ultrasonic machine, etc. 7. Home: Home computers can be used in recreational activities. Home computers can also be used in storing personal information such as birth dates, addresses, telephone numbers, photo albums, to keep the record of family expenses etc. People use home computers to listen songs, watching movies, accessing the Internet, etc. 8. Industries: Computers are extensively used in industries for automatic control of machines, process, measurement and display of electrical and physical quantities. 9. Military: Smart weapons use microprocessor-based control systems. Modern tanks, torpedoes, missiles etc. employ computerized control systems. A guided missile uses internal computer for its control. 10. Multimedia: In recording and film studio computer are used intensively. The music can be recorder in different tracks and later merge to a single music. Movie editing can be done frame by frame. 11. Office: Computers are used in preparing reports, memorandum, and copy of advertisement, letter, publicity, contracts, forms, notes, and notices etc., which are the basic works of an office. Computer helps user in preparing, storing, retrieving and displaying text. In offices accounting, billing preparation of payrolls, data analysis, auditing, investment, inventory control, preparation of budgets, sales analysis etc. are also performed. In most of the computerized offices, word processing and spread applications are used. 12. Research: Scientific and engineering design and research work involve complex and massive computations. So computers are must for this type of work. 13. Simulation: Computer simulation involves typing to predict what will happen in a real-life situation from model of that situation. Computer Aided Design (CAD) is widely used in the design of electronic circuits, ships, roads cars etc.
INPUT - OUTPUT DEVICES
Input-output (I/O) devices are the primary means by which one can interact with computer. The computer devices are also called peripheral devices. Peripheral devices of computer can be divided into internal (installed inside the computer) & external (existing outside the computer but connected electronically to it). All the peripherals of the computer are connected to the ports. An input device converts incoming data & instructions into a pattern of electronic signals in binary code i.e. combination of 0s & 1s to a digital computer, whereas an output device reverses the processing, translating the digital signals into a form understandable to the user.
INPUT DEVICES
The raw data & program are entered in the computer through input device. It reads the raw data prepared by the user & sends then into the computer as a series of electronic pulses. The devices which, read the data and program into the computer, are called input devices, i.e., data and programs are entered into the computer system for processing through input device. An input device converts input data into suitable form acceptable to a computer. So, it is a means of communication between user and the computer. Examples of input devices are keyboard, mouse, joysticks, optical character reader, light pen, touch panel.
Keyboard
A keyboard is the primary & most user-friendly input device. Both data & instructions can be entered into the computer with the help of the keyboard. It is an essential device for interactive processing in the sense that the user can issue commands through the keyboard to the CPU & receive the response immediately on monitor. Computer keyboards are similar to typewriter keyboards but contain additional keys. There are separate keys for each number, letters & symbols. The data can be sent to the CPU either serially or in parallel. The keyboards therefore can be divided into two types: serial keyboard & parallel keyboard.
Serial Keyboard It sends the data bit by bit in a serial way. Thus, there is an only one line that carries data from keyboard to the CPU. A serial to parallel converter is required at the computer end to convert serial data into parallel.
Parallel Keyboard It sends all the bits of the data simultaneously on separate lines. Thus, there are as many data lines as there are bits in the data.
At microcomputer level, IBM personal computer has three different PC keyboards-
- XT (Extended Technology): It has 83 keys.
- AT (Advanced Technology): It has 101 keys.
- Enhanced Keyboard: It has 103 keys.
Alphanumeric Keys
Letters and numbers along with Tab, Caps Lock, Backspace and Enter
Modifier Keys Shift, Ctrl, Alt Function keys F1, F2, F3, F4, F5, F6, F7, F8, F9, F10, F11, F Numeric Keypad Ten digits (0-9) and mathematical operators (*, - , /, +, %, }, {, <, >, ?, etc) Cursor-Movement Keys Arrow keys, Home/End and Page Up/Page Down
Special-Purpose Keys
Insert, Delete, Esc, Print Screen, Scroll Lock, Pause, Start and Shortcut
Mouse
Mouse was invented by Douglas Engelbert of Standford Research Center in 1963 & pioneered by Xerox in the 1970's. It is hand held device with a roller on its base is moved over the surface. As the ball is moved over the surface in any direction, the motion is converted to digital values & is used to determine the direction & magnitude of the mouse's cursor on the screen. Mouse is one of the most widely used input devices of the computer. A mouse is a small had held device whose relative motion across the surface can be measured. Because mice are the relative devices, they can be picked up; move and then put down again without any changes in reported position. For this the computer maintains the current mouse position, which is incremented or decremented by the mouse movement. Mouse is a small plastic box with two or three buttons on the top and a ball at the bottom, which rolls on a flat surface. As the ball moves across flat surface (mouse pad), the visible indicator (i.e. pointer/cursor) on the screen, moves in the direction of mouse movement. We can select the commands, draw pictures, and edit text etc. by pressing the mouse button. The mouse is important for graphical user interface because we can simply point to options or objects & click a mouse button. Such applications are often called point & click programs.
