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Operating System

Chapter 1: Introduction to Operating System 

— Definition of Operating System.

— Evolution of operating systems – simple batch systems, multi-programmed batch systems, time sharing systems.

— Functions of an operating system

— Characteristics of Operating System

— Single user and multi-user operating systems

Open-source and closed-source operating systems.

Important Questions with their Answers of “Introduction to operating system”

— MCQs and Fill in Blanks

— Short term Questions

— Long Term Questions


Chapter 2: Process Overview

Definition of process, process states, process life cycle,

Process Control Block (PCB),

Process Scheduling – Scheduling queues,

Schedulers (short term, medium term and long term).

Dispatcher.

Context Switch.

Important Questions with their Answers of “Process Overview”

— MCQs and Fill in blanks

Short term Questions

Long Term Questions


Chapter 3: CPU Scheduling

  • CPU Scheduler,
  • Preemptive and non-preemptive scheduling.
  • Scheduling criteria – CPU utilization, Throughput, Turnaround time, Waiting time, Response time.
  • Scheduling Algorithms: First-Come-First-Serve, Shortest-Job-First, Priority Scheduling, Round-Robin.

Important Questions with their Answers of “CPU Scheduling”

  • MCQs and Fill in blanks
  • Short term Questions
  • Long Term Questions

Long Term Question of “Process Overview”

Ques 1 : What are the various Process state and explain, the Process Life Cycle ?

Ques 2 : What is Scheduler and also explain, various type of Scheduler ?

Ques 3 : What is Context Switch and also explain, Context Switch Triggers ?


Ques 1 : What are the various Process state and explain, the Process Life Cycle ?

Ans : A process is an instance of a program in execution. The execution of a process must progress in a sequential fashion.

Process State

Process state are those states which tells the status of of the process. Status of the process includes whether the Process has Executed or Whether the process is Waiting for Some input and output from the user and whether the Process is Waiting for the CPU to Run the Program after the Completion of the Process.

A Process has a five States:-

  1. New State
  2. Ready State
  3. Waiting State
  4. Running State
  5. Terminated State

Process Life Cycle

When a process is executed, it passes through different states. These steps may vary in different operating systems, and the names of these states are not standardized.

In general, a process can have one of the following five states at a time.

Sr. No.   Process States  Description
1.  Start  This is the initial state when a process is first started/created.
2.  Ready  When the Process is Ready to Execute but he is waiting for the CPU to Execute then this is called as the Ready State. After the Completion of the Input and outputs the Process will be on Ready State means the Process will Wait for the Processor to Execute.
3.  Running  When the Process is Running under the CPU, or When the Program is Executed by the CPU , then this is called as the Running process and when a process is Running then this will also provides us Some Outputs on the Screen.
4.  Waiting  Process moves into the waiting state if it needs to wait for a resource, such as waiting for user input, or waiting for a file to become available.
5.  Terminated  Once the process finishes its execution, or it is terminated by the operating system, it is moved to the terminated state where it waits to be removed from main memory.

 

Ques 2 : What is Scheduler and also explain, various type of Scheduler ?

Ans : The OS scheduler determines how to move processes between the ready and run queues which can only have one entry per processor core on the system, it has been merged with the CPU. The main task of scheduler is to select the jobs to be submitted into the system and to decide which process to run.

Types of Schedulers: There are three types of Scheduler:

  1. Short Term Scheduler
  2. Medium Term Scheduler
  3. Long Term Scheduler

Short Term Scheduler:- 

  • It is also called as CPU scheduler.
  • Main objective is to increase system performance in accordance with the chosen set of criteria.
  • CPU scheduler selects a process among the processes that are ready to execute and allocates CPU to one of them.
  • Short-term schedulers, also known as dispatchers, make the decision of which process to execute next.

Medium Term Scheduler:- 

  • Medium-term scheduling is a part of swapping.
  • It removes the processes from the memory.
  • It reduces the degree of multi-programming.
  • The medium-term scheduler is in-charge of handling the swapped out-processes.

