1
GATE CSE 2013
MCQ (Single Correct Answer)
+2
-0.6
A certain computation generates two arrays a and b such that a[i]=f(i)for 0 ≤ i < n and b[i] = g (a[i] )for 0 ≤ i < n. Suppose this computation is decomposed into two concurrent processes X and Y such that X computes the array a and Y computes the array b. The processes employ two binary semaphores R and S, both initialized to zero. The array a is shared by the two processes. The structures of the processes are shown below.
Process X:
private i;
for(i = 0; i < n; i++){
 a [i] = f (i);
 Exit X (R, S);
}

Process Y:
private i;
for(i = 0; i < n; i++){
 Entry Y (R, S);
 b [i] = g (a [i] );
}
Which of the following represents the correct implementations of Exit X and Entry Y?
A
Exit X (R, S){
  P(R);
  V(S);
}
Entry Y (R, S){
  P(S);
  V(R);
}
B
Exit X (R, S){
  V(R);
  V(S);
}
Entry Y (R, S){
  P(R);
  P(S);
}
C
Exit X (R, S){
  P(S);
  V(R);
}
Entry Y (R, S){
  V(S);
  P(R);
}
D
Exit X (R, S){
  V(R);
  P(S);
}
Entry Y (R, S){
  V(S);
  P(R);
}
2
GATE CSE 2012
MCQ (Single Correct Answer)
+2
-0.6
Fetch_And_Add (X, i) is an atomic Read-Modify-Write instruction that reads the value of memory location X, increments it by the value i, and returns the old value of X. It is used in the pseudocode shown below to implement a busy-wait lock. L is an unsigned integer shared variable initialized to 0. The value of 0 corresponds to lock being available, while any non-zero value corresponds to the lock being not available.
AcquireLock(L){ 
  While (Fetch_And_Add(L,1)) 
  L = 1; 
} 
Release Lock(L){ 
  L = 0; 
}
This implementation
A
fails as L can overflow
B
fails as L can take on a non-zero value when the lock is actually available
C
works correctly but may starve some processes
D
works correctly without starvation
3
GATE CSE 2010
MCQ (Single Correct Answer)
+2
-0.6
The following program consists of 3 concurrent processes and 3 binary semaphores. The semaphores are initialized as S0=1, S1=0, S2=0. GATE CSE 2010 Operating Systems - Synchronization and Concurrency Question 13 English How many times will process P0 print '0'?
A
At least twice
B
Exaclty twice
C
Exactly thrice
D
Exactly once
4
GATE CSE 2009
MCQ (Single Correct Answer)
+2
-0.6
The enter_CS() and leave_CS() functions to implement critical section of a process are realized using test-and-set instruction as follows:
void enter_CS(X) { 
   while test-and-set(X) ; 
} 

void leave_CS(X) { 
  X=0; 
}
In the above solution, X is a memory location associated with the CS and is initialized to 0. Now consider the following statements:

I. The above solution to CS problem is deadlock-free
II. The solution is starvation free.
III. The processes enter CS in FIFO order.
IV More than one process can enter CS at the same time.

Which of the above statements is TRUE?
A
I only
B
I and II
C
II and III
D
IV only
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