A module is comprised of the interface and the design behavior.
module | macromodule identifier (port_list) ;
All module declarations must begin with the module (or macromodule) keyword and end with the endmodule keyword. After the module declaration, an identifier is required. A ports list is an option. After that, ports declaration is given with declarations of the direction of ports and the optionally type. The body of module can be any of the following:
· Any declaration including parameter, function, task, event or any variable declaration.
· Continuous assignment.
· Gate, UDP or module instantiation.
· Specify block.
· Initial block
· Always block.
If there is no instantiation inside the module, it will be treated as a top-level module
module module_1(a, b, c) ;
parameter size = 3 ;
input [size : 0] a, b ;
output [size : 0] c;
assign c = a & b;
Module declaration with a parameter declaration.
· If there are two or more modules without instantiation inside the modules, then they are treated as the top-level. This means that you can have several of top-level modules.