Question

5.14 Describe how the compare and swap() instruction can be used to provide mutual exclusion that satisfies the bounded-waiting requirement. (15 pts)

Answer 1

Answer:

Explained below

Explanation:

Compare and Swap(C&S) is simply an atomic operation whereby the compare and swap operations are automatically executed.

Now compare and Swap basically needs 3 arguments namely:

- 2 old values which we will label X and Y

- 1 new value which is written in X that we will call Z

Thus, we now have; C & S = {X, Y, Z}

To explain this well, let X be a variable where X has a value of 7.

Now, if a programmer gives a program me that X be multiplied by 2,then what C&S operation will do is;

I) Y = X where Y is a new variable.

II) Result = C&S(X, Y, X*7)

Variable X is global and this means that mere than one process and more than 1 thread can see the variable X.

Now, if a process named P1 wants multiply the variable X by 7 using C&S operation, it will first make a local copy of variable X (which in this case is now the new variable Y). After that it will atomically compare X & Y and if they are equal, it will replace X with 10X.

However, if they are not equal, P1 will re-read value of X into Y and carry of C&S instruction again.