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A disk has $200$ tracks (numbered $0$ through $199$). At a given time, it was servicing the request of reading data from track $120$, and at the previous request, service was for track $90$. The pending requests (in order of their arrival) are for track numbers.

$30\:70\:115\:130\:110\:80\:20\:25$

How many times will the head change its direction for the disk scheduling policies SSTF(Shortest Seek Time First) and FCFS (First Come First Serve)?

  1. $2$ and $3$
  2. $3$ and $3$
  3. $3$ and $4$
  4. $4$ and $4$
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Option C is correct.

Observation : R/W head is moving outside (90 to 120) initially.

For SSTF direction changes at following tracks : 120, 100 and 130.

For FCFS direction changes at following tracks : 120, 30, 130 and 20 .
Answer:

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