Q. Consider the methods used by processes P1 and P2 for accessing their critical sections whenever needed, as given below. The initial values of shared boolean variables S1 and S2 are randomly assigned.

Method Used by P1
while (S1 == S2) ;
Critica1 Section
S1 = S2;

Method Used by P2
while (S1 != S2) ;
Critica1 Section
S2 = not (S1);

Which one of the following statements describes the properties achieved?

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Q. 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?

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Q. The P and V operations on counting semaphores, where s is a counting semaphore, are defined as follows:

P(s) : s = s - 1;
if (s < 0) then wait;
V(s) : s = s + 1;
if (s <= 0) then wakeup a process waiting on s;

Assume that Pb and Vb the wait and signal operations on binary semaphores are provided. Two binary semaphores Xb and Yb are used to implement the semaphore operations P(s) and V(s) as follows:

P(s) : Pb(Xb);
s = s - 1;
if (s < 0) {
Vb(Xb) ;
Pb(Yb) ;
}
else Vb(Xb);

V(s) : Pb(Xb) ;
s = s + 1;
if (s <= 0) Vb(Yb) ;
Vb(Xb) ;
The initial values of Xb and Yb are respectively.

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Q. A process executes the following code

for (i = 0; i < n; i++) fork();

The total number of child processes created is

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Q. Consider the following statements about user level threads and kernel level threads. Which one of the following statement is FALSE?

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Q. Two processes, P1 and P2, need to access a critical section of code. Consider the following synchronization construct used by the processes:Here, wants1 and wants2 are shared variables, which are initialized to false. Which one of the following statements is TRUE about the above construct?

/* P1 */
while (true) {
wants1 = true;
while (wants2 == true);
/* Critical
Section */
wants1=false;
}
/* Remainder section */


/* P2 */
while (true) {
wants2 = true;
while (wants1==true);
/* Critical
Section */
wants2 = false;
}
/* Remainder section */

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Q. Which one of the following is FALSE?

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Q. Consider two processors P1 and P2 executing the same instruction set. Assume that under identical conditions, for the same input, a program running on P2 takes 25% less time but incurs 20% more CPI (clock cycles per instruction) as compared to the program running on P1. If the clock frequency of P1 is 1GHz, then the clock frequency of P2 (in GHz) is _________.

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Q. Barrier is a synchronization construct where a set of processes synchronizes globally i.e. each process in the set arrives at the barrier and waits for all others to arrive and then all processes leave the barrier. Let the number of processes in the set be three and S be a binary semaphore with the usual P and V functions. Consider the following C implementation of a barrier with line numbers shown on left.

void barrier (void) {
1: P(S);
2: process_arrived++;
3. V(S);
4: while (process_arrived !=3);
5: P(S);
6: process_left++;
7: if (process_left==3) {
8: process_arrived = 0;
9: process_left = 0;
10: }
11: V(S);
}

The variables process_arrived and process_left are shared among all processes and are initialized to zero. In a concurrent program all the three processes call the barrier function when they need to synchronize globally. The above implementation of barrier is incorrect. Which one of the following is true?

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Q. Barrier is a synchronization construct where a set of processes synchronizes globally i.e. each process in the set arrives at the barrier and waits for all others to arrive and then all processes leave the barrier. Let the number of processes in the set be three and S be a binary semaphore with the usual P and V functions. Consider the following C implementation of a barrier with line numbers shown on left.

void barrier (void) {
1: P(S);
2: process_arrived++;
3. V(S);
4: while (process_arrived !=3);
5: P(S);
6: process_left++;
7: if (process_left==3) {
8: process_arrived = 0;
9: process_left = 0;
10: }
11: V(S);
}

The variables process_arrived and process_left are shared among all processes and are initialized to zero. In a concurrent program all the three processes call the barrier function when they need to synchronize globally. Which one of the following rectifies the problem in the implementation?

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Q. Consider two processes P1 and P2 accessing the shared variables X and Y protected by two binary semaphores SX and SY respectively, both initialized to 1. P and V denote the usual semaphone operators, where P decrements the semaphore value, and V increments the semaphore value. The pseudo-code of P1 and P2 is as follows :

P1 :
While true do {
L1 : ................
L2 : ................
X = X + 1;
Y = Y - 1;
V(SX);
V(SY);
}

P2 :
While true do {
L3 : ................
L4 : ................
Y = Y + 1;
X = Y - 1;
V(SY);
V(SX);
}

In order to avoid deadlock, the correct operators at L1, L2, L3 and L4 are respectively

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Q. Suppose we want to synchronize two concurrent processes P and Q using binary semaphores S and T. The code for the processes P and Q is shown below.

Process P:
while (1) {
W:
print '0';
print '0';
X:
}

Process Q:
while (1) {
Y:
print '1';
print '1';
Z:
}

Synchronization statements can be inserted only at points W, X, Y and Z. Which of the following will always lead to an output staring with '001100110011' ?

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Q. Suppose we want to synchronize two concurrent processes P and Q using binary semaphores S and T. The code for the processes P and Q is shown below.

Process P:
while (1) {
W:
print '0';
print '0';
X:
}

Process Q:
while (1) {
Y:
print '1';
print '1';
Z:
}

Synchronization statements can be inserted only at points W, X, Y and Z Which of the following will ensure that the output string never contains a substring of the form 01n0 or 10n1 where n is odd?

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Q. Which of the following does not interrupt a running process?

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Q. Which of the following need not necessarily be saved on a context switch between processes?

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Q. The following two functions P1 and P2 that share a variable B with an initial value of 2 execute concurrently.

P1()
{
C = B – 1;
B = 2*C;
}

P2()
{
D = 2 * B;
B = D - 1;
}

The number of distinct values that B can possibly take after the execution is

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Q. In a certain operating system, deadlock prevention is attempted using the following scheme. Each process is assigned a unique timestamp, and is restarted with the same timestamp if killed. Let Ph be the process holding a resource R, Pr be a process requesting for the same resource R, and T(Ph) and T(Pr) be their timestamps respectively. The decision to wait or preempt one of the processes is based on the following algorithm.
if T(Pr) < T(Ph)

then kill Pr

else wait
Which one of the following is TRUE?

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Q. A process executes the following segment of code :
for(i = 1; i < = n; i++)

fork ();
The number of new processes created is  

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Q. The semaphore variables full, empty and mutex are initialized to 0, n and 1, respectively. Process P1 repeatedly adds one item at a time to a buffer of size n, and process P2 repeatedly removes one item at a time from the same buffer using the programs given below. In the programs, K, L, M and N are unspecified statements.

 P1
while (1) {     K; P(mutex); Add an item to the buffer; V(mutex);     L; } P2 while (1) {    M; P(mutex); Remove an item from the buffer; V(mutex);     N; }

The statements K, L, M and N are respectively

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Q. Consider a non-negative counting semaphore S. The operation P(S) decrements S, and V(S) increments S. During an execution, 20 P(S) operations and 12 V(S) operations are issued in some order. The largest initial value of S for which at least one P(S) operation will remain blocked is ________.

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