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Processes, Threads, System Calls and Context Switching PYQs for GATE CS

Solve 9+ Processes, Threads, System Calls and Context Switching previous year questions for GATE CS with answers and detailed solutions. Free sample questions

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Question 1

2026 Slot Set1 PYQ

Consider the following program snippet. Assume that the program compiles and runs successfully. Further, assume that the fork() system call is always successful in creating a process.int main () {   int i;   for (i = 0; i < 3; i++){      if (fork() == 0){        continue;      }      break;   }   printf("Hello!");   return 0;}The total number of times that the printf statement gets executed is ________. (answer in integer)

Question 2

2025 Slot Set1 PYQ

Suppose in a multiprogramming environment, the following C program segment is executed. A process goes into I/O queue whenever an I/O related operation is performed. Assume that there will always be a context switch whenever a process requests for an I/O, and also whenever the process returns from an I/O. The number of times the process will enter the ready queue during its lifetime (not counting the time the process enters the ready queue when it is run initially) is _______. (Answer in integer)int main(){ int x=0,i=0; scanf("%d",&x); for(i=0; i<20; i++) { x = x+20; printf("%d\n",x); } return 0;}

Question 3

2024 Slot Set2 PYQ

Consider a process P running on a CPU. Which one or more of the following events will always trigger a context switch by the OS that results in process P moving to a non-running state (e.g., ready, blocked)?

Question 4

2024 Slot Set1 PYQ

Which of the following statements about threads is/are TRUE?

Question 5

2024 Slot Set1 PYQ

Consider the following code snippet using the fork() and wait() system calls. Assume that the code compiles and runs correctly, and that the system calls run successfully without any errors. int x = 3; while(x > 0) {     fork();     printf("hello");     wait(NULL);     x--; } The total number of times the printf statement is executed is _________

Question 6

2024 Slot Set1 PYQ

Which of the following process state transitions is/are NOT possible?

Question 7

2023 PYQ

Which one or more of the following options guarantee that a computer system will transition from user mode to kernel mode?

Question 8

2023 PYQ

Which one or more of the following need to be saved on a context switch from one thread (T1) of a process to another thread (T2) of the same process?

Question 9

2021 Slot Set1 PYQ

Which of the following standard C library functions will <i>always</i> invoke a system call when executed from a single-threaded process in a UNIX/Linux operating system?

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Processes, Threads, System Calls and Context Switching PYQs for GATE CS

Solve 9+ Processes, Threads, System Calls and Context Switching previous year questions for GATE CS with answers and detailed solutions. Free sample questions below.

Chapter Roadmap: Processes, Threads, System Calls and Context Switching

Chapter Journey: 4 Topics, One Clear Path

This chapter builds your understanding of how the operating system manages execution. Here is what you will master:

Topic 1: Process States, Ready Queues and Context Switching (Current)

Understand the lifecycle of a process: new, ready, running, blocked, terminated
Learn how the OS maintains ready queues and decides which process runs next
Master context switching: what gets saved, what gets loaded, and the overhead involved
Foundation for everything else in this chapter

Topic 2: Threads and Thread Context

Threads as lightweight units of execution within a process
What threads share versus what they keep private
Thread context switching versus process context switching

Topic 3: System Calls and User-Kernel Mode Transitions

The boundary between user mode and kernel mode
How programs request OS services through system calls
The mechanism of mode transition and its triggers

Topic 4: Process Creation using Fork and Wait

The fork system call: creating child processes
Understanding parent-child relationships
The wait system call: synchronization between processes

What You Will Achieve:

By the end of this chapter, you will be able to trace process state transitions, calculate context switch overhead, distinguish between process and thread behavior, predict the number of processes created by fork loops, and identify which operations trigger user-to-kernel mode transitions.

What is a Process? The Unit of Execution

Process = Program in Execution

A process is a program in execution. It is the fundamental unit of work in a modern operating system.

Program versus Process

Aspect	Program	Process
Nature	Passive entity	Active entity
Storage	Stored on disk	Loaded in memory
Analogy	Recipe in a cookbook	The act of cooking
Lifetime	Permanent file	Temporary execution

What Makes Up a Process?

