Operating Systems Semester Exam
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Which type of operating system allows many users to share the computer simultaneously and gives each user the impression that the entire system is dedicated to them?

  • Multiprogramming Operating System
  • Time Sharing System (correct)
  • Multiprocessing Operating System
  • Batch Operating System

    • Spooling is a process where data is permanently stored in memory for long-term use.

    False

    What are the primary goals of an operating system?

    Convenience / user friendly

    ______ involves handling the creation, scheduling, and termination of processes, which are executing programs.

    <p>Process Management</p> Signup and view all the answers

    What is the main difference between Symmetric Processing and Asymmetric Processing?

    <p>Each processor is assigned a specific task or role in Asymmetric Processing.</p> Signup and view all the answers

    What is the primary function of a Real Time Operating System (RTOS)?

    <p>To process jobs within defined time constraints</p> Signup and view all the answers

    Soft Real-Time Operating Systems can afford to miss deadlines occasionally.

    <p>True</p> Signup and view all the answers

    Match the following system calls with their categories:

    <p>Create file, delete file = File management Create, delete communication connection = Communications Allocate and free memory = Process control Get time or date, set time or date = Information maintenance</p> Signup and view all the answers

    What are the two main modes of operation needed in an operating system?

    <p>User mode and Kernel mode</p> Signup and view all the answers

    What is the purpose of the mode bit in the hardware of a computer?

    <p>indicate the current mode: kernel (0) or user (1)</p> Signup and view all the answers

    Which of the following sections are part of a process?

    <p>Data Section</p> Signup and view all the answers

    A program is a passive entity by default.

    <p>True</p> Signup and view all the answers

    A process becomes active when an executable file is loaded into ________ memory.

    <p>main</p> Signup and view all the answers

    Match the following process control block (PCB) information with its description:

    <p>Process state = Defines whether the process is new, ready, running, waiting, halted, etc. CPU-scheduling information = Includes process priority, pointers to scheduling queues, etc. Memory-management information = Contains data like base and limit registers, page tables, etc. I/O status information = Includes the list of I/O devices allocated to the process, open files, etc.</p> Signup and view all the answers

    Define CPU utilization.

    <p>Keeping the CPU as busy as possible</p> Signup and view all the answers

    What is meant by throughput in CPU scheduling?

    <p>Number of processes completed per time unit</p> Signup and view all the answers

    Explain waiting time in the context of CPU scheduling.

    <p>Sum of periods spent waiting in the ready queue</p> Signup and view all the answers

    What is the response time in CPU scheduling?

    <p>Time it takes to start responding</p> Signup and view all the answers

    Define Turn Around Time (TAT) in the context of CPU scheduling.

    <p>Completion Time - Arrival Time</p> Signup and view all the answers

    What is the formula to calculate Waiting Time?

    <p>Turn Around Time - Burst Time</p> Signup and view all the answers

    What scheduling criteria should be considered when selecting a CPU-scheduling algorithm?

    <p>Response time</p> Signup and view all the answers

    What is the main problem with multi-level queue scheduling?

    <p>How to decide number of ready queues, scheduling algorithm inside the queue, and between the queues; and once a process enters a specific queue, it cannot change to another queue.</p> Signup and view all the answers

    Which of the following is a characteristic of a multilevel feedback queue scheduling algorithm?

    <p>Processes are moved between queues based on CPU burst characteristics.</p> Signup and view all the answers

    The multilevel feedback queue scheduling algorithm prevents starvation.

    <p>True</p> Signup and view all the answers

    Race condition is a situation where the output of a process depends on the ____________ of processes.

    <p>execution sequence</p> Signup and view all the answers

    Define Mutual Exclusion in the context of the critical section problem.

    <p>Mutual Exclusion refers to the rule that only one process is allowed inside the critical section at a time, ensuring exclusive access to shared resources.</p> Signup and view all the answers

    Study Notes

    Operating System Overview

    • An operating system (OS) acts as an intermediary between users and hardware, managing computer resources and providing a platform for application programs to run.
    • Goals of an OS:
      • Primary goals: Convenience, user-friendly
      • Secondary goals: Efficiency, reliability, maintainability
    • Functions of an OS:
      • Process management: handling process creation, scheduling, and termination
      • Memory management: managing allocation and deallocation of physical and virtual memory
      • I/O device management: handling I/O operations of peripheral devices
      • File management: managing files on storage devices, including information, naming, permissions, and hierarchy
      • Network management: managing network protocols and functions
      • Security and protection: ensuring system protection against unauthorized access and other security threats

    OS Structure

    • Major components of an OS:
      • Kernel: central component managing system resources and communication between hardware and software
      • Process management: process scheduler, process control block (PCB), and concurrency control
      • Memory management: physical memory management, virtual memory management, and memory allocation
      • File system management: file handling, file control block, and disk scheduling
      • Device management: device drivers and I/O controllers
      • Security and access control: authentication, authorization, and encryption
      • User interface: command-line interface (CLI) and graphical user interface (GUI)
      • Networking: network protocols and network interface

