Multiple Choice Questions on Operating Systems

Understanding the Basics of an Operating System

An operating system is a computer program that controls and manages computer hardware and other software applications that run on a computer. It provides a platform for the installation and execution of other software programs and acts as an intermediary between the user and hardware.

The Resource Manager/Allocator is the primary driver that manages system resources in a fair and unbiased manner, including hardware resources such as CPU time, system buses, and memory, as well as software resources such as access, authorization, and semaphores. It provides the functionality necessary to application programs.

The operating system has several functions, including process management, memory management, file management, I/O device management, network management, and security and protection. It controls and coordinates the use of resources amongst various application programs.

There are four types of operating systems: Batch Operating System, Multiprogramming Operating System, Multitasking Operating System, and Real-Time Operating System.

Batch Operating System is used to speed up processing jobs with similar types by running them through the processor as a group batch. Jobs are grouped according to their requirements, such as FORTRAN or COBOL jobs.

Multiprogramming Operating System keeps several jobs in a computer's main memory simultaneously. The OS picks and begins to execute one of the jobs in memory, and other jobs may have to wait for I/O operations to complete.

Multitasking Operating System allows users to play MP3 music, edit documents, and surf the web while simultaneously running multiple programs. Multiprogramming and the concept of time-sharing must be present for multitasking to occur.

Real-Time Operating System is designed for time-bound systems with defined, fixed time constraints. It must complete processing within the defined constraints or the system will fail. RTOS serves real-time applications that process data as it comes in without buffer delays.

Overall, an operating system plays a crucial role in managing computer hardware and software applications efficiently.

CPU Scheduling Algorithms

The following are examples of CPU scheduling algorithms:

Priority scheduling
Round Robin
Shortest Job First

All of the above are considered CPU scheduling algorithms.

Operating Systems and their Role as User-Friendly Interfaces

Operating systems (OS) play a crucial role as user-friendly interfaces for programmers. They provide a layer of abstraction that isolates the complexities of the underlying hardware from the programmer, making it easier to write software. Some of the ways in which OS do this include:

- Providing a graphical user interface (GUI) that allows users to interact with the system using visual elements like icons, menus, and buttons.

- Linking programs with subroutines to provide access to shared resources like disk drives, network connections, and other hardware devices.

- Allowing programmers to create flowcharts of their programs, which serves as a visual representation of the program's logic and structure.

Overall, OS are critical components for creating user-friendly software that is easy to use and manage.


The term used to define the process where the pages are copied from main memory to secondary memory as per the requirement is called Demand Paging.

//Sample code to implement demand paging.
while (!done) {
   // Check if the page is in the main memory
   if (page_table[page_num].valid_bit == TRUE) {
      // Page is in the main memory
      // Access it and continue
   } else {
      // Page is not in the main memory
      // Bring it from the secondary memory
      if (free_frame_exists()) {
         // There is a free frame available in main memory
         // Use it to load the page from secondary memory
      } else {
         // No free frame available
         // Use page replacement algorithm to free up a frame

FIFO Scheduling Type

The FIFO scheduling type is considered as a type of non-preemptive scheduling.

Real-time Operating System

A real-time operating system is the type of OS that reads and reacts in terms of real-time. It is designed to process data and respond within a guaranteed time frame. It is commonly used in embedded systems, industrial automation, and scientific research equipment.


RTOS-based systems are used in applications that require instantaneous response times, such as flight control systems, automobile engines, and medical equipment. Such systems require deterministic performance: an operation must be performed consistently, independent of other system activity or interference.

Procedure for Switching CPU to a New Process

In a system where multiple processes run in parallel in the CPU, when a higher priority process becomes available, the kernel must transfer control of the CPU to that process. This is done through a systematic procedure known as context switching.

Programming Language Used for UNIX

The programming language used for writing UNIX is C. UNIX was developed in the 1970s at AT&T Bell Laboratories.

Thread: Light weight process

In an operating system, a thread is the most basic unit of execution that can be scheduled. It's a lightweight process that enables a program to perform multiple tasks simultaneously within a single process. Unlike heavyweight processes, threads share common memory and resources, making them more efficient and faster to create and destroy. Therefore, the correct answer is: Thread is a light weight process.

Operating System Thread Classification

In terms of threading, an operating system classifies threads into two categories: User level thread, which is implemented by the user, and Kernel level thread, which is implemented by the kernel. The options provided in the given sentence don't accurately reflect the thread classification done by an operating system.

