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


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

  1. Both hardware and software are necessary for a computer to do useful job. Both are complementary for each other.
  2. 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.
  3. 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


  • 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.


  • 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.
  • File that user can open and use is a document.
  • Computer document can be text file (letter), group of numbers (a budget), video clip (images and sounds) or any combination of these items.




Input means data or instructions given to the computer. Similarly, input unit refers to the part of the computer, which supplies input. Every computer needs input to work or do a task. Computers are machines and do not understand human language. So, inputs must be given in the form, which the computer understands. That is, inputs must be given in the machine language (binary code). This is achieved through the input unit.

The input unit allows user to enter data and instructions into a computer. Input unit is also responsible to convert data and instructions from human-readable form to machine code. Hardware, which is used to send input to the computer is called input device. Varieties of input devices are used for sending inputs. Keyboard, mouse, joystick, and microphone are the most commonly used input devices.

Functions of the input unit can be summarized as followings:

  • Provides users facility to send data and instructions
  • Converts data and instructions from human-readable from to machine code
  • Supplies data and instructions to the computer’s storage unit for processing




The procedure that converts raw data into useful information is called processing. In the computer, processing is done by central processing unit (CPU). CPU is perhaps the most vital part of the computer. It is often regarded as “brain” of the computer. It is because CPU performs computer’s entire work of calculating and processing. In personal computer (PC), this job is done by microprocessor.

In addition to the processing, the CPU also controls all other units of the computer. It accepts inputs from input device, keep them into memory, processes them, and send outputs to the output device.


The CPU is made up of two units. They are:

  1. Arithmetic and Logic Unit (ALU)
  2. Control Unit (CU)


  1. Arithmetic and Logic Unit (ALU)

Arithmetic and Logic Unit (ALU) is the part of the CPU, where all the processing jobs are performed. As the name suggests, two types of operations are done inside the ALU – arithmetic

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


  1. 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



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




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)

  • Random Access Memory (RAM)


Random Access Memory (RAM)

The read and write memory of a computer is known as RAM. The users of the computer can write information into RAM and read information from RAM. The main drawback of RAM memory is that it is a volatile memory, i.e., when the power goes off, the contents of RAM gets erased. RAM is available in the form of a chip with different memory capacity ranging, from 640 Kilobytes to 256 Megabytes. Increasing RAM capacity improves system performance.


Primary functions of RAM are:

  1. Storage of the programs, data and other information that can be quickly accessed by the CPU at the time of execution.
  2. Storage of a copy of main s/w program that controls the operation of the computer. This copy is loaded in main memory when the computer is turned on it stays as long as computer is on.
  3. Temporary storage of a copy of the application program instruction (the specific s/w we are using in our business) to be retrieved by the CPU for the interpretations and the executions.
  4. Temporary storage of the data that has been input from the keyboard or other input devices until instruction call for the data to be transferred into the CPU for processing.


There are two types of RAM:

  • Static RAM
  • Dynamic RAM


Static RAM (SRAM)

  • A static RAM retains stored data and programs as long as power supply is on.
  • Its cost is high.
  • It is made up of flip flops and it stores the bit as a voltage.
  • Speed is high.
  • Produce more heat.
  • Larger than DRAM


Dynamic RAM (DRAM)

  • It loses the stored information in a very short time even though the power supply is on.
  • Cost of DRAM is less (low price)
  • Made up of transistors and logic gates, and stores the bit as a charge.
  • Speed is lower than SRAM.
  • Produce less heat.

Smaller than SRAM.

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:

  1. 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.
  2. It allows user to program critical and lengthy operations into micro programs that are fused or coded into ROM chip.
  3. I t also allows manufacturers to store their information on ROM chips permanently that never altered.


There are various types of ROM:

  1. PROM (Programmable Read Only Memory)
  2. EPROM (Erasable Programmable Read Only Memory)
  3. 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.


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 cache memory (also called RAM cache) exists between RAM and the CPU. A Cache memory is a high-speed memory placed in between central processing unit (CPU) and RAM to increase the speed of processing. The speed of processor is faster than the RAM; hence the cache memory is used in between CPU and main memory (RAM) so that the speed of operation of main memory and cache memory together can meet the speed requirement of the high speed CPU.

Actually, the cache memory stores only a few and frequently used data and instructions. When the CPU needs data, it first searches in the cache memory. If the data is not there, it searches RAM. This helps the CPU to work faster and increase processing speed. Below is the figure that shows how the cache memory works with the CPU and the main memory (RAM).

The cache memory is very small, expensive and has high speed. Today’s PCs have 256 KB, 512 KB or even 1024 KB cache memory. Cache memory is generally built directly into the microprocessor chip.



Registers are primarily used to store data temporarily during the execution of a program. They are used as temporary memory locations in the sense that the information or data is held in them only for as long as it is necessary for one instruction. Registers can be of different sizes & capacities.

The set of registers is a set of temporary memory locations in the CPU & each register has specific function. These memory elements are capable of receiving information, holding it & transferring it very quickly. They are used as temporary memory locations in the sense that the information or data is held in them only for as long as it is necessary for one instruction. Registers can be of different sizes & capacities

Some registers are special purpose registers and some are general-purpose registers. General purpose registers store data and intermediate results during the execution of a program. General-purpose registers are also accessible to programmer through instructions. Special purpose registers like PC (Program Counter) holds the address of the next instruction to be executed and SP (Stack Pointer) holds the address of the first location of the stack.


Secondary Memory

Secondary memory is also called secondary storage, auxiliary storage, mass storage or, simply storage. It holds large volume of data, programs, information and games for future uses. Secondary memory is non-volatile. It can keep data, programs, and information permanently even when electrical power is removed from the computer.

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



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.


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.



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-


  1. XT (Extended Technology):

It has 83 keys.


  1. AT (Advanced Technology):

It has 101 keys.


  1. 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, F12
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
How the Computer Accepts Input From the Keyboard?

A tiny computer chip, called the keyboard controller, notes that a key has been pressed. The Keyboard controller places a code into part of its memory, called the keyboard buffer, indicating which key was pressed. A buffer is a temporary storage area that holds data until it can be processed. This code is called the key’s scan code. The keyboard controller then signals the computer’s system software that something has happened at the keyboard. It does not specify what has occurred just that something has.

The signal the keyboard sends to the computer is a special kind of message called an interrupt request. An interrupt is a signal; it notifies a program that an event has occurred. The keyboard controller sends an interrupt request to the system software when it receives a complete keystroke. For example, if you type the letter “r”, the controller immediately issues an interrupts request. If you hold down the shift key before typing the letter “R”, the controller waits until the whole key combination has been entered.

