Q1. Suppose two processes are using shared memory as shown below. Give the possi

Question

Q1. Suppose two processes are using shared memory as shown below. Give the possible values that can be printed for b and c by EACH process. Assume all C language assignment statements are atomic:

P0 executes: P1 executes:
shared int a; shared int a;
int b,c; int b,c;
b=1; a=100;
a=b; b=a;
c=a; c=b+a;
Print(b,c); Print(b,c);

Q2. You are asked to design concurrency control mechanisms for a two different hardware architectures: the first supports a Test-and-Set and the other supports a swap instruction, as follows:

int Test-and-Set( int target)
{
int temp=target;
target=true;
return(temp);
}
void Swap (int *a, *b)
{
int temp;
temp = *a;
*a = *b;
*b = temp;
}

Does the swap works similarly as Test-and-set? why?

Explanation / Answer

"a" variable is of integer type and its shared between processes P0 and P1

i) P0 executes

shared int a;

int b,c

b = 1;

a=b; //Here value of a will be 1

c =a; //Here value of c will be 1

Print(b,c) //So it will print b as 1 and c as 1

ii) P1 executes

shared int a;

int b,c;

a = 100;

b = a; //Here value of b =1 because a is shared and its value is already set to 1 in P0 Process.

c = b +a; //Here value of c will be 2 (1 +1)

Print(b,c ) //So it will print b as 1 and c as 2

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