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ISRO | ISRO CS 2007 | Question 39
  • Last Updated : 27 Jun, 2018

Disk requests are received by a disk drive for cylinder 5, 25, 18, 3, 39, 8 and 35 in that order. A seek takes 5 msec per cylinder moved. How much seek time is needed to serve these requests for a Shortest Seek First (SSF) algorithm? Assume that the arm is at cylinder 20 when the last of these requests is made with none of the requests yet served

(A) 125 msec
(B) 295 msec
(C) 575 msec
(D) 750 msec


Answer: (B)

Explanation: The arm is at cylinder 20, so the service order = 18, 25, 35, 39, 8, 5, 3.

Seek time = (20−18) + (25−18) + (35−25) + (39−35) + (39−8) + (8−5) + (5−3)
= 2 + 7 + 10 + 4 + 31 + 3 + 2 = 59

Total seek time = 59 * 5 = 295

Option (B) is correct.

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