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Computer System



  1.  Input Devices: Through these devices user communicates (interact) with the systems like
    • Keyboard for text-based input,
    • Mouse for position-based input,
    • Scanners & Webcams for image-based input and
    • Microphone for voice-based input

2.  Processing Devices: Central Processing Unit also known as CPU, Processor or microprocessor.

  • Contains several million of transistors.
  • Transistors are like switches which could be” on” or “off” i.e., 1s or 0s.
  • The processor is like the brain of the computer.
  • It consists of three functional units:
  • Control unit (CU): CU controls the flow of data and instruction to and from memory, interprets the instruction and controls which tasks to be executed and when.
  • Arithmetic logic unit (ALU): Performs arithmetic operations such as +, -, /, * and logical comparison :<, >, <=, >=, =, <>.
  • Registers: The CPU also contains high speed working storage areas called registers, which are for temporary storage. They are: Accumulators, Address Registers, Storage Registers & Miscellaneous
    • Known as registers
    • Memory within CPU
    • Fastest
    • Very small


  • Smaller
  • Faster memory
  • That stores data and program which is used by CPU frequently.
  • Faster due to its location



  • This is read write memory
  • Information can be read as well as modified.
  • Volatile in nature i.e., information is lost as soon as power is turned off.



  • This is read only memory
  • Information can be read but not modified
  • Nonvolatile (contents remain even in absence of power)
  • Generally used by manufacturers to store small amount of information


  • Primary memory is volatile in nature, and in addition it has less storage capacity.
  • Secondary storage also known as external storage, Auxiliary Storage, Mass Storage or Storage are non-volatile and are available in bigger sizes.
  • The features of secondary memory devices are
    • non volatility,
    • greater capacity,
    • greater economy
    • Slow speed.
  • They differ in speed, cost, portability, capacity and type of access.
  • Examples: USB Pen Drives, Floppy drive, Hard Drive, CD, DVD, Blue ray Disks and Smart cards.



  • If a computer has less RAM than needed to run a program, Operating system (e.g., Windows) uses virtual memory to compensate.
  • When RAM runs low, operating system moves data from RAM to Virtual Memory.
  • Virtual Memory is not a separate device, but an imaginary memory area, supported   by some OS.
  • OS combines computer’s RAM with temporary space on the hard disk. This temporary space used to help RAM is called Virtual memory.


Output devices

These are the devices through which Computers communicate with the user. Information shown on a display device is called soft copy, while outputs through printers are called hard copy output.


  • Textual output comprises of text characters that are used to create words, sentences, and paragraphs.
  • Graphical outputs are such as drawings, charts, photographs, and animation.
  • Tactile output such as raised line drawings may be useful for some individuals who are blind.
  • Audio output is any music, speech, or any other sound.
  • Video output consists of visual and audio output.



It is defined as a set of instructions that tell the hardware what to do.

  • Without software, the hardware would not be functional.
  • Software can be broadly divided into two categories:  Operating Systems Software and Application Software.


Operating Systems Software


  • An Operating System (OS) is a set of computer programs that
    • manages computer hardware resources
    • provide a platform for running application software and
    • acts as an interface with computer applications programs.
    • Some prominent Operating systems used nowadays are Windows 7, Windows 8, Linux, UNIX, etc.


Activities executed by Operating systems:

  • Performing hardware functions: Operating system acts as an intermediary between the application program and the hardware.
  • User Interfaces (UI): An important function is to provide USER INTERFACE.

Which could be Text based as in DOS or can be Graphical User Interface (GUI) as in windows.

·Hardware Independence:

Programmers need not understand inner working of hardware.  The same program can run on different hardware as far as the Operating System is same.

Thus, OS gives us hardware independence.

·Memory Management:

OS allows to control how to access and maximize available memory and storage. Some OS also provides virtual memory by allocating an area of hard disk.

·Task management:

allows optimum utilization of resources.

this facilitates a user to work with more than one application at a time i.e.  multitasking and

allows more than one user to use the system at a time i.e., timesharing.

  • Networking Capability: It provides capability to connect computers and form a network. E.g., Linux & Windows 8

·Logical access security:

Provides logical security by Identification and authentication e.g., User ID and Password.

It also provides security control through logs.

  • File management: The OS keeps track where each file is stored and who can access it and how to retrieve the file.

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