CPU Scheduling Algorithms

In CPU scheduling algorithms, First-Come-First-Serve (FCFS) is the algorithm that allocates the CPU first to the process that requests the CPU first. This implies that the process which arrives first gets executed first. The other algorithms mentioned are:

  • Shortest Job First (SJF)
  • Priority Scheduling

The answer is FCFS.

AT Operating Modes

The two types of operating modes for AT are Real mode and Protected mode.

// Example code for switching between Real and Protected mode

#include <iostream>
#include <conio.h>
using namespace std;

int main()
    cout << "Switching to Real mode..." << endl;
    _asm {
        mov ax, 0x0000
        mov ds, ax
        mov es, ax
        mov ss, ax
        mov sp, 0xFFFF
    cout << "Real mode activated!" << endl;
    cout << "Switching to Protected mode..." << endl;
    _asm {
        mov ax, 0x10
        mov ds, ax
        mov es, ax
        mov ss, ax
        mov sp, 0xFFFF
    cout << "Protected mode activated!" << endl;
    return 0;

Note: The code above is for demonstration and testing purposes only. It is incomplete and inadequate for production use.

Thread Scheduling by Operating System

In a computer system, thread scheduling is done by the operating system. The operating system is responsible for managing the threads that are running on the CPU. It decides which thread gets the CPU time and when. This decision is made based on various scheduling algorithms used by the operating system. The virtual memory and input operations are not involved in thread scheduling. Therefore, the correct answer is that thread scheduling is done by the operating system.

Understanding the Ready State of a Process

The ready state of a process refers to the state in which the process has all the necessary resources required for its execution when the CPU is allocated. This means that the process is ready to execute but is waiting for the CPU to become available.

In simple terms, the process is waiting in a queue for its turn to get executed by the CPU. Once the CPU becomes available, the process will move from the ready state to the running state, where it will execute its instructions.

It is important to note that the ready state does not mean that the process is currently using the CPU or that it is scheduled to run immediately. It simply means that the process is waiting for its turn to get executed and has all the resources it needs to do so.

Identifying Spooled Device

A spooled device is a device that uses a buffer to store data, allowing slow devices to operate at their natural speed while other processes continue at a faster pace.

Out of the given options, the line printer that prints the output of a number of jobs is an example of a spooled device.

//example code of spooling

#include <stdio.h>

int main() {
   int a, b;

   printf("Enter two numbers to add\n");
   scanf("%d%d", &a, &b);

   printf("Sum of entered numbers = %d\n", a+b);

   return 0;

The above code is an example of spooling which uses a buffer to store data when waiting for input from the user, allowing the user to enter the two numbers at their own pace, while the rest of the code keeps running.

Main Memory of a Computer System

The main memory of a computer system is volatile.

Volatile memory involves storage that is lost when the computer is turned off or loses power. This means that any information stored in volatile memory is temporary and considered highly volatile, as it can be lost at any moment.

Examples of volatile memory in a computer system include RAM (Random Access Memory) and CPU (Central Processing Unit) cache. These memory types are essential for running programs and tasks while the computer is on.

In contrast, non-volatile memory retains data even when the computer is turned off, and examples of it include ROM (Read-Only Memory), hard drives, and solid-state drives (SSD).

Knowing the difference between volatile and non-volatile memory is crucial when it comes to understanding how a computer system operates and how to properly manage and store data.

Banker's Algorithm Purpose

The purpose of the Banker's Algorithm is to prevent deadlock in a computer system that uses a resource allocation scheme. The algorithm ensures that processes that request resources do not end up in a situation where they cannot obtain the necessary resources to complete execution.


//Initialize variables
int procCount = 5;   //number of processes
int resCount = 3;    //number of resources
int maxRes[5][3] = { {7, 5, 3}, {3, 2, 2}, {9, 0, 2}, {2, 2, 2}, {4, 3, 3} }; //maximum required resources for each process
int currRes[5][3] = { {0, 1, 0}, {2, 0, 0}, {3, 0, 2}, {2, 1, 1}, {0, 0, 2} }; //current resources allocated to each process
int availRes[3] = { 3, 3, 2 }; //total number of available resources

//Calculate the need matrix
int need[5][3];
for (int i = 0; i < procCount; i++) {
  for (int j = 0; j < resCount; j++) {
    need[i][j] = maxRes[i][j] - currRes[i][j];