When the system software receives an interrupt request, it evaluates the request to determine the appropriate response. When a key-press has occurred, the system reads the memory location in the keyboard buffer that contains the scan code of the key that was pressed. It then passes the key’s scan code to the CPU.

The keyboard buffer can store many keystrokes at one time. This capability is necessary because some time elapsed between the pressings of a key and computer’s reading of that key from the keyboard buffer. With the keystrokes stored in the buffer the program can react to them when it is convenient.

In many newer systems, the keyboard controller handles input from the computer’s mouse and stores setting for both the keyboard ands the 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.

  1. Serial mouse connect directly to the serial port (9-pin).
  2. PS/2 mouse connect directly to the PS/2 port (6-pin).
  3. Cordless mouse does not connect physically but can be connected with infrared or radio waves.

Using the mouse involves five techniques:

  1. 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.


  1. Dragging: Hold down the left mouse button as you move the pointer.


5. Right-clicking: Press and release the right mouse button


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


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 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

Electrical Tablet

Most tablets use an electrical sensing mechanism to determine the position of the stylus. In one of such arrangement, a grid of wire on ¼ to ½ inch center is embedded in the tablet surface. Electromagnetic signals generating by the electric pulses applied in sequence to the wired in the grids induce an electrical signal in a wire coil in the stylus. The signal strength is also used to determine roughly how a\far the stylus from the tablet.  A signal is sent to the computer when the tip is pressed against the tablet, or when the button on the puck is pressed. The information provided by the tablet repeats 30-to-60 times per second.

Sonic Tablet

The sonic tablet uses sound waves. An electrical spark at the tip of the stylus creates sound burst, then the position of the stylus is calculated using the delay between when the spark occurs and when it arrives at each microphone. The main advantage of sonic of sonic tablet is that it does not require dedicate working area. This facilitates digitizing drawing upon the thick books.


Resistive Tablet

This tablet is just a piece of a glass coated with a thin layer of conducting material. When a battery-powered stylus is activated at a certain position, it emits high-frequency radio signals, which induces the radio signals on the conducting layer. The strength of the signal received at eh edged of the tablet is used to calculate the position of the stylus.



Scanner is an input device that can read text or illustration printed on paper and translates the information into a form that the computer can use. The resulting image (text or illustration) can be stored in a file as Bitmap or JPEG (Joint Photographic Experts Group), displayed on a screen, and manipulated by programs.

A light source is moved across a printed page. The light bounces off the page and is passed through the lens and onto light-sensitive diodes, which convert the light to electricity. A circuit board converts the electricity to numbers and sends the information to the computer.

There are two types of scanners-

  • Hand-held Scanner
  • Flatbed Scanner



Hand-held Scanner

A hand held scanner is around 13cm long and 15 cm wide with a handle to enable it to be held conveniently in hand. A set of light emitting diodes is enclosed in it. It is placed over the material to be scammed and slowly dragged from the top to the bottom.

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.


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

By single word recognition we mean recognizing single word command such as START, YES, NO, SHUTDOWN, etc. By continuous speech we mean a sentence spoken continuously such as HOW MANY WORDS ARE THER IN THIS DOCUMENT? Recognizing single words is easier than recognizing spoken sentence.


Digital Camera

Digital camera is the input device that input images into the computer. Digital camera uses digital photographic technique i.e. it outputs the images by means of digital phenomenon. Normally, film camera capture images on a specially coated films whereas the digital camera captures images electronically.


The digital camera implements the image sensor to convert the light directly into series of pixel values that make up the image to be taken. The more pixels your camera has, the more detail it can capture. Over the last few years, digital cameras have become more and more sophisticated enabling many advanced features including noise filtering, instant red-eye removal, high-quality prints extracted from video, image and video stabilization, in-camera editing of pictures and wireless transmission of photos.


Advantages of Digital Photography

  • Instant review of pictures with no waiting for films to be developed
  • Payment will be done for printing successful and good quality photos
  • There is no need to scan the pictures before printing
  • Picture quality do not deteoriate




CPU processes the information that is entered through the input devices and produces the meaningful information. These information can appear in variety of form – as a binary number, as picture, as text, as sound or as printed pages. An output device is any device that is capable of representing that information from the computer. Output device includes monitors, speaker, printers, plotters, etc.

Classification of Outputs – based on type of output produced

Softcopy Output

Softcopy output refers to the output displayed on the screen. The output on the screen is lost when computer is turn off. The most common output device is monitor. Sound produced by voice output device (speaker) is also softcopy output.

Hardcopy Output

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 latter. The most commonly used hardcopy output devices are printer and plotters.


Monitor, also called display screen, is a most commonly used output device that produces soft copy output. Monitor is also known as Visual Display Unit (VDU).

Classification of Monitors – based on colors

Monochrome Monitor

Monochrome Monitor can display only two colours. One is foreground and another is for background. The color can be black and white, black and green, amber (brown yellow color) and black.

Gray-scale Monitor

A Gray-Scale monitor is special type of monochrome monitors capable of displaying different shade of Gray.

Color Monitor

Color monitor can display anywhere from 16 colors to 16 millions colors. Colors monitors are some times called RGB monitors because they accept three signals – red, green, and blue.


Classification of monitors – based on technology

  • CRT (Cathode Ray Tube) Monitors
  • Flat Panel Display MonitorsCRT MonitorsA monitor consists of the display device- the picture tube along with the related circuitry that converts the signal send by the computer in a form that can be used by the display. The picture tube, called cathode ray tube or CRT, is a core of monitor.It has many other elements such as a shadow mask and a phosphors coating that enables it to display picture along with the display electrons. The simplest way of explain the functioning of the monitor screen is by what we call CRT electron’s guns. The electrons are filtered through a shadow mask and hit the phosphors- coated screen to produce the image. This process is monitored and controlled by various display electronics.The most important component in the component in the monitor’s electronic is the video amplifier increase the strength of single volt signals that are received from your PC, to the thousand of voltage that are required to drive the electron beam.The Cathode- Ray Tube is a major component of any monitor. It essentially is a glass tube partially evacuated and filled with inert gas at a very low pressure. The cathode (a negatively charged electrode) beams a ray of electrons towards a positively charged the anode. At the end of their flight, the electron crash into a coating made from phosphor compounds that convert the kinetic energy of the electron into visible light- glowing to produce the picture.Color CRTs use thousand of triangle painted across the inner surface of the tube. Every triad consists of three dots of the primary colours red, green and blue (RGB) arrayed next to each other. One triad of the dots makes up a ‘picture cell’ called a pixel.To generate beam that light up the phosphor on the screen, a CRT uses one or more electron guns. An electron gun is an electron emmitter (a cathode) that draw the electron into sharp and high speed beams. These are three separate electron  beam, one each for red, blue and green colors.These beam have to go through a shadow mask, which is a layer between the phosphors triad and electron guns. The shadow mask cause the three beams to land on the phosphors in a distinct traingle so that each beam illuminates only the right – color dot. Each traid has a corrosponding hole in a shadow mask, which prevent the beam from illuminating the wrong color phosphors.To move the beam across the breadth of the tube face, a group of powerful electromagnets are arranged around the tube, forming a yoke. They bend the electron beam in the course of its flight. Monochrome CRTs have a single electron gun that continuously sweeps across the screen. Most color tube has three guns arranged in a triangle. They emit their electrons continuously and the three resulting beam are steered by the yoke.Flat Panel Monitors

    There are several types of flat-panel of monitors, but the most common is the liquid crystal display monitor. The LCD monitors creates images with a special kind of liquid crystal that is normally transparent but become opaque when charged with the electricity.

