As mentioned in the last example, any circuit can be constructed using NOR components. This example demonstrates how VHDL can be used for the structure modeling . Those concepts are in VHDL strictly distinguished. The diverse interpretation of those concepts can be best explained upon an example.
All electronic circuits consist of a large number of different components.
Each single component can be observed as a entity as long as this component does not belong to a circuit. Those components who are being used for the construction of a certain circuit are the components of the circuit. