# GATE | GATE-CS-2009 | Question 51

A hard disk has 63 sectors per track, 10 platters each with 2 recording surfaces and 1000 cylinders. The address of a sector is given as a triple (c, h, s), where c is the cylinder number, h is the surface number and s is the sector number. Thus, the 0th sector is addressed as (0, 0, 0), the 1st sector as (0, 0, 1), and so on
The address <400,16,29> corresponds to sector number:
(A) 505035
(B) 505036
(C) 505037
(D) 505038

Explanation: The data in hard disk is arranged in the shown manner. The smallest division is sector. Sectors are then combined to make a track. Cylinder is formed by combining the tracks which lie on same dimension of the platters.
Read write head access the disk. Head has to reach at a particular track and then wait for the rotation of the platter so that the required sector comes under it.
Here, each platter has two surfaces, which is the r/w head can access the platter from the two sides, upper and lower.
So,<400,16,29> will represent 400 cylinders are passed(0-399) and thus, for each cylinder 20 surfaces (10 platters * 2 surface each) and each cylinder has 63 sectors per surface.
Hence we have passed 0-399 =  400 * 20 * 63 sectors + In 400th cylinder we have passed 16 surfaces(0-15) each of which again contains 63 sectors per cylinder so 16 * 63 sectors. + Now on the 16th surface we are on 29th sector.
So, sector no = 400x20x63 + 16×63 + 29 = 505037.

Reference :

https://www.ilbe.com/1144674842

This solution is contributed by Shashank Shanker khare.

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