CH 04 : Operating Systems And Computer Architecture

Operating Systems

• ==Operating System(OS)==: It is a computer software running in the background when the computer is on, and performs various operations.
  • It is also responsible for the communication between hardware and software.
  • Operating systems make it alot easier to use computers.
• Some of the tasks an OS performs:
  • Multitasking
  • Multiprogramming
  • Error handling
  • Loading and running software
  • Memory management
  • Security (passwords etc)
  • Input and output control
• Windows and Linux are examples of OS.
• When a computer is turned on:
  • The initial starting programs are loaded from ==Read Only Memory(ROM),==
  • Different components of the computer are checked for errors,
  • The OS is loaded into the computer memory.

Interrupts And Buffers

• ==Interrupt==: It is a signal sent to the Processor from a device or software, to make the Processor ^^temporarily^^ stop its current operation and service another.

  • An Interrupt is sent when for example, a page is jammed in the printer, or when a software error occurs.
  • Interrupts allow computers to ^^multitask^^.
  • When an Interrupt occurs, the status of the currently running task is saved using an ==Interrupt Handler.== so that the processor can resume it after the Interrupt is serviced.

• ==Buffer==: It is a ^^temporary^^ memory area.

  • Buffers are filled from computer memory and emptied to the hardware, allowing the processor to perform other tasks in the meanwhile.

• Example to explain how Buffers and Interrupts are used together, while a printer is being used:

  

Computer Architecture

• ==Von Neumann Architecture==: A computer system which can store programs in a memory. The idea was developed by ^^John von Neumann^^.

  • Before Neumann, computers had to be fed data while they were running, so for every task the data had to be fed immediately before-hand.

• In a von Neumann system, a ==Memory Unit, Processor(containing a Control Unit)== and the hardware are connected through ==Buses== to perform different tasks.

  

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• ==Buses==: The connections which transport data around the system and also maintain data ^^synchronization^^.

  • ==Address Bus:== ^^Unidirectional^^ Bus carrying signal related to the addresses of the memory contents.
  • ==Data Bus:== ^^Bidirectional^^ Bus responsible for moving the data and contents of the memory around the system.
  • ==Control Bus==^^: Unidirectional^^ Bus carrying signals related to control and coordination of all activities in the computer.

• ==Registers==: High speed storage areas within the computer system, which are needed to store and ^^manipulate^^ data.

  • ==Memory Address Register(MAR)==: Address which contains the location of the memory contents.
  • ==Memory Data Register(MDR)==: Register which contains the content from memory.
  • ==Arithmetic Logic Unit(ALU)==: Register which allows the ^^calculations and logic operations^^ to be performed within the computer.
  • ==Program Counter(PC)==: Register which contains the instructions for the next task to be performed.
  • ==Current Instruction Register(CIR)==: Register which contains the instructions for the current task.

  

• ==Memory Unit:== It contains all the data to be processed and its addresses as well.

• ==Processor==: The part where data is processed and the necassary logic operations are carried out.

• ==Control Unit==: It controls the operation of the memory, processor and input/output devices.

  • Control Unit reads an instruction from memory and tells the other components how to function, through Control Bus.

• ==Input/Output Devices==: Ways of feeding data into a computer and/or getting it out.

Fetch-Execute Cycle

• To perform a task, instructions for the said task are ^^Fetched^^ from the memory and Processed before being ^^Executed^^.
• ==Fetch==:
  • Instructions for the next task are fetched from the PC and put into CIR through Data Bus.
  • PC is ^^incremented^^ by 1.
  • Instructions are sent to the Processor(through Data Bus) to be decoded.
• ==Execute==:
  • Processor sends the decoded instructions as ^^control signals^^ using the Control Bus, to other components of the system, so that the instructions can be executed.

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