2014年6月3日 星期二

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. 

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