    Classification of Operating Systems

    • Batch operating system: early computers, no interaction, jobs are processed in batches
    • Multiprogramming operating system: multiple jobs in memory, CPU switching between jobs
    • Multiprocessing operating system: multiple CPUs, each processing a separate job
    • Multitasking operating system: multiple tasks, time sharing, and job scheduling
    • Real-time operating system: strict deadlines, predictable responses
    • Distributed operating system: multiple nodes, networked, and loosely coupled

    CPU Scheduling

    • Scheduling concepts: process states, process transition diagram, and schedulers
    • Performance criteria: CPU utilization, throughput, and turnaround time
    • Scheduling algorithms: first-come, first-served (FCFS), shortest job first (SJF), and priority scheduling

    Concurrent Processes

    • Process concept: principle of concurrency, producer/consumer problem, and mutual exclusion
    • Classical problems in concurrency: dining philosopher problem, sleeping barber problem, and semaphores
    • Interprocess communication models and schemes

    Memory Management

    • Basic bare machine: resident monitor, multiprogramming with fixed and variable partitions
    • Protection schemes: paging, segmentation, and paged segmentation
    • Virtual memory concepts: demand paging, performance, and page replacement algorithms

    I/O Management and Disk Scheduling

    • I/O devices and I/O subsystems: I/O buffering, disk storage, and disk scheduling
    • File system: file concept, file organization, and file access mechanisms
    • File system implementation issues: file allocation methods, free-space management, and file protection

    Microkernel Approach

    • Structures the OS by removing nonessential components from the kernel and implementing them as system and user-level programs
    • Benefits: easier to extend the OS, smaller kernel, and fewer modifications required### Command Interpreters and Interfaces
    • A command interpreter is a special program that runs when a job is initiated or when a user first logs on.
    • On systems with multiple command interpreters, they are known as shells.
    • Examples of shells include Bourne shell, C shell, Bourne-Again shell, and Korn shell.

    Graphical User Interfaces (GUIs)

    • A GUI is a user-friendly interface that allows users to interact with the operating system using a mouse and windows.
    • GUIs are characterized by a desktop with icons, files, and directories.
    • Users can interact with the GUI by clicking on icons, selecting files, and pulling down menus.

    Interface Choice

    • The choice of whether to use a command-line interface or a GUI is mostly a matter of personal preference.
    • System administrators and power users often use command-line interfaces because they are more efficient.
    • Some systems only provide a subset of system functions via the GUI, leaving the less common tasks to command-line users.

    System Calls

    • System calls provide a way for a user program to ask the operating system to perform tasks on its behalf.
    • System calls are an interface to the services provided by an operating system.
    • They are available as routines written in C and C++, and specify the functions and parameters available to an application programmer.

    Types of System Calls

    • Process control: end, abort, load, execute, create process, terminate process, etc.
    • File management: create file, delete file, open, close, read, write, reposition, etc.
    • Device management: request device, release device, read, write, reposition, etc.
    • Information maintenance: get time or date, set time or date, get system data, set system data, etc.
    • Communications: create, delete communication connection, send, receive messages, transfer status information, etc.

    Modes of Operation

    • Two modes of operation: User mode and Kernel mode (also called supervisor mode, system mode, or privileged mode).
    • A mode bit is added to the hardware to indicate the current mode.

    Process

    • A process is a program in execution.
    • A program is not a process by default; it becomes a process when loaded into main memory and its PCB is created.
    • A process consists of text, stack, data, and heap sections.
    • A process can be in one of the following states: new, running, waiting, ready, or terminated.

    Process Control Block (PCB)

    • A PCB is a data structure that represents a process in the operating system.
    • It contains information such as process state, program counter, CPU registers, CPU-scheduling information, memory-management information, accounting information, and I/O status information.

    Process States

    • New: the process is being created.
    • Running: instructions are being executed.
    • Waiting (Blocked): the process is waiting for some event to occur.
    • Ready: the process is waiting to be assigned to a processor.
    • Terminated: the process has finished execution.

    Schedulers

    • Schedulers: a process migrates among the various scheduling queues throughout its lifetime.
    • Long-term scheduler: determines which processes enter the ready queue from the job pool.
    • Medium-term scheduler: swaps processes in and out of memory to optimize CPU usage and manage memory allocation.
    • Short-term scheduler: selects from among the processes that are ready to execute and allocates the CPU to one of them.

    CPU Scheduling

    • CPU scheduling is the process of determining which process in the ready queue is allocated to the CPU.
    • Various scheduling algorithms can be used, such as First-Come-First-Served (FCFS), Shortest Job Next (SJN), Priority, and Round Robin (RR).
    • Different algorithms support different classes of processes and favor different scheduling criteria.

    Context Switch

    • Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process.
    • This task is known as a context switch.

    CPU Scheduling Algorithms

    • Non-Preemptive: once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU willingly.
    • Preemptive: a process will leave the CPU willingly or it can be forced out.

