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Vrilog HDL: Nonblocking Procedural Assignments


Nonblocking Procedural Assignment

 Nonblocking assignments allow scheduling of assignments without blocking execution of
the statements that follow in a sequential block. A <= operator is used to specify nonblocking assignments. Note that this operator has the same symbol as a relational operator, less_than_equal_to. The operator <= is interpreted as a relational operator in an expression and as an assignment operator in the context of a nonblocking assignment. To illustrate the behavior of nonblocking statements and its difference from blockinglets take an example.

Example:

reg x, y, z;
reg [15:0] reg_a, reg_b;
integer count;
//All behavioral statements must be inside an initial or always block
initial
begin
x = 0; y = 1; z = 1; //Scalar assignments
count = 0; //Assignment to integer variables
reg_a = 16'b0; reg_b = reg_a; //Initialize vectors

reg_a[2] <= #15 1'b1; //Bit select assignment with delay
reg_b[15:13] <= #10 {x, y, z}; //Assign result of concatenation
//to part select of a vector
count <= count + 1; //Assignment to an integer (increment)
end

In this example, the statements x = 0 through reg_b = reg_a are executed sequentially at
time 0. Then the three nonblocking assignments are processed at the same simulation
time.

1. reg_a[2] = 0 is scheduled to execute after 15 units (i.e., time = 15)
2. reg_b[15:13] = {x, y, z} is scheduled to execute after 10 time units (i.e.,
time = 10)
3. count = count + 1 is scheduled to be executed without any delay (i.e., time = 0)
Thus, the simulator schedules a nonblocking assignment statement to execute and
continues to the next statement in the block without waiting for the nonblocking
statement to complete execution. Typically, nonblocking assignment statements are
executed last in the time step in which they are scheduled, that is, after all the blocking
assignments in that time step are executed.

In the above example, blocking and nonblocking assignments are mixed to illustrate their
behavior. However, it is recommended not to be mixed the blocking and nonblocking assignments in the same always block.





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