Mapping of Parameters

The examples presented in this section have used what is refered to as positional association to describe how actual parameters are paired with formal parameters of the subprogram.

Positional association is a quick and convenient way to describe the mapping of parameters, but it can be error-prone.

For this reason, you might want to write your subprogram references using an alternate form of port map called named association. Named association guarantees that the correct parameters are connected, and it also gives you the ability to re-order the parameters as needed.

The following example shows how the same subprogram might be referenced using both positional and named association:

    dff(Rst,Clk,Data,Result);

    dff(Rst=>Rst,C=>Clk,D=>Data,a=>Result);

The special operator => indicates exactly which lower-level ports are to be connected to which higher-level signals.