Processes that execute on shared data may experience data incoherence if no synchronization is provided. To illustrate the problem consider two processes that share variable X, which has been initialized to 0. The processes each execute the following code: Load R, X /* copy value of variable X to register R */ Add R, 1 /* add 1 to the value in the register */ Store R, X /* copy value in register back to variable X */

a) What will the final value of X be if the processes execute the code serially?

b) What are the possible final values of X if the processes execute the code concurrently (interleaved instructions)? Show the instruction interleaving that causes each possible value of X.