Which one or more of the following options guarantee that a computer system will transition from use

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GATE-CS is a competitive examination conducted in India. This question is from the 2023 GATE-CS Operating Systems paper.

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Question

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

Accepted Answer

Correct Answer: (C) and (D)Concept -- User Mode to Kernel Mode Transitions (from Silberschatz, Galvin, Gagne, Operating System Concepts):(A) FALSE -- Function Call: A regular function call (within user space, such as calling a library function or a user-defined function) stays in user mode. It simply modifies the stack and instruction pointer within the process's own address space. No mode switch occurs.(B) FALSE -- malloc Call: malloc is a C library function (from glibc or similar) that runs in user space. It manages a heap region in user space. While malloc may occasionally call a system call like sbrk or mmap when it needs more memory from the OS, the malloc call itself does not always trigger a mode switch. The call to malloc does not guarantee a transition to kernel mode.(C) TRUE -- Page Fault: A page fault is a hardware exception that always causes a transition from user mode to kernel mode. The CPU detects that the requested page is not present in physical memory (or the access is invalid), raises a fault, and the hardware automatically switches to kernel mode and transfers control to the OS page fault handler.(D) TRUE -- System Call: A system call (e.g., read(), write(), fork()) is an explicit request from a user-space program to the OS kernel to perform a privileged operation. System calls always cause a mode switch from user mode to kernel mode via a software interrupt, trap instruction, or special instruction (like SYSCALL/SYSENTER in x86).

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

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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

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