Long Term Scheduler:- 

  • It is also called a job scheduler.
  • A long-term scheduler determines which programs are admitted to the system for processing.
  • It selects processes from the queue and loads them into memory for execution.
  • Process loads into the memory for CPU scheduling.
  • The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound.
  • It also controls the degree of multi-programming.
  • If the degree of multi-programming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system.

Difference Between Short Term, Medium Term and Long Term Scheduler

Sr. No.  Short Term Scheduler  Medium Term Scheduler  Long Term Scheduler
1. It is a CPU scheduler. It is a process swapping scheduler. It is a job scheduler.
2. Speed is fastest than other two. Speed is in between both short and long term scheduler. Speed is lesser than short term scheduler.
3. It is minimal in time sharing system It is a part of Time sharing systems..  It is almost absent or minimal in time sharing system.
4. It provides lesser control over degree of multi-programming. It reduces the degree of multi-programming. It controls the degree of multi-programming.
5. It selects those processes which are ready to execute. It can re-introduce the process into memory and execution can be continued. It selects processes from pool and loads them into memory for execution.

 

Ques 3 : What is Context Switch and also explain, Context Switch Triggers ?

Ans: Context Switch is the mechanism to store and restore the state or context of a CPU in Process Control block so that a process execution can be resumed from the same point at a later time. This is a feature of a multitasking operating system and allows a single CPU to be shared by multiple processes.

In the above diagram, initially Process 1 is running. Process 1 is switched out and Process 2 is switched in because of an interrupt or a system call. Context switching involves saving the state of Process 1 into PCB1 and loading the state of process 2 from PCB2. After some time again a context switch occurs and Process 2 is switched out and Process 1 is switched in again. This involves saving the state of Process 2 into PCB2 and loading the state of process 1 from PCB1.

Context Switching Triggers

There are three major triggers for context switching. These are given as follows:

  • Multitasking: In a multitasking environment, a process is switched out of the CPU so another process can be run. The state of the old process is saved and the state of the new process is loaded. On a pre-emptive system, processes may be switched out by the scheduler.
  • Interrupt Handling: The hardware switches a part of the context when an interrupt occurs. This happens automatically. Only some of the context is changed to minimize the time required to handle the interrupt.
  • User and Kernel Mode Switching: A context switch may take place when a transition between the user mode and kernel mode is required in the operating system.

Context Switching Steps

The steps involved in context switching are as follows:

  • Save the context of the process that is currently running on the CPU. Update the process control block and other important fields.
  • Move the process control block of the above process into the relevant queue such as the ready queue, I/O queue etc.
  • Select a new process for execution.
  • Update the process control block of the selected process. This includes updating the process state to running.
  • Update the memory management data structures as required.
  • Restore the context of the process that was previously running when it is loaded again on the processor. This is done by loading the previous values of the process control block and registers.

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‘Operating System’ Practical

  1. To install and configure MS Windows 7/8/10 on a computer
  2. To get familiar with general Windows commands – ECHO, CLS, DIR, TREE, MD, CD, TYPE, RD, DEL, MOVE, COPY , REN, ATTRIB, COLOR, DATE, TIME, ERASE, FIND, FC, MORE, CMD, PATH, SYSTEMINFO, SHUTDOWN, TASKLIST, TASKKILL, VER, VOL, CHKDSK
  3. To use wildcard characters for copying, moving, renaming, and deleting files and directories in a given hierarchical directory structure under Windows’s command prompt.
  4. To get familiar with windows control panel components.
  5. To use Windows backup and restore features.
  6. To get familiar with commonly used Windows PowerShell cmdlets like Get-ChildItem, GetContent, Get-Command, Get-Help, Clear-Host, Copy-Item, Move-Item, Remove-Item, Rename-Item, Get-Location, Set-Location, Write-Output, Get-Process, Stop-Process.
  7. To write scripts in Windows PowerShell to automate tasks.