Exercise 1

Assume we have two concurrent threads/processes, each with simple straight-line code and sharing a global variable x. Assume that each individual statement is atomic.

 1  thread0:
 2  {
 3    x=0
 4    x=x+1
 5  }

 6  thread1:
 7  {
 8    x=0
 9    x=x+1
10    x=x+2
11  }
        

Without using SPIN, give one trace illustrating each of the possible final values of x.

Exercise 2

Answer the previous exercise, except with different atomicity. Assume that each variable lookup, variable assignment, and addition are atomic, as in a typical machine language. Specifically, use the following code:

 1  thread0:
 2  {
 3    x  =  0
 4    r0 =  x
 5    r0 += 1
 6    x  =  r0
 7  }

 8  thread1:
 9  {
10    x  =  0
11    r1 =  x
12    r1 += 1
13    x  =  r1
14    r1 =  x
15    r1 += 2
16    x  =  r1
17  }