Operating System Multiple Choice Questions on “Semaphores”.
1. An un-interruptible unit is known as ____________
a) single
b) atomic
c) static
d) none of the mentioned
Answer: b
Clarification: None.
2. TestAndSet instruction is executed ____________
a) after a particular process
b) periodically
c) atomically
d) none of the mentioned
Answer: c
Clarification: None.
3. Semaphore is a/an _______ to solve the critical section problem.
a) hardware for a system
b) special program for a system
c) integer variable
d) none of the mentioned
Answer: c
Clarification: None.
4. What are the two atomic operations permissible on semaphores?
a) wait
b) stop
c) hold
d) none of the mentioned
Answer: a
Clarification: None.
5. What are Spinlocks?
a) CPU cycles wasting locks over critical sections of programs
b) Locks that avoid time wastage in context switches
c) Locks that work better on multiprocessor systems
d) All of the mentioned
Answer: d
Clarification: None.
6. What is the main disadvantage of spinlocks?
a) they are not sufficient for many process
b) they require busy waiting
c) they are unreliable sometimes
d) they are too complex for programmers
Answer: b
Clarification: None.
7. The wait operation of the semaphore basically works on the basic _______ system call.
a) stop()
b) block()
c) hold()
d) wait()
Answer: b
Clarification: None.
8. The signal operation of the semaphore basically works on the basic _______ system call.
a) continue()
b) wakeup()
c) getup()
d) start()
Answer: b
Clarification: None.
9. If the semaphore value is negative ____________
a) its magnitude is the number of processes waiting on that semaphore
b) it is invalid
c) no operation can be further performed on it until the signal operation is performed on it
d) none of the mentioned
Answer: a
Clarification: None.
10. The code that changes the value of the semaphore is ____________
a) remainder section code
b) non – critical section code
c) critical section code
d) none of the mentioned
Answer: c
Clarification: None.
11. The following program consists of 3 concurrent processes and 3 binary semaphores. The semaphores are initialized as S0 = 1, S1 = 0, S2 = 0.
Process P0
while(true)
{
wait(S0);
print '0';
release(S1);
release(S2);
}
Process P1
wait(S1);
release(S0);
Process P2
wait(S2);
release(S0);
How many times will P0 print ‘0’?
a) At least twice
b) Exactly twice
c) Exactly thrice
d) Exactly once
Answer: a
Clarification: None.
12. Each process Pi, i = 0,1,2,3,……,9 is coded as follows.
repeat
P(mutex)
{Critical Section}
V(mutex)
forever
The code for P10 is identical except that it uses V(mutex) instead of P(mutex). What is the largest number of processes that can be inside the critical section at any moment (the mutex being initialized to 1)?
a) 1
b) 2
c) 3
d) None of the mentioned
Answer: c
Clarification: Any one of the 9 processes can get into critical section after executing P(mutex) which decrements the mutex value to 0. At this time P10 can enter critical section by incrementing the value to 1. Now any of the 9 processes can enter the critical section by again decrementing the mutex value to 0. None of the remaining processes can get into their critical sections.
13. Two processes, P1 and P2, need to access a critical section of code. Consider the following synchronization construct used by the processes.
Process P1 :
while(true)
{
w1 = true;
while(w2 == true);
Critical section
w1 = false;
}
Remainder Section
Process P2 :
while(true)
{
w2 = true;
while(w1 == true);
Critical section
w2 = false;
}
Remainder Section
Here, w1 and w2 have shared variables, which are initialized to false. Which one of the following statements is TRUE about the above construct?
a) It does not ensure mutual exclusion
b) It does not ensure bounded waiting
c) It requires that processes enter the critical section in strict alternation
d) It does not prevent deadlocks but ensures mutual exclusion
Answer: d