GATE | Gate IT 2005 | Question 83

Q 81_Part B

A disk has 8 equidistant tracks. The diameters of the innermost and outermost tracks are 1 cm and 8 cm respectively. The innermost track has a storage capacity of 10 MB.

If the disk has 20 sectors per track and is currently at the end of the 5^th sector of the inner-most track and the head can move at a speed of 10 meters/sec and it is rotating at constant angular velocity of 6000 RPM, how much time will it take to read 1 MB contiguous data starting from the sector 4 of the outer-most track?

(A) 13.5 ms
(B) 10 ms
(C) 9.5 ms
(D) 20 ms


Answer: (A)

Explanation: Radius of innermost track = 0.5 cm
Radius of outermost track = 4 cm
Distance traversed by header = 4 – 0.5 = 3.5 cm = 3.5 / 100 m = 350 m
Time taken by header = 350 / 10 = 3500 s = 3.5 ms (speed: 10 meters/sec)

It is given that header at the end of the 5th sector of the innermost track.
To start from front of 4th sector it must rotate upto 18 sector.


Thus, header does 6000 rotations in 60,000ms (60 seconds) and 1 rotation in 10 ms.
So, to traverse sector 18 it takes 9 ms.

Storage capacity = 10 MB
Thus, 10 MB data is read in 10 ms.
1 MB can be read in 1 ms.

Total Time = Seek + Rotation + Transfer.
Total time taken to read 1 MB contiguous data = (1 + 9 + 3.5) ms = 13.5ms


Thus, option (A) is correct.

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