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.

  

• 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.