Disk requests come to disk driver for cylinders 10, 22, 20, 2, 40, 6 and 38 in that order at a time when the disk drive is reading from cylinder 20. The seek time is 6 msec per cylinder. Compute the total seek time if the disk arm scheduling algorithm is first come first served is
876 msec
First-come-first-served disk scheduling serves requests strictly in arrival order, so the head moves from its current cylinder to each requested cylinder in turn. Total seek time = total cylinders crossed × seek time per cylinder.
Step 1 — Head movements from cylinder 20: 20 → 10 = 10, 10 → 22 = 12, 22 → 20 = 2, 20 → 2 = 18, 2 → 40 = 38, 40 → 6 = 34, 6 → 38 = 32.
Step 2 — Total distance \(= 10 + 12 + 2 + 18 + 38 + 34 + 32 = 146\) cylinders.
Step 3 — Total seek time \(= 146 \times 6 = 876\) msec.
The values 868, 884 and 892 msec correspond to 144.7, 147.3 and 148.7 cylinders, which are not whole-number movements, so they come from mis-added distances.
876 msec option is correct.
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