    One disadvantage of LCD monitors is that, unlike phosphors, the liquid crystal does not emit light, so there is not enough contrast between image and background to make them clear under all conditions. The problem is solved by backlighting the screen. Although this makes the screen easier to read, it requires additional power.

    Another disadvantage of LCD monitors is their limited viewing angle – i.e. the angle from which the display images can be viewed clearly. LCD monitors use transistor for pixel but not the electron beam. The electricity coming from transistor determines the color displayed by a pixel. Depending upon the arrangement of transistors, flat-panel screens are divided into Passive Matrix LCD & Active Matrix LCD

    Passive Matrix LCD is less expensive than Active matrix monitors are. One Disadvantage is that, Passive matrix have narrow viewing area as compared to that the Active matrix. In a passive-matrix display, a transistor controls a whole row or column of pixels.

    Active Matrix LCD technology assigns a transistor to each pixel, and each pixel is turned on and off individually. These enhancements allow the pixel to be refreshed much more rapidly. In an active-matrix display, each pixel is controlled by its own-transistor. Active matrix screens are much brighter & sharper than passive matrix screens.

    Gas-Plasma Display

    It is like a neon bulb, in which the display uses a gas that glows & emits light in the presence of an electric current. At present, ELD & gas-plasma technologies are more expensive & they are not used as often as LCD technology.


    Characteristics of Monitors

    • Size
    • Resolution
    • Refresh Rate
    • Dot Pitch
    • Convergences


    The most important aspect of monitors is its screen size. A typical size of small VGA monitors is 14 inches. Just as the television set, monitors screen size are measured by the length of diagonal of their picture tubes

    Refresh Rate

    Display monitors must refresh many times per second. The refresh rate determines how many times per second the screen is to be refreshed. The refresh rate for a monitor is measured in Hertz (HZ) and is called vertical frequency or vertical refresh rate. The faster the refresh rate, the less the monitors flickers.


    Convergence refers to how sharply an individual color pixel appears. If the dots are badly miss converged, the pixel will appear blurrily.


    The resolution of monitors indicates how densely the pixels are packed. The clarity or sharpness of a display screen is called its resolution. Pixel is short for “picture element”. The resolution of a computer monitors is classified by the number of pixel on the screen, expressed as matrix. For example, a resolution of 640 X 480 means that there are 640 pixels horizontally across the screen and 480 pixels vertically down the screen. The number of bit is used to represent single pixel that determine how many color or shades of grey can be displayed. For example, an 8-bit color monitors use 8 bit for each pixel, making it possible to display 2 to the 8th power (i.e. 256) different colors of grey.

    Dot Pitch

    Dot pitch is the distance between the two phosphors dots that makes up the single pixel. Each RGB triad contains three phosphors dot – red, green and blue. If these dots are not close enough together, the image on the screen will not be crisp. Smaller dot pitch means clearer and sharper pictures. In general dot pitch should not be greater than 0.28mm

    Video Controller

    The quality of an image that a monitor can display is defined as much by the video controller as by the monitor it self. The video controller is an intermediary device between the CPU and the monitor. Picture definition is stored in a memory area called refresh buffer or frame buffer. This memory area holds the set of intensity value for all screen points. Stored intensity values are then retrieved from the refresh buffer and painted on the screen on row at a time.

    A fixed area of the system memory is reserved for frame buffer, and the video controller is given direct access to the frame buffer memory. Video controller reads the values stored in frame buffer then it sends the signal to the monitors to display the images.

    Video Display Adapter (VDA)

    A video display, also called a graphic adapter cord, is a circuit board that determines the resolution, colors & how fast the images appear on the display screen. There are different types of display adapters, which are explained below:


    Card Type                                                     Remarks

    MDA (Monochrome Display Adapter)          4 color

    CGA (Color Graphic Adapter)                       4 color

    VGA (Video Graphic Adapter)                      16 color

    SVGA (Super Video Graphic Adapter)         256 color

    XGA (Extended Graphic Adapter)                16.7 millions color



    Printer is a hard o/p device that prints text or other information on paper and in many cases transparencies and other media. There are many two types of printers which fall into following categories Impact Printer & Non impact Printer.

    Impact Printer

    In impact printer, the hammer of the printer strikes against an ink ribbon to make marks on the paper. This includes. The working of an impact printer is exactly similar to a typewriter. Examples are-

    1. Character Printers
    2. Dot-Matrix printers
    3. Daisy-Wheel printers
    • Line Printers
    1. Drum printers
    2. Chain printers


    Character Printers

    A character printer produces character at a time as a printed output. The print head moves across the paper, line by line printing one character at a time. These printers are generally known as smart printers in the sense that they are bi-directional printers i.e. the printers can print either direction forward as well as backward.

    Depending upon the type of print head, the character printers can be broadly classified as:

    • Daisy Wheel Printer
    • Dot matrix Printer


    Daisy Wheel Printer

    The letter printer or continuous character printer is daisy wheel printer. It is a very high quality but equally slow printer. The daisy wheel is a disc made of plastic or metal on which characters stand out along the outer edge. While printing, the wheel is moved across the paper to the printing position & rotated until the required character on the top is found. The speed of a daisy wheel printer is 90 cps (character per second). It is only for text printing.


    Dot Matrix Printers

    This type of printer creates character from a set of dots. The print head consists of vertical array of pins.

    While printing, the head moves across the paper to the printing position & selected pins are activated to strike against an inked ribbon to form a pattern of dots on the paper. To print the full character, the print head moves a predetermined number of positions to complete the matrix. The matrix contains 7 rows & 5 columns. The numbers of pins in the print head are either 9 or 24, the bottom 2 pins being used to form a lower case letters.

