ITPEC FE Subject A April 2024 Question 9
When an HDD has the specifications below, what is the average access time for an HDD to transfer 1 MB of data in ms? Here, the average access time can be calculated as the sum of the average seek time, controller overhead, rotational latency, and transfer time. Other overheads can be ignored.
Explanation
27.92 ms — add the seek time, controller overhead, average rotational latency and transfer time.
- •One rotation: 60,000 ÷ 5,000 = 12 ms, so the average rotational latency is half of that, 6 ms
- •Transfer time: 1 ÷ 60 s ≈ 16.67 ms
- •Total: 4 + 1.25 + 6 + 16.67 = 27.92 ms
Answer (c)
Why not others:
- •(a) 21.93 — leaves out the rotational latency: 4 + 1.25 + 16.67 ≈ 21.92
- •(b) 26.67 — leaves out the controller overhead: 4 + 6 + 16.67
- •(d) 33.92 — takes a full rotation, 12 ms, as the latency instead of half of one
Key rule: Average rotational latency is half a revolution, and one revolution takes 60,000 ÷ rpm milliseconds.
Computer Component · AI-written, may contain errors
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