A disk has cylinders 0 to 199. The head is at cylinder 70 and the request queue is 120, 35, 180, 10, 95, 150, 60. The head moves towards higher cylinders under C-SCAN. Counting the return sweep from 199 to 0 as head movement, what is the total?
Correct answer D. 388 cylinders
Explanation
C-SCAN serves requests in one direction only. It moves from 70 to 199 (129) serving 95 to 180, returns from 199 to 0 (199) without serving anything, then sweeps up to 60 serving 10, 35 and 60 (60). The total is 129 + 199 + 60 = 388 cylinders. C-SCAN gives more uniform waiting times than SCAN, because the cylinders just passed are not revisited immediately.
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