GATE CS 2023 Set 1 — Question 23

MSQ+1 / -0MediumUser vs Kernel ModeOS Structure & System CallsOperating System

Operating System → OS Structure & System Calls → User vs Kernel Mode

Last updated 5 September 2026

Question

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

Correct answer (C) Page Fault; (D) System Call Solution A transition from user mode to kernel mode occurs when the CPU needs to handle an event that requires privileged access to hardware or OS services. Function Call : A standard function call within a program's own code executes entirely in user mode. malloc Call : malloc is a library function that manages a memory pool in user space. It only makes a system call (like brk or sbrk) when it needs to request more memory from the OS. Thus, it does not guarantee a transition every time it is called. Page Fault : This is a hardware-generated exception (trap) that occurs when a process attempts to access a memory page not currently in RAM. It forces an immediate transition to kernel mode so the OS can handle the fault. System Call : This is an explicit request by a user program for an OS service, which always involves a transition to kernel mode via a software interrupt or specialized instruction. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (C) Page Fault; (D) System Call Solution A transition from user mode to kernel mode occurs when the CPU needs to handle an event that requires privileged access to hardware or OS services. Function Call : A standard function call within a program's own code executes entirely in user mode. malloc Call : malloc is a library function that manages a memory pool in user space. It only makes a system call (like brk or sbrk) when it needs to request more memory from the OS. Thus, it does not guarantee a transition every time it is called. Page Fault : This is a hardware-generated exception (trap) that occurs when a process attempts to access a memory page not currently in RAM. It forces an immediate transition to kernel mode so the OS can handle the fault. System Call : This is an explicit request by a user program for an OS service, which always involves a transition to kernel mode via a software interrupt or specialized instruction.

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