    Scheduling Criteria

    • CPU utilization: keeping the CPU as busy as possible.
    • Throughput: the number of processes that are completed per time unit.
    • Waiting time: the sum of the periods spent waiting in the ready queue.
    • Response time: the time it takes to start responding.

    First-Come-First-Served (FCFS) Scheduling

    • The process that requests the CPU first is allocated the CPU first.
    • Implementation is managed by a FIFO queue.
    • It is always non-preemptive in nature.

    Advantages and Disadvantages of FCFS

    • Advantages: easy to understand, and can be used for background processes where execution is not urgent.
    • Disadvantages: suffers from convoy effect, which means smaller processes have to wait for larger processes, resulting in large average waiting times.### CPU Scheduling Algorithms
    • FCFS (First-Come-First-Served) algorithm:
      • Non-preemptive algorithm
      • Process with the highest arrival time gets the CPU first
      • High average waiting time and Turnaround Time (TAT) compared to other algorithms
    • SJF (Shortest Job First) algorithm:
      • Non-preemptive and preemptive versions
      • CPU is assigned to the process with the smallest burst time
      • In non-preemptive version, once a process is scheduled, it cannot be preempted
      • In preemptive version, the process with the smallest burst time is executed first
      • Guarantees minimal average waiting time, making it an optimal algorithm
    • Priority Scheduling algorithm:
      • Each process is assigned a priority
      • CPU is allocated to the process with the highest priority
      • Non-preemptive and preemptive versions
      • In non-preemptive version, once a process is scheduled, it cannot be preempted
      • In preemptive version, the process with the highest priority is executed first
      • Tie is broken using FCFS order
    • Round Robin (RR) algorithm:
      • Designed for time-sharing systems
      • Each process is allocated a fixed time slice (time quantum)
      • If a process completes within the time quantum, the next process is executed
      • If a process does not complete within the time quantum, it is preempted and added to the end of the ready queue
      • Each process must wait no longer than (n-1) x time quantum for its next time quantum
    • Multi-Level Queue (MLQ) Scheduling algorithm:
      • Partitions the ready queue into several separate queues
      • Each queue has its own scheduling algorithm
      • Processes are permanently assigned to one queue based on their properties and requirements
      • Scheduling among the queues is commonly implemented as fixed-priority preemptive scheduling or round robin with different time quantum
    • Multi-Level Feedback Queue (MLFQ) Scheduling algorithm:
      • Allows processes to move between queues
      • If a process uses too much CPU time, it is moved to a lower-priority queue
      • If a process waits too long in a lower-priority queue, it may be moved to a higher-priority queue
      • Prevents starvation

    Process Synchronization and Race Condition

    • Process Synchronization:
      • Multiple processes competing for limited resources
      • Concurrent access to shared data may result in data inconsistency
    • Race Condition:
      • A situation where the output of a process depends on the execution sequence of processes
      • If the order of execution of different processes is changed, the output may change
    • Critical Section Problem:
      • A section of code where a process accesses shared resources
      • Mutual Exclusion: No two processes should be present in the critical section at the same time
      • Progress: If no process is executing in its critical section, some processes may participate in deciding which process will enter the critical section next
      • Bounded Waiting: There exists a bound on the number of times a process is allowed to enter its critical section
    • Solutions to Critical Section Problem:
      • Two-Process Solution: Using Boolean variables, Boolean arrays, and Peterson's Solution
      • Operating System Solution: Using semaphores (counting semaphores and binary semaphores)
      • Hardware Solution: Using test and set lock, and disabling interrupts

    Solutions to Critical Section Problem (Two-Process Solution)

    • Using Boolean variable turn:
      • Initialization: turn = 0 or 1 randomly
      • Process 0 and Process 1 execute the following code:
        • While (turn != 0); // Process 0
        • While (turn != 1); // Process 1
        • Critical Section
        • turn = 1; // Process 0
        • turn = 0; // Process 1
        • Remainder Section
      • Follows Mutual Exclusion, but suffers from strict alternation
    • Using Boolean array flag:
      • Initialization: flag = F
      • Process 0 and Process 1 execute the following code:
        • flag = T; // Process 0
        • flag = T; // Process 1
        • While (flag); // Process 0
        • While (flag); // Process 1
        • Critical Section
        • flag = F; // Process 0
        • flag = F; // Process 1
        • Remainder Section
      • Follows Mutual Exclusion, but may lead to deadlock
    • Dekker's algorithm:
      • Uses a combination of Boolean flags and turn variable
      • Ensures Mutual Exclusion, Progress, and Bounded Waiting
    • Peterson's solution:
      • Uses a combination of Boolean flags and turn variable
      • Ensures Mutual Exclusion, Progress, and Bounded Waiting
      • Classic Software-based solution to the critical-section problem for two processes

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    OS in 6 hours.pdf

    Description

    This quiz covers the fundamentals of operating systems, including classification, structure, and CPU scheduling. It is designed for a semester exam and tests knowledge of key concepts and terminology.

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