//Apply the Banker's Algorithm
bool finish[5] = { false, false, false, false, false };
int safeSequence[5];
int count = 0;
while (count < procCount) {
  bool found = false;
  for (int i = 0; i < procCount; i++) {
    if (!finish[i]) {
      int j;
      for (j = 0; j < resCount; j++) {
        if (need[i][j] > availRes[j]) {
      if (j == resCount) {
        for (int k = 0; k < resCount; k++) {
          availRes[k] += currRes[i][k];
        safeSequence[count++] = i;
        finish[i] = true;
        found = true;
  if (!found) {

//Print the safe sequence
cout << "The safe sequence is: ";
for (int i = 0; i < count; i++) {
  cout << "P" << safeSequence[i];
  if (i != count - 1) {
    cout << " -> ";
cout << endl;

Device Drivers and Disk Drivers

Device drivers are essential software components that allow hardware devices to communicate seamlessly with the computer's operating system. Without device drivers, the computer would be incapable of controlling and communicating with hardware devices properly.

In particular, a disk driver is a type of device driver that allows a specific disk drive to communicate with the rest of the computer. It acts as an interface between the operating system and the disk controller and ensures that data can be read from and written to the disk drive.

Disk drivers are crucial in ensuring that the computer can access information stored on disk drives such as hard drives or floppy disks. Therefore, disk drivers are required in devices that use disk storage, including main memory and cache memory, to ensure that the information stored on them can be accessed reliably and quickly.

Deallocation of Register Context and Thread Stack

In a thread, the register context and stack are deallocated when the thread terminates, which means that all the operations for a specific thread have been executed. After the thread has finished running, the system frees up the memory space that was used to store the register context and the stack of the thread.

Thread-Specific Memory

Each thread in a process has its own program counter and stack, therefore threads of the same process do not share program counters and stacks. This is because each thread might have its own execution sequence/code and would need to push/pop its program counter contents onto its own separate stack. Therefore, the answer is both program counter and stack.

Jacketing Technique for Non-Blocking System Calls

The jacketing technique is used to convert a blocking system call into a non-blocking system call. It involves wrapping the system call with a function that will allow it to return immediately if the requested operation cannot be immediately performed, instead of waiting for it to complete. This is useful in situations where the calling thread needs to continue running uninterrupted while waiting for the system call to complete.

Resource Sharing for Processes and Threads

Resource sharing is used for all of the mentioned purposes, including sharing memory and resources and compressing the address space. In particular, resource sharing is used to allow an application to have several threads of activity all within the same address space, each of which can access the shared resources and memory of the process to which the threads belong. This can improve the efficiency and effectiveness of an application, making it more responsive and easier to manage. Therefore, resource sharing is an essential aspect of modern computing systems and software development.

Advantages of Many-to-One Model

The many-to-one model has an advantage when the program does not require multithreading and only a single processor is available. This is because only one thread can access the kernel at a time, making it impossible for multiple threads to run in parallel on multiprocessors. Therefore, if a program doesn't need multithreading, the many-to-one model is more efficient.

// No code provided for this answer

Identifying System Calls that Do Not Return Control on Termination

In the context of system calls, certain functions do not return control on termination to the original calling point. We can identify these top system calls with the following list:

  1. exec
  2. fork
  3. longjmp
  4. ioctl

After review, it is concluded that the exec system call is the only one in the list that does not return control to the calling point.

Understanding Zombie Processes in Linux

In Linux, a zombie process or a defunct process is a process that has completed its execution using the exit system call. However, its entry still exists in the process table, making it a process in the "Terminated state".

Here is an example program:

#include <stdio.h> 
#include <stdlib.h> // Required for exit() 

int main() 
    pid_t child_pid = fork(); 

    if (child_pid > 0) {
        sleep(100); // Parent process sleeps 
    } else if (child_pid == 0) {
        exit(0); // Child process exits 

    return 0; 

In this program, the


function uses the


system call to create a child process. If


returns a value greater than 0, the parent process goes to sleep for 100 seconds. Meanwhile, the child process immediately exits.

When a child process exits before the parent, the kernel Init process becomes the parent process of the child process. The child process then becomes a zombie process since it still has an entry in the process table, yet it has finished its execution.

