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GO Classes DPP | OS |DISK

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A disk drive has $\mathbf{200}$ cylinders $($numbered $0$ to $199)$. The disk head is currently positioned at cylinder $\mathbf{50}$, and the previous request serviced was at cylinder 40. The queue of pending requests in FIFO order is$: \mathbf{80, 140, 25, 170, 35.}$

Which of the following statement(s) is/are TRUE?

  1. Using the SSTF (Shortest Seek Time First) algorithm, the total head movement is $185$ cylinders.
     
  2. Using the SCAN algorithm (moving toward higher cylinders first), the total head movement is $220$ cylinders.
     
  3. Using the LOOK algorithm, the disk head will not travel to cylinder $199$ if no request exists there.
     
  4. The C-SCAN algorithm starts from beginning after reaching end.

1 Answer

1 1 vote

(A) SSTF (Shortest Seek Time First):

Path: $50 \rightarrow 35 \rightarrow 25 \rightarrow 80 \rightarrow 140 \rightarrow 170$

Total Movement: $15+10+55+60+30=\mathbf{1 7 0}$ cylinders.

(B) SCAN Algorithm:

Path: $50 \rightarrow 80 \rightarrow 140 \rightarrow 170 \rightarrow 199 \rightarrow 35 \rightarrow 25$

Total Movement: $(199-50)+(199-25)=149+174=\mathbf{3 2 3}$ cylinders.

(C) LOOK Algorithm:

  • Unlike SCAN, the LOOK algorithm reverses immediately after the last request in a direction is serviced $(170)$, without going to the end of the disk $(199)$.
     
  • Conclusion: $(C)$ is TRUE.
     

Option C,D are true.

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