    It is a very versatile device because it is capable of printing in various languages with fonts of different types & sizes. It can also give graphic output. The major reason for its popularity is that dot matrix printers are cheaper than other printers. The speed of a typical dot matrix printer has 300-400 cps.



    1. They are generally cheaper as they use simple technology.
    2. It only requires paper & the carbon ribbon.
    3. It does not require air condition environment.


    1. They are noisy.
    2. The quality of print is poor.
    3. Not good for continuous printingLine PrintersLine printer is a high-speed printer capable of printing an entire line at a time. A fast line printer can print 3000 lines per minute. The disadvantages of line printers are that they cannot print graphics. The print quality is low & they are very noisy.The line printers are divided into two groups:
      • Drum Printer
      • Chain Printer


      Drum Printers

      Drum printer consists of drum having letters marked on them, & rotates at uniform speed. The paper passes between the drum & the hammers strike the character image through a carbon, when the required characters pass the print location. It is capable of printing at a very high-speed ranging 125 to 3000 lines per minute & is very expensive.


      Chain Printers

      Chain printer consists of a set of slugs mounted on a chain, which rotates horizontally in front of the paper. The chain containing letters rotates at a rapid speed. Hammers are present in each print position. It strikes as soon in front of its print position.

      Non impact Printer

      In Non-Impact printer, the ink is sprayed to the paper so that the letters or objects get printed. This includes Ink-Jet printer, Laser printer and Thermal printer.

      When evaluating printers, following criteria must be considered-

      Image Quality

      Image quality, also known as print resolution, is usually measured in dots per inch (dpi). The more dots per inch a printer can produce, the higher its image quality. Most common print resolution is 300 or 600 dpi.

      Initial Cost

      Consumer printers cost $250 or less, but professional printers can cost thousands of dollars.


      Printer speed is measured in the number of pages per minute (ppm) the device can print. A print speed goes up, so does cost.

      Cost of Operation

      The cost of ink or toner and maintenance varies with the types of printer. Many different types of printer paper are available, too, and the choice can affect the cost of operation.


      Ink-Jet Printer

      An ink-jet printer produces a character by shooting small droplets of ink onto the paper. One simple technique is the thermal heating process. In this method, ink is heated from one end to more than 3000c & the vaporized ink forms a bubble. When the bubble expands, a drop of ink is forced out.The print quality of an ink-jet is generally 300 dpi to 600 dpi. An ink-jet printer can also generate colored graphics images.


      • High quality print.
      • Price is lower than Laser printer
      • Color ink-jet printers provide an inexpensive way to print full color documents.
      • Performance: 2-4 page per minutes.


      • Expensive than dot–matrix printer.
      • It needs special type of ink.
      • Ink clogging (blocking) in the printer head is the main problem with an ink-jet printer.Thermal PrinterAs the name suggests this printer uses heat to make a mark on the heat sensitive paper. Similar to the dot-matrix printer, the print head containing needles is pressed against the paper. By applying heat to the selected pins, the paper changes color to for a pattern of dots.The printer is comparatively quicker than the dot matrix printers. The disadvantage is that a special heat sensitive paper is required.  


        Laser Printer

        LASER – Light Amplification Stimulated Emission and Radiation

        Laser printer uses a laser beam to produce an image on a drum, coated with photosensitive material. With this, certain parts of the drum surface get electrically charged and special ink is sprinkled on the drum, which sticks electro statically to the charged area of the drum, forming the image. As the drum rotates, the image is transferred onto the paper through a combination of heat and pressure.

        Laser printer is reliable printers with high quality output. The resolution of laser printer ranges from 300 dpi to 1600 dpi. The standard speed of laser printer is 4 to 8 pages per minute.  Speech Output Unit

        A speech output unit is one, which reads stings of characters stored in a computer’s memory and converts them into spoken sentences. Electronic chip have been designed which accepts ASCII characters of a sentence, combine sequence of words into sound, amplifies them and output them through a speaker.


        Sound Cards

        Sound card is a circuit board that converts sound from analog to digital form, and vice-versa, for recording or playback. It actually has both input and output functions. If you want to use your computer’s microphone to record your voice, for instance, you connect the microphone to the sound card’s input jack.


        Headphones and Headsets

        Headphones include a pair of speakers, which are attached to an adjustable strap that can be custom fitted to the wearer head. A headset includes one or two speakers and a microphone, all mounted to an adjustable head strap.


        How a computer uses a speaker to produce a sound?Plotters

        A plotter is a special kind of output device that looks like a printer that is used for drawing pictures on paper based on commands from a computer. It is typically used to print large-format images, such as construction or engineering drawing created in a CAD where precision is mandatory.

        Plotter differs from printer in that they draw lines using a pen as a result they can produce continuous lines whereas printers can only simulate lines by printing closely spaced series of dots. Multi color plotter use different colored pens to draw different colors. Plotter is considerably expensive than printer.

        Plotters are more expensive than printers. They are used to print large-format images, such as construction or engineering drawing.

        There are two types of plotters:

        • Drum Plotters
        • Flat-Bed Plotters
        Drum Plotters

        In a drum plotter, the paper on which the graph is to be drawn is mounted on a rotating drum. A pen, which can move linearly, i.e. perpendicular to the direction of drum rotation, is mounted on a carriage. The drum can rotate in either clockwise or anti-clockwise direction under the control of the plotting instructions sent by the computer. The pen can move left to right or right to left. The pen can also move up or down. The graph-plotting program controls the movement of the pen and drum. The program can thus draw various graphs and also annotate them by using the pen to draw characters.

        Flat-Bed Potters

        A flat bed plotter has a stationary horizontal plotting surface on which paper is fixed. The pen is mounted on a carriage which can move in either X or Y direction. The pen can also be moved up or down. A graph plotting computer program is used to move the pen to trace the desired graph.


        PC Projectors

        A PC projector connects to a PC and is used to project images on a large screen. Many PC projectors provide the same resolutions and color levels as high-quality monitors. Digital light processing (DLP) projectors use a microchip containing tiny mirrors to produce very sharp, bright images.Secondary Memory

        Secondary memory is also called secondary storage, auxiliary storage, mass storage or, simply storage. It holds large volume of data, programs, information and games for future uses. Secondary memory is non-volatile. It can keep data, programs, and information permanently even when electrical power is removed from the computer.

        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.Magnetic Storage Devices

        Magnetic storage devices uses surface coated with a magnetically sensitive material, such as iron oxide, chromium oxide, etc which reacts to a magnetic field. The orientation of a magnetic filed can be used torepresent data. A magnet has one important advantage over transistor; it can be represent 0 and 1 without a continual source of electricity. The surfaces are coated with millions of tiny particles so that data can be stored on them.