A process in memory contains:

Text section: The compiled program code
Program counter: Points to the next instruction to execute
Stack: Temporary data, function parameters, return addresses, local variables
Data section: Global variables
Heap: Dynamically allocated memory (grows during execution)

Process Control Block (PCB)

The operating system tracks each process using a Process Control Block (PCB). Think of it as the process identity card. It contains:

Process state: current state (new, ready, running, blocked, terminated)
Program counter: address of next instruction
CPU registers: contents of all registers when process was last running
CPU scheduling information: priority, pointers to scheduling queues
Memory management information: base and limit registers, page tables
Accounting information: CPU time used, time limits, process number
I/O status information: list of I/O devices allocated, open files

The PCB is the key data structure that allows the operating system to support multiple processes simultaneously.

Processes, Threads, System Calls and Context Switching: Solved Questions with Step-by-Step Explanations (9 Problems)

Question 1 · Operating System · 2026_Set1 NAT

Consider the following program snippet. Assume that the program compiles and runs successfully. Further, assume that the fork() system call is always successful in creating a process.

int main () {
   int i;
   for (i = 0; i < 3; i++){
      if (fork() == 0){
        continue;
      }
      break;
   }
   printf("Hello!");
   return 0;
}

The total number of times that the printf statement gets executed is ________. (answer in integer)

Question 2 · Operating System · 2025_Set1 NAT

Suppose in a multiprogramming environment, the following C program segment is executed. A process goes into I/O queue whenever an I/O related operation is performed. Assume that there will always be a context switch whenever a process requests for an I/O, and also whenever the process returns from an I/O. The number of times the process will enter the ready queue during its lifetime (not counting the time the process enters the ready queue when it is run initially) is _______. (Answer in integer)

int main()
{
int x=0,i=0;
scanf("%d",&x);
for(i=0; i<20; i++)
{
x = x+20;
printf("%d\n",x);
}
return 0;
}

Question 3 · Operating System · 2024_Set2 MSQ

Consider a process P running on a CPU. Which one or more of the following events will always trigger a context switch by the OS that results in process P moving to a non-running state (e.g., ready, blocked)?

A.
P makes a blocking system call to read a block of data from the disk
B.
P tries to access a page that is in the swap space, triggering a page fault
C.
An interrupt is raised by the disk to deliver data requested by some other process
D.
A timer interrupt is raised by the hardware

Question 4 · Operating System · 2024_Set1 MSQ

Which of the following statements about threads is/are TRUE?

A.
Threads can only be implemented in kernel space
B.
Each thread has its own file descriptor table for open files
C.
All the threads belonging to a process share a common stack
D.
Threads belonging to a process are by default not protected from each other

Question 5 · Operating System · 2024_Set1 NAT

Consider the following code snippet using the fork() and wait() system calls. Assume that the code compiles and runs correctly, and that the system calls run successfully without any errors.

int x = 3;
while(x > 0) {
    fork();
    printf("hello");
    wait(NULL);
    x--;
}

The total number of times the printf statement is executed is _________

Question 6 · Operating System · 2024_Set1 MSQ

Which of the following process state transitions is/are NOT possible?

A.
Running to Ready
B.
Waiting to Running
C.
Ready to Waiting
D.
Running to Terminated

Question 7 · Operating System · 2023 MSQ

Which one or more of the following options guarantee that a computer system will transition from user mode to kernel mode?

A.
Function Call
B.
malloc Call
C.
Page Fault
D.
System Call

Question 8 · Operating System · 2023 MSQ

Which one or more of the following need to be saved on a context switch from one thread (T1) of a process to another thread (T2) of the same process?

A.
Page table base register
B.
Stack pointer
C.
Program counter
D.
General purpose registers

Question 9 · Operating System · 2021_Set1 MSQ

Which of the following standard C library functions will <i>always</i> invoke a system call when executed from a single-threaded process in a UNIX/Linux operating system?

A.
exit
B.
malloc
C.
sleep
D.
strlen

Try a question

Login to view the complete previous-year questions and solutions

Personalised first. High quality throughout.

Built around you, not around a syllabus PDF

Revision that hits your weak spots

Questions calibrated to the real exam

Notes written for recall, not for volume

One place for everything

Honest progress

Unlock the whole course

Processes, Threads, System Calls and Context Switching PYQs for GATE CS

Chapter Roadmap: Processes, Threads, System Calls and Context Switching

Chapter Journey: 4 Topics, One Clear Path

What is a Process? The Unit of Execution

Process = Program in Execution

Program versus Process

What Makes Up a Process?

Process Control Block (PCB)

Processes, Threads, System Calls and Context Switching: Solved Questions with Step-by-Step Explanations (9 Problems)

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