GATE - 2004 | OS | The storage area of a disk has innermost diameter of 10 cm and

GATE - 2004 [Operating System]

Question:

The storage area of a disk has innermost diameter of 10 cm and outermost diameter of 20 cm. The maximum storage density of the disk is 1400 bits/cm. The disk rotates at a speed of 4200 RPM. The main memory of a computer has 64-bit word length and 1µs cycle time. If cycle stealing is used for data transfer from the disk, the percentage of memory cycles stolen for transferring one word is

A	0.5%
B	1%
C	5%
D	10%

  

Solution:

Option (C) is Correct.

y μs is cycle time.
x μs is data transfer time.
% of time CPU idle is,
y/x × 100
Maximum storage density is given, so consider innermost track to get the capacity
= 2 × 3.14 × 5 × 1700 bits
= 3.14 × 14000 bits
Rotational latency = 60/4200 s = 1/70 s
Therefore, to read 64 bits, time required
(10⁶ × 64)/(70 × 3.14 × 17000) μs = 20.8 μs
As memory cycle time given is 1μs,
Therefore, % CPU cycle stolen = (1/20.8) × 100 ≈ 5%

Thank You