GATE CS 2024 Set 2 Q15 — Processes and Threads

To determine which events always force the running process $P$ into a non-running state (such as ready or blocked), let us analyze each option:

• Option A (Correct): A blocking system call means process $P$ cannot proceed until the requested disk I/O completes. The OS must move $P$ to the blocked state and context switch to another ready process.
• Option B (Correct): A page fault for a page in the swap space requires disk I/O to bring the page into main memory. Since disk access is slow, the OS blocks $P$ and context switches to another process to utilize the CPU. Thus, $P$ always moves to the blocked state.
• Option C (Incorrect): An interrupt for another process is handled by the Interrupt Service Routine (ISR). After the ISR finishes, the OS can resume the execution of $P$ directly if $P$ still has the highest priority, without moving $P$ to a non-running state.
• Option D (Incorrect): A timer interrupt triggers the scheduler, but if $P$ is the only ready process in the system, the scheduler will immediately resume $P$. Thus, it does not always force $P$ to transition to a non-running state.