Concurrent Process

In single processor systems or in multiple processor systems processes may execute concurrently. We here first discuss with concurrent processing in uni processor systems. Two process are said to be concurrent if their execution overlaps in time.Consider statements
S1: a= x+y
S2: b= z
S3: c=a+b
S4: d=c-1

Here statements S1&S2 are independent they can be executed concurrently, whereas statement S3 is depends on S2, S1. S4 is depends on S3. If you look at the precedence graph it is looks like

If we define two sets Read operation R(s), write operation W(s) then the conditions for concurrency are
R(s) ∩ W(s) = fi

W(s) ∩ R(s) = fi

W(s) ∩ W(s) = fi

R(s) ∩ R(s) ≠ fi