ITPEC FE Subject A April 2026 Question 15

11.5 ms — build the Round Robin schedule with a 5 ms slice, then subtract each job's own execution time from its completion time.

Execution order with bursts P1=5, P2=13, P3=4, P4=8:

• •0–5 P1 runs 5 ms and finishes at 5
• •5–10 P2 runs 5 ms (8 ms left)
• •10–14 P3 runs 4 ms and finishes at 14
• •14–19 P4 runs 5 ms (3 ms left)
• •19–24 P2 runs 5 ms (3 ms left)
• •24–27 P4 runs 3 ms and finishes at 27
• •27–30 P2 runs 3 ms and finishes at 30

Waiting time = completion − execution time:

• •P1: 5 − 5 = 0
• •P2: 30 − 13 = 17
• •P3: 14 − 4 = 10
• •P4: 27 − 8 = 19

Average: (0 + 17 + 10 + 19) ÷ 4 = 46 ÷ 4 = 11.5

Answer (c)

Why not others:
- (d) 19 — the average turnaround time ((5 + 30 + 14 + 27) ÷ 4), which forgets to subtract each job's own execution time
- (a) 7.5 — the average execution time ((5 + 13 + 4 + 8) ÷ 4), not a waiting time at all
- (b) 10 — results from stopping after the second round and letting P2 complete at 24, ignoring its remaining 3 ms

Key rule: Waiting time is time spent ready but not running: completion time minus the job's own CPU time. Build the full timeline first — the last round is easy to drop.

AI-generated — may contain errors