Blocking and Non-blocking Assignments

Procedure assignment can be evaluated in two ways: Blocking and nonblocking assignments. 

We will try to swap values in registers a and b, assume the value of a and b before the clock is a = 0 and b = 1.

always@ (posedge clk) begin a = b; b = a; end //(1)both a and b becomes 1.	always@ (posedge clk) begin a <= b; b <= a; end //(2)a becomes 1 and b becomes 0.
always@ (posedge clk) fork a = b; b = a; join //(3)both a and b becomes 1.	always@ (posedge clk) fork a <= b; b <= a; join //(4)both a and b becomes 1.

Here, code (1) and code (3) didn't give us required result. 

Code (2) and code (4) will simulate correctly.

Use non-blocking assignments to implement sequential logic and use blocking assignments to implement combinational logic.