To avoid creating zombie processes, a parent process must handle the child process's termination using the




system calls. These calls allow the parent process to obtain details about the child's termination and remove its entry from the process table.

In conclusion, without proper management from the parent process, creating child processes within a program can lead to zombie processes that can negatively impact system performance if left unchecked.

C program with fork() function


int main () {
    fork(); // First call to fork() function
    fork(); // Second call to fork() function
    printf("code "); // Print "code"
    return 0;

The output of the above program will be:

code code code code

code code code

code code



The first call to fork() function creates a child process which in turn calls the second fork() function creating two child processes. So, in total, there are 4 processes in the system where each process executes the print statement. Hence, "code" is printed 4 times.

Identifying System Calls without Error

// The getpid() function gets the process ID of the calling process.
pid_t pid = getpid();

Out of the given system calls, the call getpid() does not return an error. It gets the process ID of the calling process and returns the process ID on success. On failure, it sets the errno variable to indicate the error.

Thread Cancellation

Thread cancellation is the process of terminating a thread before its execution.

Understanding Thread Termination in Java

In Java, a thread can be terminated in different ways. One of the ways is terminating a target thread immediately. Here is the corrected version of the given question and answer:

Question: What is the term used for when a thread terminates some target thread immediately?

Answer: Asynchronous termination.


Synchronous termination involves waiting for the target thread to complete its work before termination. In contrast, asynchronous termination abruptly stops the execution of the target thread without waiting for it to finish.

Deferred cancellation, on the other hand, refers to a mechanism where the termination of a thread is postponed until a specific point in time.

It is essential to understand the different methods of thread termination in Java to avoid unexpected behavior and errors in multi-threaded applications.

Signal Types

In this context, signals of the same type are sent together. They can also be queued or stacked, but they must be of the same type. If there are no signals of a particular type, then the answer is "none."

UNIX Command for Sending a Signal

In UNIX, the command used for sending a signal is



Comparing speed of data writing in magnetic tape disks and disk drives

The statement is true. Both magnetic tape disks and disk drives can have comparable speeds when writing data.

It's important to note that the speed can depend on the type of data being written and the specific models of the magnetic tape disk and disk drive being used.

Alternative Names for Command Interpreter

In addition to "command interpreter," this program is also referred to as a shell.The correct definition of spooling is that it holds a single copy of data.

The User Process State Transition

The user process can initiate the state transition by itself only to 'block' state. Whenever the user process triggers an input/output request, it goes into the ‘block’ state until the request is completed. The other state transitions, such as 'dispatch' and 'wakeup', are handled by the operating system.

Process Execution Steps

In process execution, there are two main steps which are CPU and I/O burst. During the CPU burst, the processor executes instructions of the process. This process includes fetching instructions from memory, decoding them, and executing them. After the CPU burst, there is an I/O burst where the process waits for input/output operations to complete. During this time, the processor is idle and not executing any instructions. Then, the process returns to the CPU burst step to execute the next set of instructions.

Reasons for CPU Scheduling

The purpose of CPU scheduling is to increase the efficiency and utilization of the CPU so that multiple processes can be executed without having to wait too long. This leads to a faster response time for computer users and allows the system to handle more tasks within a given time frame.

The primary goal of CPU scheduling is to minimize the amount of time the CPU spends idle and maximize the number of processes that can be executed in a given time period. This is achieved by assigning priorities to different processes and allocating CPU time based on their priority levels.

By optimizing CPU utilization, scheduling reduces costs and increases productivity in the system. Effective CPU scheduling reduces wasted CPU cycles and resources, which ultimately leads to improved performance and reduced energy consumption.

Optimal CPU Scheduling Algorithm

In computer science, the term "optimal" usually refers to the algorithm that produces the best result in terms of time and space complexity. As for the CPU scheduling algorithm, the most optimal one is the Shortest Job First (SJF) algorithm.

SJF algorithm gives the least average waiting time for each process by selecting the process with the shortest expected processing time. It minimizes the waiting time of all the processes in the ready queue and improves the overall turnaround time of the system.

Therefore, the answer to the question is Shortest Job First.

Solving the Critical Section Problem

In order to solve the critical section problem, a minimum of two variables are required to be shared between processes. These variables are:

  • Turn variable: This variable decides which process has the permission to enter the critical section.
  • Boolean flags: These flags are used to indicate whether a process is ready to enter the critical section or not.

