ITPEC FE Morning April 2022 Question 12
What is the approximate average access time in milliseconds (ms) of a magnetic disk with the specifications shown in the table below? Approximate average access time is the sum of average seek time, track-to-track seek time, and average rotational delay. Here, the controller overhead can be ignored.
Explanation
12.87 ms — add the two seek times and half a rotation, as the question defines the access time.
- •One rotation: 7,200 rpm is 120 rotations per second, so 1,000 ÷ 120 ≈ 8.33 ms
- •Average rotational delay: half a rotation, 8.33 ÷ 2 ≈ 4.17 ms
- •Access time: 7.5 + 1.2 + 4.17 ≈ 12.87 ms
Answer (b)
Why not others:
- •(a) 11.67 — 7.5 + 4.17, leaving out the track-to-track seek time that the definition includes
- •(c) 15.83 — 7.5 + 8.33, a full rotation instead of half, and without the track-to-track seek time
- •(d) 25 — larger than any combination of the given figures; even both seek times plus a full rotation are only 17.03 ms
Key rule: The average rotational delay is half the time of one rotation: 60 ÷ rpm seconds per turn, halved.
Computer Component · AI-written, may contain errors
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