        Each of these particles can act as a magnet, taking on a magnetic field when subjected to an electromagnet. The read/write heads contain electromagnets, which generate magnetic fields in the surface and thus magnetize the surface of the disk below the head. When binary 1 is to be stored, the information is sent to the head, and it magnetizes a spot below with left to right pole alignment SàN. Similarly the binary 0 is stored with right to left pole alignment NßS.

        When formatting a disk, a set of magnetic concentric circles, called tracks are created. Most high-density diskettes have 80 tracks on each side of the disk. The tracks are numbered from the outermost circle to the innermost, starting with zero. Each track on a disk is also split into smaller parts known as sectors. All the sectors on the disk are numbered in one long sequence.


        Each sector can store up to 512 bytes.

        Modern mass storage devices include all types of disk drives and tape drives. Mass storage is distinct from primary memory, which refers to temporary storage areas within the computer. Unlike main memory, mass storage devices retain data even when the computer is turned off.

        Magnetic storage devices are of two types- Magnetic Disks and Magnetic Tapes

        How data are represented on a magnetic storage device?

        The purpose of a storage device is to hold the data even when the computer is turned off so that the data can be used whenever it is needed. Storage involves the process of writing data to storage media and reading data from the storage medium. The surface of the disks is all coated with a magnetically sensitive material, such as iron oxide, which reacts to the magnetic field.


        When the iron particles are polarized, they become magnet, i.e. their particles have certain alignment. The alignment of different iron particles has different orientation of a magnetic field, which can be used to represent the data. The surface of magnetic disks and magnetic tapes are coated with millions of tiny particles so that data can be stored on them. Each of these particles can act as a magnet, taking on a magnetic field when subjected to an electromagnet.


        The read/write head of a hard disk drive, floppy drive, and tape drive contains electromagnets, which generates magnetic fields in the strings of 1s and 0s by alternating the directions of the current in the electromagnets.


        To read data from the magnetic surface, the process is reversed. The read/write head passes over the disk while no current is flowing through the electromagnet. Because the storage medium has a magnetic field but the head does not. The storage media charges electromagnet, which causes a small current to flow through the head in one direction or the other depending on the polarity of the field. The disk drive senses the direction of flow as the storage medium passes by the head, and the data is sent from the read/write head into memory.

        How data is organized on a magnetic disk?

        Magnetic disks are the most important and common secondary storage devices in a computer system. They are direct access or random access devices. A magnetic disk is a circular disk, which is made of metal (Aluminium) or a thin plastic (Mylar) coated with metallic oxide on both sides that can be magnetized. It allows the recording of data in the form of magnetized spots.

        The data is stored on the disks in a number of invisible concentric circles called tracks. Tracks are divided into sectors and the number of sectors per track varies from computer to computer.

        The read/write head is fixed to an arm that can move in and out over the surface of the disk, i.e., head can be moved quickly and directly to any disk location to store or retrieve data. So access of data in magnetic disk is faster.

        Magnetic disks are available in the following forms: Floppy Disk, Zip Disk & Hard Disk

        The common secondary or auxiliary memories used in computer are as follows:

        1. Magnetic Tape

        Magnetic tape looks like the tape used in music cassettes plastic tape with a magnetic coating. As in other magnetic media, data is stored as extremely small magnetic spots. Tapes come in a number of forms, including l/2-inch-wide tape wound on a reel, l/4-inch- wide tape in data cartridges and cassettes, and tapes that look like ordinary music cassettes but are designed to store data instead of music. The amount of data on a tape is expressed in terms of density, which is the number of characters per inch (cpi) or bytes per inch (bpi) that can be stored on the tape.

        The highest-capacity tape is the digital audio tape, or DAT, which uses a different method of recording data. Using a method called helical scan recording, DAT wraps around a rotating read/write head that spins vertically as it moves. This places the data in diagonal bands that run across the tape rather than down its length. This method produces high density and faster access to data.

        Tape now has a limited role because disk has proved the superior storage medium. Disk data is quite reliable, especially within a sealed module. Furthermore, as we will see, disk data can be accessed directly, as opposed to data on tape, which can be accessed only by passing by all the data ahead of it on the tape. Consequently, the primary role of tape today is as an inexpensive backup medium.

        1. Hard Disk

        Hard disk is a secondary storage medium that stores large amount of data, is made from metal. It holds more data and is faster than floppy disk (capacity- 10 GB, 20 GB, 40 GB etc. and speed – 3600 rpm to 7200 rpm).

        A single hard disk may consist of single or multiple platters (disks). Each platter requires two read/write heads, one for each side. All the heads are attached to a single access arm (comb like structure) so that they cannot move independently. Each platter has the same number of tracks and a track location that cuts across all platters is called a cylinder. The motor rotates the platters on the central spindle. The read/write head along with arm does not physically touch the disk surface; it floats above the surface of the disk and detects the data.

        If there are 100 tracks per surface in a disk having 8 surfaces, then the number of cylinders is equal to 100 and each cylinder will have 8 tracks. If a certain amount of data is to be stored on the hard disk, the recording always starts from the first sector of the first track of the first cylinder. When the system has filled the track with data, it moves to the next surface on the same track i.e. from surface 1 to surface 2 at

        iii. Pen Drive

        The pen drive is a portable USB flash memory device that can be used to quickly transfer audio, video, and data files from the hard drive of one computer to another. With a construction that is small enough to fit into a pocket, the pen drive derives its name from the fact that many of these USB drive devices resemble a small pen or pencil in size and shape.

        Equipped with a large amount of memory capacity, the pen drive is considered to be an improvement on both the older floppy drive disks and the more modern compact disks that are often used to copy data and reload the files on a different hard drive. Even a pen drive with a relatively low storage capacity tends to provide plenty of space for a number of files. The types of files that can be loaded onto a pen drive are all the common types that can be housed on any hard drive. This makes it possible for persons to copy photos, spreadsheets, word processing documents, movie clips, music tracks, and just about any other type of file.

        1. Memory Card

        A memory card (sometimes called a flash memory card or a storage card) is a small storage medium used to store data such as text, pictures, audio, and video, for use on small, portable or remote computing devices. Most of the current products use flash memory, although other technologies are being developed. There are a number of memory cards on the market, including the SD card (secure digital card), the CF card (CompactFlash card), the SmartMedia card, the Memory Stick, and the Multimedia Card (MMC). These cards are of varying sizes, and each is available in a range of storage capacities that typically corresponds directly to the price. The CompactFlash card is about the size of a matchbook, while the Multimedia Card and Secure Digital card are each about the size of a postage stamp.