There are basically three types of mice-
Mechanical Mouse Mechanical mouse has a rubber or metal ball on its underside that can roll in all directions. Mechanical sensors within the mouse detect the direction of rolling ball & move on the screen accordingly.
Optomechanical Mouse
Optomechanical mice are same as a mechanical mouse but uses optical sensors to detect motion of the ball. A ball rolls on two shafts. The shaft turns optical shaft-angle encoders to convert motions to electrical signals. This type of mouse is easier to clean as compared to clean a mechanical mouse.
Optical Mouse The optical mouse is used on a special pad having grids of altering light and dark lines. A LED on the bottom of the mouse directs a beam of light down onto the pad, from which it is reflected and sensed by the detectors on the bottom of the mouse. As the mouse is moved, the reflected light beam is broken each time a dark line is crossed. The number of pulses so generated, which is equal to the number of lines crossed, is used to report mouse movements to the computer.
Mouse can be connected to PC in one of the three ways.
- Serial mouse connect directly to the serial port (9-pin).
- PS/2 mouse connect directly to the PS/2 port (6-pin).
- Cordless mouse does not connect physically but can be connected with infrared or radio waves. Using the mouse involves five techniques:
- Pointing : Move the mouse to move the on-screen pointer.
- Clicking : Press and release the left mouse button once.
- Double-clicking : Press and release the left mouse button twice.
- Dragging : Hold down the left mouse button as you move the pointer.
- Right-clicking : Press and release the right mouse button
Trackball
A trackball is a pointing device that works like an upside-down mouse. A track ball is really a variation on the mouse. You rest your thumb on the exposed ball and your fingers on the buttons. The track ball is held stationary while the ball is manually rotated by hand in any direction. The track balls are intended for use where the desktop space is limited or not available. To move the pointer around the screen, you roll the ball with your thumb. The advantage over mice is that it is stationary so it does not require much space to use and it can be place on any type of the surface
Joystick
A joystick is a device consisting of a hand held stick that pivots about one end and can be moved left or right, forward or backward. It lets the user move an object on the screen. A potentiometer senses the movements of the stick. A potentiometer is an instrument for measuring or comparing the movement of mouse from original position. As the stick is moved around, the movements are translated into binary instructions with the help of electrical contacts in its base. Children can play with a set in two crossed grooves & can be moved left or right, forward or backward. A joystick is generally used to control the velocity of the screen cursor movement rather than its absolute position.
Light Pen Light pen is an input device that contains a photocell mounted in pen-shaped tube. When the pen is moved over the surface of the screen, it detects the light emitted from the screen and generates electric pulses. The pulses are transmitted to a processor that identifies the pixel (picture element) the light pen is pointing to. It enables a user to write words or draw pictures directly into the computer. It is useful for graphic work like CAD (Computer Aided Designing) package.
Because of the following drawbacks the light pens are not popular now-a-days:
It obscures the screen image as it is pointing to the required spot. The light pen cannot report the coordinates of a point that is completely black It gives sometimes-false reading due to background lighting. Prolong use of it can cause arm fatigue.
Digitizer
Digitizer also known as tablet is a flat surface ranging in size from about 6" X 6" up to 48" X 72"
or more, which can detect the position of a movable stylus (pencil-shaped device) or puck
(mouse-type device). The stylus has a pressure sensitive switch on its tip, which closes when the
stylus is pressed. The puck has thin cross-hair cursor for accuracy in digitizing drawing that
is placed on the tablet and push button for command entry.
A tablet digitizes an object detecting the position of the stylus or puck. They are used to scan
over the object and input the set of the discrete coordinate position. These positions can then be
joined with the straight lines segment to approximate the shape of an original object.
There are mainly three types of the tablets-
Electrical Tablet
Sonic Tablet
Resistive Tablet
Flatbed Scanner A flat bed scanner consists of a box with a glass plate on top and cover, which covers the glass plate. The document to be scanned is placed above the glass plate. The light beam is situated below the glass plate and is moved from left to right horizontally.
