A shared variable A , initialized to 0, is operated on by 4 processes w,x,y,z. P

Question

A shared variable A , initialized to 0, is operated on by 4 processes w,x,y,z. Processes w and x increment A by 1 while processes y and z decrement it by 2. Each process before reading performs a wait on a semaphore S and signal on S after store or writing.

Ex: Process W () {

wait(S);

Read A to a local var;

increment local var by 1;

Store value from local var to A.

signal(S);

}

Same code for process x. For y,z, the increment is changed to decrement by 2.

i. Find what would be the MAX value of A after all the 4 processes complete execution Fully, if Semaphore S were to be initialized with 1. Explain your answer with the ordering of the processes.

ii. What would be the MAX value of A after all 4 processes complete fully, if Semaphore S were to be initialized with value 2.  Explain your answer with the ordering of the processes.

Explanation / Answer

(i) If semaphore is initialized with the 1,then processed would be executed correctly and then we get the final value of A as '-2'.

Answer : ' -2 '

(ii) Since initial value of semaphore is 2,

W and X be the processes. W and X incremented A by 1 and, y and Z decrement A by 2. So, the final value of A is 1 and not -1 and the two Signal operations make semaphore value 2 again.
Semaphore S is initialized to the value 2

Process W executes the S=1, A=1 but it does not update the A variable.

Then the process Y executes S=0, it decrements A, now A = -2 and
signal semaphore is S=1

Now process Z executes S=0, A=-4, signal semaphore S=1
Now process W updates the A=1, S=2

Then process A executes A=2

Answer :: ' 2 '

   /// *** Thank You *** ///

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