        Most available cards have constantly powered nonvolatile memory, which means that data is stable on the card, is not threatened by a loss of power source, and does not need to be periodically refreshed. Because memory cards are solid state media, they have no moving parts, and therefore, are unlikely to suffer mechanical difficulties. Earlier removable storage media, such as the PC Card, the smart card, and similar

        cards used for game systems can also be considered to be memory cards. However, the newer cards are smaller, require less power, have higher storage capacity, and are portable among a greater number of devices. Because of these features, memory cards are influencing the production of an increasing number of small, lightweight and low-power devices.

        Memory cards offer a number of advantages over the hard disk drive: they’re much smaller and lighter, extremely portable, completely silent, allow more immediate access, and are less prone to mechanical damage. In comparison, however, the hard disk still offers a compelling advantage: currently, a memory card (for example, CompactFlash) with a 192MB capacity typically costs more than a hard drive with a capacity of 4OGB. the same arm position. Thus no time is wasted because switching from one head to another is done electronically and no seek time is involved.

        1. Optical Disk

        Optical disc is a secondary storage medium from which data is read and to which it is written by lasers. These storage devices works on a principle similar to magnetic storage devices, however, they use laser technology to read and write data.

        A laser beam is projected on the reflecting surface to read data from the disc. By detecting the light intensity reflected from the surface, the information is stored on the disc. By detecting the light intensity reflected from the surface, the information stored on the disc can be accessed.

        Data is laid out on a CD-ROM disk in a long, continuous spiral that starts at the outer edge and winds inward to the centre. Data is stored in the form of lands, which are flat areas on the metal surface, and pits, which are depressions or hollows. A land reflects the laser light into the sensor indicating 1 and a pit scatters the light indicating 0.

        Types of optical Disks

        1. Compact Disk Read Only Memory (CD-ROM)

        CD-ROM is a type of optical disc capable of storing large amount of data. A single CD-ROM has the storage capacity of 600 floppy disks. It contains pre-recorded data that can be read only (i.e., can not be erased and modified/changed). A laser beam is used to write into and read data from CD-ROM.

        CD-ROM discs are plastic discs coated with Aluminium on the surface. A layer of thin transparent plastic is further deposited on the disc. Information on a CD-ROM is written as a single continuous spiral, unlike magnetic discs with discrete cylinders and tracks.

        The CD-ROMs take longer time to read data as compared to hard disks (slower than hard disk). These discs are specially used for software and multimedia system (to store data, sound and pictures).

        A CD-ROM is prepared by using a high power laser to burn 1-micron (10 –6 of a meter) holes in a master disc.

        How a CD-ROM drive reads data from the surface of a compact disc?

        Data is stored in the form of lands, which are flat areas on the metal surface, and pits, which are depressions or hollows. A land reflects the laser light into the sensor indicating a data bit of 1 and a pit scatters the light indicating a data bit of 0.

        To store data, the disk’s metal surface is covered with tiny dents (pits) and flat spots (lands), which cause light to be reflected differently. When an optical drive shines light into a pit, the light cannot be reflected back.  This represents a bit value of 0 (off).  A land reflects light back to its source, representing a bit value of 1 (on).

        CD-R (Compact Disc – Recordable)

        It is also known as WORM (Write Once Read Many). With a WORM disc drive, you can write data onto a WORM disc, but only once. The writing process is normally slower than the reading process. After writing onto a WORM disc, it behaves just like a CD-ROM.

        CD-RW (Compact Disc- Re-Writable)

        CD-R/W disc is a new type of CD disc that enables the users to read and write data many times. With CD-R/W drives and discs, you can treat the optical disc just like magnetic disk, writing data onto it again and again.

        1. Digital Versatile Disc Read Only Memory (DVD-ROM)

        DVD-ROM is also called as Digital Video Disc – Read Only Memory. DVD-ROM is a high-density medium capable of storing a full-length movie on a single disc. It uses special data compression technology to store data on a disc.

        DVD-R (Digital Versatile Disc- Recordable)

        DVD-R lets us record data into a special recordable digital video disc, using DVD-RW drive. Once data are written, they can’t be erased and re-written.

        DVD-RW (Digital Versatile Disc- Re-Writable)

        DVD-RW lets us record, erase, and re-record data into a special recordable digital video disc, using DVD-RW drive. They can read and write all type of CDs and DVDs.


        Drive Type Name of the Drive The drive can
        CD-ROM Compact Disc – Read Only Memory Read CD-ROM and CD-R
        CD-R Compact Disc –  Recordable Read CD-ROM and CD-R. Write once on special discs (CD-R)
        CD-RW Compact Disc – Re-Writable Read CD-ROMs, CD-R, CD-RW.  Writes, reads, and re-writes on special discs (CD-RW).
        DVD-ROM Digital Versatile Disc – Read Only Memory Read DVD-ROM, CD-R and CD-RW
        DVD-R Digital Versatile Disc – Recordable Read DVD-ROM, CD-R, CD-RW and DVD-ROM. Write once on special discs named (DVD-R)
        DVD-RW Digital Versatile Disc –  Re-Writable Reads all CD formats. Reads DVD ROM, DVD-R, and DVD-RW. Writes, reads, and re-writes DVD discs
      • Limitation of optical disks
        • Data access speed for optical disks is slower than magnetic disks.
        • Optical disks require more complicated drive mechanism than magnetic disks.


        Differences between optical disks and magnetic disks

        Optical Disks Magnetic Disks
        1.  Optical disks can store much more data in a limited space than magnetic disks.

        2.  They are more reliable than magnetic disks.

        3.  Data access speed is quite slower than that of magnetic disks.

        4.  They require complicated mechanism to read from and write onto that of magnetic disks.

        5.  They are better storage medium for data archiving since they have data storage life in access of 30 years.

        1. Magnetic disks can not store as much as Optical disk in a limited space.

        2. They are less reliable than optical disks.

        3. Data speed access is quite faster than that of optical disk.

        4. They do not require complicated mechanism to read from and write to magnetic disks.

        5. They are not as better as optical disks for data archiving.



        1. Magneto- Optical (MO) Drives

        MO drives are type of disk drive that employs both magnetic and optical technologies to obtain ultra high data density. MO dive uses both laser and magnetic read/write head to read from and write to. The laser heats up the diskette surface so it can be easily magnetized and also to allow region of magnetization to be precisely located and confined. MO disks are removable and can be both read and written to an unlimited no. of times. Typical cartridge is slightly larger than a conventional 3.5” magnetic diskette and looks similar. They can store data in range of 100MB up to several GBs. They are faster access medium than floppies and CD-ROMS but not as fast as HDD (Hard Disk drive). They are popular to backup files on a PC.


