GATE CS 2023 Q13 — System Calls

To determine which options guarantee a transition from user mode to kernel mode, let us analyze each choice:

1. Function Call: A standard user-space function call (e.g., calling a user-defined function) is managed entirely within the user-space stack. It does not involve the operating system kernel and thus remains in user mode.
2. malloc Call: malloc() is a standard library function (not a system call) that manages a memory pool in user space. If the requested memory can be allocated from this pre-allocated user-space pool, malloc() returns a pointer without invoking the operating system. It only makes a system call (such as brk or mmap) when the pool is exhausted. Therefore, it does not guarantee a transition to kernel mode.
3. Page Fault: A page fault is a hardware exception that occurs when a program attempts to access a virtual memory page that is not currently mapped to physical RAM. The hardware immediately transfers control to the operating system's page fault handler, which runs in kernel mode. This guarantees a transition to kernel mode.
4. System Call: A system call is the programmatic way a user application requests a service from the kernel (e.g., file I/O, process creation). Executing a system call triggers a software interrupt or trap instruction, which immediately transitions the CPU from user mode to kernel mode to execute the service. This guarantees a transition to kernel mode.

Thus, only Page Fault and System Call guarantee a transition to kernel mode.