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ISRO CS 2025 Q.65: solution and answer

ISRO 2025 · CSQ.65 · 1 mark · MCQ

Which of the following transitions is not possible between the process states shown below?

  1. (A)

    Run → Ready

  2. (B)

    Run → Terminated

  3. (C)

    Run → Waiting

  4. (D)

    Waiting → Run

See the question in the paper
ISRO CS 2025 question 65Tap to open full size

Answer and solution

ANSWER

The transition that is not possible is Waiting → Run, option (D).

Official answer: DSource: ISRO's final answer key of 15 May 2026

  1. In the standard five-state process model the states are New, Ready, Running, Waiting (Blocked) and Terminated.

  2. The allowed transitions are: New → Ready (admit), Ready → Run (scheduler dispatch), Run → Ready (time slice expires or pre-emption), Run → Waiting (the process requests I/O or waits for an event), Waiting → Ready (I/O or event completes), Run → Terminated (the process exits).

  3. Option (a) Run → Ready is possible: a timer interrupt or pre-emption moves the running process back to the ready queue.

  4. Option (b) Run → Terminated is possible: the process finishes or is killed while running.

  5. Option (c) Run → Waiting is possible: a blocking system call such as an I/O request puts the process to sleep.

  6. Option (d) Waiting → Run is not possible directly: when the awaited event completes, the process moves to Ready and must be chosen by the scheduler before it runs.

  7. So the impossible transition is Waiting → Run.

KEY IDEA

A process that has finished waiting always returns to the Ready queue first. Only the scheduler's dispatch moves a process from Ready to Run.

WATCH OUT FOR

Thinking a process whose I/O has completed resumes running at once. It must rejoin the ready queue and wait for the CPU.

From the ISRO CS 2025 paper. Every question of this paper · The full paper (PDF, ISRO) · All 3 ISRO Processes & threads questions, with notes