ITPEC FE Morning April 2022 Question 17
An OS chooses a process for CPU execution based on the First-Come-First-Serve (FCFS) scheduling algorithm. There are four processes, P1, P2, P3, and P4, and their-arrival times and burst times are shown in the table below. Which of the following is the appropriate combination of waiting times for the processes to be executed? Here, the arrival time is the time at which a process enters the ready queue and is ready for execution, the burst time is the time a process requires for CPU execution, and the waiting time is the period of time a process spends in the ready queue waiting for the CPU to start execution.
Unit: milliseconds
Explanation
0, 1, 4, 10 ms — run the processes in arrival order; each waits from its arrival until the previous one finishes.
Start = the later of the arrival time and the previous finish; waiting = start − arrival.
Answer (a)
Why not others:
- •(b) 0, 2, 4, 5 — these are the arrival times, not the waiting times
- •(c) 0, 3, 5, 7 — each value is the burst time of the process just before, ignoring both the earlier queue and the arrival time
- •(d) 0, 3, 8, 15 — these are the start times; the arrival time is not subtracted
Key rule: Waiting time = start time − arrival time; build the start and finish timeline first, then subtract.
Software · AI-written, may contain errors
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