Semaphores

Question 1
A
Both 1 and 2 are true
B
1 is true but 2 is false
C
2 is true but 1 is false
D
Both 1 and 2 are false
       Operating-Systems       Semaphores       Gate 2005-IT
Question 1 Explanation: 
P1 can be starves on P, then P2 loops forever.
P2 can be starves on P, then P1 loops forever.
Both statements (i) and (ii) are True.
Question 2
At a particular time of computation the value of a counting semaphore is 7. Then 20 P operations and 15 V operations were completed on this semaphore. The resulting value of the semaphore is:
A
42
B
2
C
7
D
12
       Operating-Systems       Semaphores       Gate-1992
Question 2 Explanation: 
Let the semaphore be S.
Initial value of S is 7.
So after 20P operations and 15V operations the value of semaphore S will be,
S = 7-20+15 = 2
There are 2 questions to complete.