Synchronization

Consider three concurrent processes P1, P2 and P3 as shown below, which access a shared variable D that has been initialized to 100. $$\begin{array}{|c|c|c|} \hline P1 & P2 & P3 \\ \hline \vdots & \vdots & \vdots \\ D = D + 20 & D = D - 50 & D = D + 10 \\ \vdots & \vdots & \vdots \\ \hline \end{array}$$ The processes are executed on a uniprocessor system running a time-shared operating system. If the minimum and maximum possible values of D after the three processes have completed execution are X and Y respectively, then the value of Y – X is _____.

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 ________.

The following two functions P1 and P2 that share a variable B with an initial value of 2 execute concurrently.

P1( ) {                 P2( ) {
    C = B - 1;              D = 2 * B;
    B = 2 * C;              B = D - 1;
}                       }

The number of distinct values that B can possibly take after the execution is _____.