UGC NET CSE | January 2017 | Part 2 | Question: 36

Question

There are three processes $P_{1}, P_{2}$ and $P_{3}$ sharing a semaphore for synchronizing a variable. Initial value of semaphore is one. Assume that negative value of semaphore tells us how many processes are waiting in queue. Processes access the semaphore in following order :

1. $P2$ needs to access

2. $P1$ needs to access

100. $P3$ needs to access

500. $P2$ exits critical section

5. $P1$ exits critical section

The final value of semaphore will be :

• A. $0$
• B. $1$
• C. $-1$
• D. $-2$

Comments

sem=1 => P2 accessed makes sem = 0

                P1 accessed makes sem=-1

               P3 accessed makes sem = -2

               P2 exits CS makes sem = -1

                P1 exits CS makes sem = 0

Final sem = 0

Answer 1

initial value of semaphores S=1

1. P2 needs to access decreases it to 0    (no one is waiting)
2. P1 needs to access decreases it to -1   (one process is waiting)
3. P3 needs to access decreases it to -2 ( 2 process r waiting)
4. P2 exits critical section increases it to -1 ( one process is waiting)
5. Pi exits critical section increases it to 0   ( no process is waiting)                                                                                                                    so ans is A

Answer 2

Initially the value of semaphore S is 1.

After (a) S=0
(b) S=-1
(c) S= -2
(d) S= -1
(e) S= 0

  ans is A

Comments

3 process operation

So initial value is 1-3+2=0

 

Ans is A

Answer 3

3 p operation and 2 v operation , so initial value is 1 -3+2=0

Answer 4

Option 1

sem=1 => P2 accessed makes sem = 0

                P1 accessed makes sem=-1

               P3 accessed makes sem = -2

               P2 exits CS makes sem = -1 

                P1 exits CS makes sem = 0

Final sem = 0