By modifying these shared variables, a process can enter and exit the critical section without conflicts with other processes.

Atomic Operations in Computing

In computing, an atomic operation is known as an uninterruptible unit. When a process starts executing, it will not stop or switch to another process until its execution is complete. Therefore, the answer to the question is atomic.

An atomic operation is a program operation that can be executed with no interference from other processes. It is an operation that cannot be divided or interrupted, ensuring that it will always complete successfully. Atomic operations are essential in concurrent programming, where multiple processes are running simultaneously and may access the same data.

In summary, atomic operations provide essential support for concurrency control in computing.

Understanding Semaphores in Multithreading

In multithreading, a semaphore is a synchronization tool that controls access to a shared resource. It is an integer variable that can be accessed by multiple threads, and it helps to solve the problem of critical sections. So, option 3 "integer variable, critical section" is the correct choice for the given question.

Two Types of Atomic Operations Performed by Semaphores

In semaphore operation, two types of atomic operations can be performed: wait and signal. The wait operation decrements the semaphore value and blocks the process if the value becomes negative. The signal operation increments the semaphore value and wakes up a blocked process if any. These two operations are used by processes to acquire and release shared resources in a mutually exclusive manner. Therefore, the correct answer is: wait, signal.

Semaphore Types

In semaphore, there are two types - Binary and Counting semaphores.The correct value for initializing a binary semaphore is 1.

Identifying System Calls for Resource Management

In computer science, the release and request of resources are classified as system calls. These system calls are used to request the utilization of specific resources such as memory, files, or other hardware components. When a program wants to use such resources, it makes a system call to the operating system requesting those resources. Once the program is finished utilizing the resources, it will make another system call to release them back to the operating system.

Therefore, the correct answer is: system calls.

Is Mutual Exclusion Necessary for Shareable Resources?

The answer is no. Mutual exclusion is not always necessary for shareable resources. Sometimes multiple processes can access and modify the shareable resources concurrently without causing any conflicts, while in other cases where conflicts can occur, mutual exclusion is necessary to ensure the correctness and consistency of shared data.

Understanding Unsafe States

Unsafe states are those states in a system where a process may access a shared resource that is already being used by another process, leading to inconsistencies in the system. These states are not always deadlocks, as there may be cases where unsafe states do not lead to deadlocks. However, it is important to identify and prevent these states to ensure the stability and reliability of the system. Therefore, it is crucial to implement synchronization techniques in code to avoid unsafe states and prevent deadlocks.

Memory Address Binding

Memory address binding can be done at compile time, load time, or execution time.

Identifying the Base Register

In computer architecture, the base register is also referred to as the relocation register.

The other options listed - regular register, delocation register, and basic register - are not synonymous with the base register.

Identifying Operating System

Out of the given options, Oracle is not an operating system. It is actually a relational database management system popularly known as Oracle Database. The other options such as Linux, DOS, and Windows are all operating systems used in computers.

// No code provided in the original prompt

Identifying a Single-User Operating System

Out of the given options, the single-user operating system is Ms-DOS.

// Sample code to print "Hello, World!" in Ms-DOS
#include <stdio.h>

int main() {
  printf("Hello, World!");
  return 0;

Ms-DOS is a command-line operating system released by Microsoft in 1981. It is a single-user, single-tasking operating system, which means that only one user can execute one program at a time.

Windows and MAC, on the other hand, are multi-user operating systems that can support multiple users running multiple programs simultaneously.

System Calls: The Interface to Access Operating System Services

In order to access the services of the operating system, the interface is provided by the system calls. These system calls serve as the gateway to interact with the operating system. Open, Close, Read, and Write are some of the commonly used system calls. They enable programmers to access low-level resources in the operating system.

Using an API or library can simplify access to system resources, but ultimately they rely on the underlying system calls to perform their functions. As such, understanding system calls is crucial for low-level programming and system-level debugging.

Determining the size of virtual memory

Virtual memory size is dependent on the address bus.

Example of a real-time operating system

One example of a real-time operating system is Lynx. It is designed to process tasks with strict time requirements. This kind of OS ensures that processing is done within specific timeframes without any delay.

On the contrary, Process Control is not an RTOS since processing time requirements, including any OS delay, are measured in tenths of seconds or shorter increments of time. Similarly, MS DOS and Windows XP are not real-time operating systems since they are not designed to respond to immediate processing demands.

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