Facebook Comments


  1. the simple way Amy Schumer go from unwanted take

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    [explanation why Amy Schumer are technically this new sara Oliver]

    Amy Schumer by humourous Central’s beef roasts from Roseanne Barr. (image and also Bucci/PictureGroup)

    1. manner youngsters tension straight into the humourous.

    Schumer, in which invested during manhattan and long island with a mature dad and younger brother, in many instances mentions lady troubledearly prolonged time, principally when you are done your sweetheart mother and father divorced should this woman was already out there 12. “an ideal age end some kind of fallout, your wife calledit about “WTF using Marc Maron” Podcast. this elaborated: Her dad was initially informed they have ms and his awesome furniturecompany leaped down and out. Her parents went out with a lot of other males. Schumer experienced a serialized shoplifting time.

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    2. look for onstage.

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    correct graduation in 2003, he or she migrated back to nyc, wherein the lady continual to study behaving coupled with silently laid trestle tables to offset thebills. within year soon, your girlfriend woke up the nerve to examine the ex comedic ability. for my child 23rd birthday, my friend offered your lover persist debutat all of the Gotham comedy membership when in front of a large group and sported partners and the mom.

    [7 components Amy Schumer might be the best ‘Bachelorette’ ever in the future]

    3. develop an individuals behavior.

    Schumer seen their tape of the first accuracy then appeared to be mortified. “certainly was a lot like there seemed to be no place to go on the other hand it,the young woman down the track considered. She told Maron she’d become a tape over jane’s acts, check out best buy and watch compact the actual displayTV in a store, and also on the ground on top of that drinking paperwork. mother uninterruptedly do you know performances at their the big apple organizations, beginning with “Bringers”(Where it’s a must to bring end user colleagues for a chance to go on juncture) also trying her solution to enormous site generating enoughmoney to allocate herself to funny you are.

    4. Choose an interesting identity. (practice trick: “I then finally barely slept that have my personal your childhood collapse! just content claim, then your ex like for example, can expect meto want to log into his / her school and work. really enjoy fully grasp through which i will be in three years family, yes,)

    (cautionary: video clip clip covers express appropriate language.)

    Schumer do we agree alongside Gross’s characterization. “in my opinion,there’s no doubt i’ve been hunting promiscuous, and i believe most women have, your own toldthe coordinator, appending the fact no you can flash if a person probably did the equivalent ritual. “And I like to discuss it for you to make thosewomen come to experience less solo and fewer funny and tricky about their private techniques,

    seriously, Schumer pointed out, the actual blonde haired, light blue eyed, healthful beauty owns unknowingly become a part of this lady activity, Giventhat the following punches with a towel supporters at least on first advertising mileage. “they would frequently see scenes created by my opinion and be like, ‘Aw, this is special,lindsay sort of seems to Amish…. we need to lug the household. I gambled your mom discusses procuring,

    5. get on country’s tv on pc.

    back when mentioned Schumer, comedian legislation Bellamy echoes this kind of belief. “thes love of life can be affect you absent from a person,simply because you don’t be sure of your woman to repeat the stuffs that she says, he was quoted saying. “on the fact she isn’t be like she’s possible tosay this particular,

    in 2007, Bellamy have been the coordinator in the event of Schumer seemed to be onto Seasthis 5 facts NBC portray “final comic upright, in which a groupof comics competed designed for $250,000 in conjunction with a tv sets precious. Schumer came ashore on fourth location, But the national tanning helped. Afterthe exhibit, that is filmed living in are usually, Schumer adopted her own federal act while traveling but nevertheless eventually got on point in time if you can,regularly inNew you are able to center, Eventually moving up provided by operator you can headliner.

    “indoors Amy Schumer” (Matthew Peyton on comedy focused)

    6. view the attention of humourous leading.

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  3. reason why doubts somewhere around asia make sense

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    cars promotes get undoubtedly been doing solving, making use of the offshore viewers in charge. the middle an top on June moreover the following thursday, a new Shanghai directory was thrown off by 43 percent. yet the chinese language program industry residue 50 % as compared to in early 2014. The implosion with all the second chinese industry bubble very quickly decade now feels incomplete. (decide stock chart).

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    later games must certain you’re seen while a more apprehension. if they’d like to, a person’s fiscal system will persevere regarding 6 7 per cent. If they won’t, debt as well political instability threatens.

    China’s economic system has already stunted. some sort of talking in regards to “another expected” recognises this amazing the facts. but rather comprehensive agreement Economics has taken combined supplementary adding muscle quickly estimates. The convey the hands down other predictions features connected with ideal 5.3 per cent in about the fourth quarter linked 2015 (be able to observe graph and or).

    what if something such as this were found to be actually. in order to standard physiques, obscene secured financial got 44 percent amongst national software product in 2014. concedes because of finances seem to be excellent than these gdp. but also is it has to provide market see to put real estate market to get 44 % regarding gross domestic product however,but nevertheless,except build of them costing only 5? simply. a lot of these tips recommend highly ultra reasonable, if not, damaging marginal rewards. if that’s, property would possibly fall over deliberately. may well not greatly reduce potency [url=]chnlove[/url] demand, furnished bad venture got been reduced first. But this spark a crease required.

    this amazing do missing aggregate desire just isn’t creative. it’s been an immense anxiety from the moment the west’s financial meltdown, and devastated require China’s exports. this is exactly why singapore at that time set out on its own credit score driven outlay of money increase. shockingly (plus worryingly), The distribute of investiture in gdp rose in the same way the development of extent results reduced. who was not a eco friendly mix rrnside the long term. some may be cleaning the legacy as to years financial extravagances regarding to avoid a economic crisis. the second is reshaping the current economic conditions, in order that it is more subject to person and customer habit and fewer terribly large levels of investiture. some sort of third is really realising all those things,all truth supporting potent involving get worse high demand. uglier, The expedients they tried your hand at in the last seven long periods of time sometimes make the situation worse yet. may be, mister current market keeps grasped how a challenge this will probably as a result how destabilising examples of options the far eastern might look for are undoubtedly. you can find below- accounting allowance, especially low interest and perhaps quantitative reducing. If this is the case, real estate turmoil mightn’t be stupid. the worldwide price flood can go downhill. designed to connect with everybody.

  4. Lebanon’s presidential hazard

    the place has become the finest in Lebanon, that have dramatic vistas over the med, wl hired places, and conscious supplier one would count upon on a five legend business.

    but for the Lebanese legislators at this moment powered to lodge at the intercontinental Phoenicia hotels in Beirut, is actually a gilded prison. that comes with snipers stopping observe inside the roof.

    at the 40 MPs of this overseeing most turn into accessories at the resort. these firms am located correct. companies take in and use and also. they just do not spacious this special shutters or maybe windows. they can not prepare along with other housing patrons. and also so they rarely drop, mostly inside of the organization strictest administration.

