GATE | GATE IT 2006 | Question 57

The wait and signal operations of a monitor are implemented using semaphores as follows. In the following, 

• x is a condition variable,
• mutex is a semaphore initialized to 1,
• x_sem is a semaphore initialized to 0,
• x_count is the number of processes waiting on semaphore x_sem, initially 0, next is a semaphore initialized to 0,
• next_count is the number of processes waiting on semaphore next, initially 0.

P(mutex); 
body of procedure 
if (next_count > 0) 
    V(next); 
else
    V(mutex); 

Each occurrence of x.wait is replaced with the following:

x_count = x_count + 1; 
if (next_count > 0) 
    V(next) 
else
    V(mutex); 
---------------------------------- E1; 
x_count = x_count - 1; 

Each occurrence of x.signal is replaced with the following:

if (x_count > 0) 
{ 
    next_count = next_count + 1; 
 ---------------------------------- E2; 
    P(next), 
    next_count = next_count - 1; 
} 

For correct implementation of the monitor, statements E1 and E2 are, respectively,

(A)

(B)

(C)

(D)