# Operating system ..ace academy test series

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A disk is double interleaved. It has 8 secret per track of 512 byte each.rotation rate is 300 rpm.assume that arm is correctly positioned and 1/2 rotation is needed to get secter 0 under the head .the time needed to read all 8 section of track in order is .....(in milisecond)
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Is it 600ms?
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what is meaning of double interleaved?
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srestha  This is a new term for me also..

This means that two sectors which were supposed to be consecutively placed is interleaved with two other sectors.

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No correct answer is 650 ms
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Ok Yes i did a mistake..now getting 650 ms
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How?

300 rpm  => 300 rotations --> 60 secs

1 rotation --> 0.2 secs

Now in one rotation we can scan the entire track. But in double interleaved disk the sectors are not arranged sequentially like 0,1,2,3...7. They are separated by 2 interleaved sectors. And it is given that the scanning should be done in order.

See this :

In one rotation the head moves over 8 sectors which takes 0.2s (i.e. time for 1 rotation).

So for 1 sector it takes 0.2/8 =0.025s. Let us take t=0.025s.

It is given that 1/2 a rotation is needed to place the head over sector 0.

1 rotation as we calculated takes 0.2s. So 1/2 rot will take 0.1s.

Now head is moved to beginning of sector 0.

Recall, Time taken to scan 1 sector is t. So t secs is needed to scan sector 0.

Now we have to move to sector 1. This will require moving past sector no. 3,6 i.e. 2 sector time i.e. 2t. To scan sector 1 another t. So to finish scanning sector 1 we need 3t secs.

Now we have to move to sector 2. This will require moving the head over 4,7 i.e. 2 sectors. Time taken is 2t and to scan sector 2 another t sec. So again 3t to finish scanning sector 2.

Likewise we can see for scanning all the sectors from 1 to 7 it takes 7*3t=21t.

Total time taken = Time taken to position head over sector 0 + time to scan sector 0 + 21t

=  0.1 s + t +21t = 0.1 +22t = 0.1 +22*0.025 = 0.1 +0.55=0.65 s =650ms.

edited

see figure 1 .  it is non interleaving so when we have to read the data of single track , we can read the all track in single rotation (which we do normally )

see figure 2    it is single interleaving so when we have to read the data of single track, we can read all data of single track  in 2 round

because

in single interleaved disk the sectors are not arranged sequentially like 0,1,2,3...7. They are separated by 1 interleaved sectors.

means

when we read 0 sector  after we have to pass through 4 to read sector 1 and to read sector 2 we have to pass through 5

so

to read total data (one track data) it will take 2 rotation

see figure 3    it is double interleaving so when we have to read the data of single track, we can read all data of single track  in 2.75 round   why ?? watch the hint

hint watch the 7 in the figure means when we read all the 8 sector we have to cover 2 full rotation + 3/4 rotation

now come to question

300 rpm  => 300 rotations --> 60 secs

1 rotation --> 0.2 secs

and  time for arm for correct it at position  0 sector is =1/2 of  full rotation =0.1 s

.so total time to read all data =time to set arm + time to read data

= 0.1 s+2.75 * 0.2 s

=0.65 second

=650 ms

edited
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