Operating Systems | Set 1

Following questions have been asked in GATE CS exam.

1. Which of the following is NOT a valid deadlock prevention scheme? (GATE CS 2000)
(a) Release all resources before requesting a new resource
(b) Number the resources uniquely and never request a lower numbered resource than the last one requested.
(c) Never request a resource after releasing any resource
(d) Request and all required resources be allocated before execution.

Answer: (c)
References:
http://www.cs.jhu.edu/~yairamir/cs418/os4/sld013.htm
http://en.wikipedia.org/wiki/Deadlock

2. Let m[0]…m[4] be mutexes (binary semaphores) and P[0] …. P[4] be processes.
Suppose each process P[i] executes the following:

  wait (m[i]); wait(m[(i+1) mode 4]);

  ------

  release (m[i]); release (m[(i+1)mod 4]);

This could cause (GATE CS 2000)
(a) Thrashing
(b) Deadlock
(c) Starvation, but not deadlock
(d) None of the above

Answer: (b)
Explanation:
You can easily see a deadlock in a situation where..
P[0] has acquired m[0] and waiting for m[1]
P[1] has acquired m[1] and waiting for m[2]
P[2] has acquired m[2] and waiting for m[3]
P[3] has acquired m[3] and waiting for m[0]

3. A graphics card has on board memory of 1 MB. Which of the following modes can the
card not support? (GATE CS 2000)
(a) 1600 x 400 resolution with 256 colours on a 17 inch monitor
(b) 1600 x 400 resolution with 16 million colours on a 14 inch monitor
(c) 800 x 400 resolution with 16 million colours on a 17 inch monitor
(d) 800 x 800 resolution with 256 colours on a 14 inch monitor

Answer: (b)
Explanation:
Monitor size doesn’t matter here. So, we can easily deduce that answer should be (b) as this has the highest memory requirements. Let us verify it.
Number of bits required to store a 16M colors pixel = ceil(log2(16*1000000)) = 24
Number of bytes required for 1600 x 400 resolution with 16M colors = (1600 * 400 * 24)/8 which is 192000000 (greater than 1MB).

4 Consider a virtual memory system with FIFO page replacement policy. For an arbitrary page access pattern, increasing the number of page frames in main memory will (GATE CS 2001)
a) Always decrease the number of page faults
b) Always increase the number of page faults
c) Some times increase the number of page faults
d) Never affect the number of page faults

Answer: (c)
Explanation:
Incrementing the number of page frames doesn’t always decrease the page faults (Belady’s Anomaly). For details see http://en.wikipedia.org/wiki/Belady%27s_anomaly

5. Which of the following requires a device driver? (GATE CS 2001)
a) Register
b) Cache
c) Main memory
d) Disk

Answer: (d)

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