Q. Which of the following is FALSE about SJF (Shortest Job First Scheduling)?

S1: It causes minimum average waiting time
S2: It can cause starvation

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Q. Two concurrent processes P1 and P2 use four shared resources R1, R2, R3 and R4, as shown below.

P1 P2
Compute: Use R1; Use R2; Use R3; Use R4; Compute; Use R1; Use R2; Use R3;. Use R4;

Both processes are started at the same time, and each resource can be accessed by only one process at a time The following scheduling constraints exist between the access of resources by the processes:

P2 must complete use of R1 before P1 gets access to R1
P1 must complete use of R2 before P2 gets access to R2.
P2 must complete use of R3 before P1 gets access to R3.
P1 must complete use of R4 before P2 gets access to R4.

There are no other scheduling constraints between the processes. If only binary semaphores are used to enforce the above scheduling constraints, what is the minimum number of binary semaphores needed?  

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Q. We wish to schedule three processes P1, P2 and P3 on a uniprocessor system. The priorities, CPU time requirements and arrival times of the processes are as shown below.

 Process  Priority  CPU time required  Arrival time (hh:mm:ss)
 P1 10(highest)  20 sec  00:00:05
 P2  9  10 sec  00:00:03
 P3  8 (lowest)  15 sec  00:00:00

We have a choice of preemptive or non-preemptive scheduling. In preemptive scheduling, a late-arriving higher priority process can preempt a currently running process with lower priority. In non-preemptive scheduling, a late-arriving higher priority process must wait for the currently executing process to complete before it can be scheduled on the processor. What are the turnaround times (time from arrival till completion) of P2 using preemptive and non-preemptive scheduling respectively.

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Q. Consider an arbitrary set of CPU-bound processes with unequal CPU burst lengths submitted at the same time to a computer system. Which one of the following process scheduling algorithms would minimize the average waiting time in the ready queue?

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Q. Assume every process requires 3 seconds of service time in a system with single processor. If new processes are arriving at the rate of 10 processes per minute, then estimate the fraction of time CPU is busy in system?

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Q. Consider the following code fragment:

if (fork() == 0)
{ a = a + 5; printf("%d,%d\n", a, &a); }
else { a = a –5; printf("%d, %d\n", a, &a); }

Let u, v be the values printed by the parent process, and x, y be the values printed by the child process. Which one of the following is TRUE?

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Q. The atomic fetch-and-set x, y instruction unconditionally sets the memory location x to 1 and fetches the old value of x in y without allowing any intervening access to the memory location x. consider the following implementation of P and V functions on a binary semaphore .

void P (binary_semaphore *s) {
unsigned y;
unsigned *x = &(s->value);
do {
fetch-and-set x, y;
} while (y);
}

void V (binary_semaphore *s) {
S->value = 0;
}

Which one of the following is true?

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Q. Three concurrent processes X, Y, and Z execute three different code segments that access and update certain shared variables. Process X executes the P operation (i.e., wait) on semaphores a, b and c; process Y executes the P operation on semaphores b, c and d; process Z executes the P operation on semaphores c, d, and a before entering the respective code segments. After completing the execution of its code segment, each process invokes the V operation (i.e., signal) on its three semaphores. All semaphores are binary semaphores initialized to one. Which one of the following represents a deadlockfree order of invoking the P operations by the processes?

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Q. A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes W and X reads x from memory, increments by one, stores it to memory, and then terminates. Each of the processes Y and Z reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P operation (i.e., wait) on a counting semaphore S and invokes the V operation (i.e., signal) on the semaphore S after storing x to memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution?

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Q. A process executes the code
fork();
fork();
fork();
The total number of child processes created is

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Q. 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;
}
ReleaseLock(L){
L = 0;
}

This implementation

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Q. The time taken to switch between user and kernel modes of execution be t1 while the time taken to switch between two processes be t2. Which of the following is TRUE?

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Q. A thread is usually defined as a "light weight process" because an operating system (OS) maintains smaller data structures for a thread than for a process. In relation to this, which of the following is TRUE?

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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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