Bar Code Reader
Bar Code Reader is a photoelectric scanner that read bar code, an identification code printed on product containers that gives its value (price) and other information (quality, manufacturer etc.). Bar code is a printed series of parallel bars or lines of varying width that is used for entering data into a computer system. They are mostly used by supermarkets, bookshops etc.
The bars are typically black on a white background & their width & quantity vary according to applications. The bars are used to represent 0 & 1, sequence of which in turn can represent numbers from 0 to 9 & be processed by a digital computer. An optical scanner reads bar code information, which is apart of a computer system.
Magnetic Ink Character Reader
This method of input is very common in computerized banks. MICR reads cheques & deposit slips magnetically of the banking industry. The account number of the customer is pre-printed on the cheque using " magnetic ink ". Magnetic ink is a special type of ink containing iron-oxide particles, which is used to write characters. MICR reads the cheque first magnetizing the magnetic characters printed on the cheque & the sensing the signal induced by each passing character under a reading head.
The data so captured is fed to the computer system.
Optical Character Recognition
Optical character recognition is a process of recognizing pre-printed characters & distinguishes one character from other provided characters to meet OCR standards. OCR can also read characters produced by typewriters. There are many OCR front standards such as OCR-A, OCR-B etc. It can be observed that OCR reader can read alphabets, numbers & special characters that are printed or typed on the paper. An optical reader uses photoelectric device to scan the input. OCR is an input device, which is used to read an image, characters of different fonts printed on any paper. The printed characters are examined by passing them under a light and lens system. A light source converts the alphabets, number and special characters into electrical pulses, which are then sent to the computer for processing. If there is no dark spot it is represented by 0 and if there is a dark spot it is represented by 1.Such representation is called the bit map of the image. Nowadays, advanced OCR system can also read handwritten text.
Optical Mark Recognition
It is an input device that can detect the presence or absence of a pencil or pen mark on a paper. Light is shown onto the marked paper and the reflected light is observed. The presence of a mark is confirmed due to lesser light being reflected from that portion of the paper.
OMRs are mostly used for:
Objective type answer papers in examinations.
Order forms containing a small choice of items Market survey, population survey etc. Time sheets of factory employees in which start and stop time may be marked.
Touch Screen
Touch screens allow the user to point directly, usually to select from a menu of choices on the screen. Most touch screen computers use sensors in or near the computers' screen that can detect the touch of a finger. Touch screen accept input by allowing the user to place a fingertips directly on the computer screen. Touch screen are appropriate in environments where dirt or weather would render keyboards & pointing devices useless & where a simple interface is important. Many techniques have been used to make the screen sensitive-
Infrared screens employ light-emitting diodes (LED) and photo detector cells forming matrix of beams covering the screen in the horizontal and vertical dimensions. LEDs emit infrared light, and photo detectors receive it. When the user touches the screen, some light beams are interrupted, and the computer then senses the position of the finger. Capacitive screen uses a device, which can sense changes in capacitance when and where the user touches the screen with stylus or finger. In pressure sensitive mechanism, two layers of Mylar containing rows of invisible wires are separated by a small space. The sheets are placed in such a way that the wires run horizontally in one sheet and vertically in the other. When the user applies pressure on the screen, the wires at that point make contact and a circuit is closed. This is sensed and fed to the computer.
Disadvantages
Although touch screens provide a natural interface for computer novices, they are unsatisfactory for most applications because the finger is such as relatively large object. It is impossible to point accurately to small areas of the screen. In additional, it is tiring job to the arm for using it a long period.
Speech Input Devices A unit, which takes as its input spoken words, and converts them to a form, which can be “understood” by a computer, is called a speech input unit. Now, that sound capabilities are a standard part of computers, microphones are becoming increasingly important as input devices. A sound card can translate the electrical signals from the microphones into a digitized form that the computer can store & process. Sound cards can also translate digitized sounds back into Analog signals that can be sent to the speaker. There is also demand for translating spoken words into text, much as there is a demand for translating handwriting into text. Translating voice to text is a capability known as voice recognition. With it you can speak with computer rather than to type & you can control the computer with simple commands such as "shut down". Voice recognition software takes the individual sound in a language, called phoneme & translates into text or commands. By understanding we mean that the unit can uniquely code each spoken word and can interpret and initiate action based on the word. Giving a spoken command is much quicker than typing out such a command. Speech input unit is particularly useful in situation where commands are to be given to a remote computer using a telephone or when one‟s hand are not free. Speech input units may be classified as:
Single word recognition unit and Continuous speech recognition unit
