Which one or more of the following options guarantee that a computer system will transition from user mode to kernel mode?
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GATE CSE 2023 · Q23
MSQ1 markEasy
Time
Which one or more of the following options guarantee that a computer system will transition from user mode to kernel mode?
Select all that applyOne or more correct
Pick every correct option
Solution
Step-by-step verified solution
Official answer: C) Page Fault; D) System Call
System CallsA computer system transitions from user mode to kernel mode via hardware interrupts, software interrupts/traps (such as system calls), or exceptions.
(A) Function Call: A standard user-space function call simply manipulates the user-space call stack and program counter. It does not cause a switch to kernel mode.
(B)
mallocCall:mallocis a C standard library function that manages memory in user space. If previously allocated heap space is available,mallocsatisfies the allocation request entirely within user mode without invoking a system call (likebrkormmap). Hence, it does not guarantee a transition to kernel mode.(C) Page Fault: A page fault is a hardware exception generated by the Memory Management Unit (MMU) when accessing a page not present in main memory (or on permission violation). Servicing this fault requires the OS page fault handler, guaranteeing a transition to kernel mode.
(D) System Call: A system call explicitly triggers a software trap/interrupt to request operating system services, guaranteeing a transition from user mode to kernel mode.
Thus, options C and D are correct.
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Examiner trap
The misconception this question tests
Consider which events trigger hardware/software traps or exceptions that must be handled exclusively by privileged OS handlers rather than user-space code.
This System Calls previous year question is Q23 of the GATE CSE 2023 paper. Find more GATE CSE PYQs on System Calls in Operating Systems, or browse all GATE CSE PYQs.
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System Calls in GATE CSE
System Calls has 4 GATE CSE PYQs across 4 papers from 2011 to 2024. 2 of them are 1-mark MSQs like this one, and it is the only one in GATE CSE 2023.
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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.