Behavioral modeling is the highest level of abstraction in the Verilog HDL. The other modeling techniques are relatively detailed. They require some knowledge of how hardware, or hardware signals work. The abstraction in this modeling is as simple as writing the logic in C language. This is a very powerful abstraction technique. All that designer needs is the algorithm of the design, which is the basic information for any design.
Most of the behavioral modeling is done using two important constructs: initial and always. All the other behavioral statements appear only inside these two structured procedure constructs.
The initial Construct
The statements which come under the initial construct constitute the initial block. The initial block is executed only once in the simulation, at time 0. If there is more than one initial block. Then all the initial blocks are executed concurrently. The initial construct is used as follows:
reset = 1'b0;
clk = 1'b1;
clk = 1'b1;
In the first initial block there are more than one statements hence they are written between begin and end. If there is only one statement then there is no need to put begin and end.
The Always Construct
The statements which come under the always construct constitute the always block. The always block starts at time 0, and keeps on executing all the simulation time. It works like a infinite loop. It is generally used to model a functionality that is continuously repeated.
#5 clk = ~clk;
clk = 1'b0;
The above code generates a clock signal clk, with a time period of 10 units. The initial blocks initiates the clk value to 0 at time 0. Then after every 5 units of time it toggled, hence we get a time period of 10 units. This is the way in general used to generate a clock signal for use in test benches.
always @(posedge clk, negedge reset)
a = b + c;
d = 1'b1;
In the above example, the always block will be executed whenever there is a positive edge in the clk signal, or there is negative edge in the reset signal. This type of always is generally used in implement a FSM, which has a reset signal.
a = ( b + c )*d;
e = b | c;
In the above example, whenever there is a change in b, c, or d the always block will be executed. Here the list b, c, and d is called the sensitivity list.
In the Verilog 2000, we can replace always @(b,c,d) with always @(*), it is equivalent to include all input signals, used in the always block. This is very useful when always blocks is used for implementing the combination logic