Q98 — Operating Systems

Operating Systems

Q98. Marks: +2.0 UGC NET Paper 2: Computer Science and Application 26th June 2025 Shift 1

📄 Passage

Consider the following disc Queue with requests for I/O to block on cylinders:

45, 81, 185, 33, 175, 99, 150, 77

(Consider that the head starts from location 58)

What shall be the total head movement of cylinders if the FCFS disc scheduling method is used?

1.536

2.594

3.647 ✓ Correct

4.700

Given:

Request queue (arrival order): 45, 81, 185, 33, 175, 99, 150, 77

Initial head position: 58

Policy: FCFS (First-Come, First-Served) → service strictly in the order listed.

Step-by-step head movements

Compute the absolute distance for each hop, starting at 58 and following the queue:

45 → 81 → 185 → 33 → 175 → 99 → 150 → 77

Starting from 58, movement should be: 58 → 45 → 81 → 185 → 33 → 175 → 99 → 150 → 77

Step-by-step movement

Total Head Movement $13 + 36 + 104 + 152 + 142 + 76 + 51 + 73 = \mathbf{647}$

✅ Answer: 647 cylinders

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