Question

Consider the following two threads of a process, to be run concurrently in a shared memory (all variables are shared between the two threads): Assume the following: 1. a single-core system 2. load and store are atomic (i.e. they start and finish without interruption) 3. x is initialized to 0 before either thread starts, and 4. x must be loaded into a register before being incremented (and stored back to memory afterwards). The following questions consider the final value of x after both threads have completed. 1a.: State the upper and lower bounds of value of x when both threads have completed. Explain your answer.

Answer 1

Thread A, for each execution it is making X value increment by 1, it is executing 5 times so it increments x value X+5.

Thread B, for each execution it increments x value as X+2, it is executing 5 times so it increments x value as X+10.

Each incrimination of done as (i) Load the value of x into register (ii) Increment value of x by 1(or)2 based on the thread A/B (iii) Again stare back the value in X. \r\n In any concurrent Execution if these 3 steps are atomic then after completion of Thread A_1 Thread B x = 15. \r\n If there is any interrupt after incrimination, before store then (i) Min value of X: Suppose Thread A is Executing after completion of 5 iterations and before storing x = 5 it got interrupted So Thread B Execution starts and finishes all its Execution, now thread A store back result 5 into x and finishes Execution So now x = 5 \r\n (ii) Max value of x: if Thread A, Thread B Executes in any sequence without an interrupt then x = 15

If there is a possibility of interrupt between these 3 operations thn consider the below scenarios for max and min value of X.