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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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4 Answers

Best answer
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48 votes

Answer is (C)

SSTF: $(90) \ 120 \ 115 \ 110 \ 130 \ 80 \ 70 \ 30 \ 25 \ 20$

Direction changes at $120,110,130$

FCFS: $(90) \ 120 \ 30 \ 70 \ 115 \ 130 \ 110 \ 80 \ 20 \ 25$

direction changes at $120,30,130,20$

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Here it is mentioned in the question that the disk was servicing request from track 120 and previously it has serviced track 90 so
from 90 to 120 it must move in one direction (lets consider this direction -->)  then from 120 it must read 115(SSTF) which is in (<---) direction.
So the answer is C.
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According to Shortest Seek Time First:
90-> 120-> 115-> 110-> 130-> 80-> 70-> 30-> 25-> 20
Change of direction(Total 3); 120->15; 110->130; 130->80
According to First Come First Serve:
90-> 120-> 30-> 70-> 115-> 130-> 110-> 80-> 20-> 25
Change of direction(Total 4); 120->30; 30->70; 130->110;20->25
Answer:

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