Consider a process $P$ running on a CPU. Which one or more of the following events will always trigger a context switch by the OS that results in process $P$ moving to a non-running state (e.g., ready, blocked)?

A running process must leave the running state and undergo a context switch if it can no longer make progress on the CPU until some external operation completes:

• (A) is correct: When process $P$ issues a blocking system call for disk I/O, it cannot proceed until the requested data is fetched from the secondary storage. The OS moves $P$ to the blocked/waiting state and schedules another ready process to run.

• (B) is correct: A page fault occurs when $P$ accesses a page currently in swap space. Fetching the page from disk is a high-latency I/O operation; the OS immediately transitions $P$ to the blocked/waiting state while the page is brought into memory, triggering a context switch.

• (C) is incorrect: An external I/O interrupt for another process is handled in the kernel mode via an Interrupt Service Routine (ISR). Once the ISR completes, if $P$ remains the highest-priority runnable process, execution of $P$ resumes immediately without moving $P$ to a non-running state.

• (D) is incorrect: A timer interrupt occurs periodically to update system clocks and accounting. It only preempts $P$ and moves it to the ready state if $P$'s time slice has expired; if time remains or if $P$ is the only runnable task, $P$ continues execution without switching to a non-running state.