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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Q. In the working-set strategy, which of the following is done by the operating system to prevent thrashing?

1.It initiates another process if there are enough extra frames.
2.It selects a process to suspend if the sum of the sizes of the working-sets exceeds the total number of available frames.

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Q. Processes P1 and P2 use critical_flag in the following routine to achieve mutual exclusion. Assume that critical_flag is initialized to FALSE in the main program.

get_exclusive_access ( ) { if (critical _flag == FALSE) { critical_flag = TRUE ; critical_region () ; critical_flag = FALSE; } }

Consider the following statements.
i. It is possible for both P1 and P2 to access critical_region concurrently.
ii. This may lead to a deadlock.
Which of the following holds?

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Q. An operating system implements a policy that requires a process to release all resources before making a request for another resource. Select the TRUE statement from the following:

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Q. If the time-slice used in the round-robin scheduling policy is more than the maximum time required to execute any process, then the policy will

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Q. Consider the following C code for process P1 and P2. a=4, b=0, c=0 (initialization)
P1 P2
if (a < 0) b = 10;
c = b-a; a = -3;
else
c = b+a;
If the processes P1 and P2 executes concurrently (shared variables a, b and c), which of the following cannot be the value of ‘c’ after both processes complete?

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Q. Which of the following is NOT a valid deadlock prevention scheme?

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Q. More than one word are put in one cache block to

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Q. Consider Peterson’s algorithm for mutual exclusion between two concurrent processes i and j. The program executed by process is shown below.

repeat
flag [i] = true;
turn = j;
while ( P ) do no-op;
Enter critical section, perform actions, then exit critical
section
flag [ i ] = false;
Perform other non-critical section actions.
until false;

For the program to guarantee mutual exclusion, the predicate P in the while loop should be.

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Q. Which combination of the following features will suffice to characterize an OS as a multi-programmed OS?

(a) More than one program may be loaded into main memory
at the same time for execution.
(b) If a program waits for certain events such as I/O,
another program is immediately scheduled for execution.
(c) If the execution of program terminates, another program
is immediately scheduled for execution.

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