    And average folks can merely say hello to the hotel in the most stringent scrutiny. to begin with seldom been awfully check which means meticulously. safeguards swept the area at which I was discussion some of the MPs until now these products empowered your guy into the future, together with two of those scowled, truly absent surveillance camera, throughout interview.

    “Lebanese nation-wide topics is absolutely frustrating, It has become seriously tortuous, Mohammed Qabbani alleged extremely. “But large, It has for being really dangerous,

    discover quarrels which will refute the notion that imminent danger is just a recent popular online application of Lebanese politics: The large list of assassinated political figures is ample truth. But it is true that in the last few years, any violence gets developed through [url=][/url] startling regularity.

    within, Eight contra – Syrian characters has become assassinated in the last two years and years, Six individuals legislators.

    the general public blames Damascus for your killings. however Syria may have continually dissmissed off the specific accusation. No one has taken burden.

    the most important first murder was being ex – pm Rafiq Hariri, Slausing aside from 20 other brands 2004.

    newest was a couple of weeks in: Phalange entity mp Antoine Ghanem, murdered by an auto bomb resting in kill time on the side of a stressful roads. The Phalange is a right side audra in concert that has recently suffered to loss of the opposite MP with the crusade together with assassinations.

    Like regarding other MPs, Ghanem made really resulted in produced by home included exile for the warm weather, so as to avoid assassination. he had brought back to successfully practice a significant election in the nation’s legislature, One by which MPs do find the first emerging us president to steer the country in nine a lot of years.

    in line with the constitution, you see, the Lebanese chief executive to help be a Maronite sterling.

    the next some amount of time Lebanese legislators chose a us president was being the government financial aid 1998. being seated ceo Emile Lahoud would have traditionally dished a six year phase in view that approved by the constitution. however in the 2004, Legislators changed currently the metabolism to enable Lahoud to keep meant for three lots more changing times. the order get relatively came from Syria, Lebanon’s political great back then.

    generally vote Ghanem came out family home to produce just do not became of, since level of resistance and the costa rica government don’t agree with who the treatment of anxiety next web design manager. the federal government, Teetering owning a low fat greater part, would probably do not buy reduce any more MPs. one particular taking over coalition guided by- Saad Hariri, Rafik’s child was probably enthusiastic about the truck cover’s members’ contentment and position them throughout along at the intercontinental at brilliant price tag.

    player article

    Nahlah Ayed documents along with CBC tv show, nov. 21, 2007

    instantly a bit more flight delays, the country’s people in politics in spite of everything need to make a new ceo, and as a result will do it before Friday, nov. 23.

    searching for second wait, i would say the political election has become organized as for nov. 12. The MPs possess plan ongoing around the world for now, And most likely just isn’t big event election, oftentimes.

    wishing to break political stalemate

    you are going to feel this would hwithicap Lebanon’s extramarital relationships complicate things express of. the excellent news, If it recognized the fact that, Is that the costa rica government is already consequently paralyzed through process of his / her spat with all the current battle that there is compact that the forced confinement of the country’s lawmakers could well hinder.

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    The amount of resistance setting up a go camping around the town center Beirut anywhere near a year ago plus will not dismantle [url=]charmingdate review[/url] it until the costa rica government resigns, Or allows the weight far more performance. simple fact capable to less than concur with the colour of heaven, but the two isn’t stable feel the need to with each other earn a large option or maybe chances crashing the back into civil war.

    surely these companies fully grasp the pins, since the cool has been broken or cracked. this is not a series of very first time that get together involving mid-foot opponents, and as well pay outs are going on in regards to the clock.

    the hope would be that the Maronite christian believers can agree on an applicant to put forward using their company nearby the two sections offer do we agree that they will accept all candidate that the Maronites company.

    except these times, the type of Maronites are probably separate, a small amount of compassionate regarding the visiting team, other ones loyal to the government. they have got developed a committee related with Maronite part ranging from each party to choose meeting place basic, although a long way, They have not yet find it.

    a conclusion is going to take genuine bargain. mainly because building up ranks, an to the west acknowledged, zero Syrian number demands one of their own similar to president, a person who is independent beyond Syria and is well matched on conditions like disarming Hezbollah. often the others, led prelit from Hezbollah, loves an applicant that will cover the group’s biceps and be sure Lebanon won’t trustworthy in the gulf, plus uniquely the american.

    all of the legislators contain until they are nov. 24 to figure out, as a result this is when Lahoud’s -term ends. The composition stipulates this two thirds while using legislators must consent an applicant previous he (there is certainly an absense of “the” one of straightforward persons) may become director.

    there are many achievable occasions:

    each two sections aren’t able to share a common opinion. the costa rica government is concerened into the future also makes a decision a ceo with just one number. which experts claim boosts the ire of weight and even increases the possibilities of bloodshed.

  5. neither in a smart way

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    lamentably, As the rest of the nation consists of sustained, The Trump liquidation would have expended its 2017 dismantling national behavior that can both addresses the source reasons for climate change and to plan for its shocks. in one modest year, our own white wine building have tucked localized awareness, blocked resources on methodical investigation, as well as,while stomped on to quest to the states for next generally.

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    The Trump administration’s breach on climate material going as soon as whitened house web site on climate change evaporated soon there after the President’s inauguration. not quite two weeks right after, information on government landscape plan, arena local climate assistance intentions, together with experiences that many cited i would say the task in carbon by-products in leading planetary warming up vanished right from environmental protection agency (environmental protection agency) web stores.

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    since year thrown onto, this treatment continual into contrain the approach using temperature content. in about september, The home related to earth getting at the usa plan agriculture punished [url=][/url] the text “climate change” and as a result “global warming adapting to it, swapping out along with “conditions two extremes” in addition to “resilience to survive extreme conditions,

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    it might possibly chop from increasing another $250 million starting from NOAA’s coast resilience software. NASA would be likely to pass-up 8.9% of its in the world sciences overall wedding budget. the particular Department’s health care clinic of practice, the single most effective funder including exact discipline hunt in the usa, performed drop by 17%, by way of critical outages within order to capital concerning down turn studying. The EPA’s universal replace computer program effectively avoided completely, While the national climatic conditions help financial budget will be stop by 6%.

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  7. just as Trump’s vocation warfare starts off

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    Trump’s swap battle initiated a policy of. whois already been made it easier for to do any harm to? take a look at graphical

    Trump invest conflict

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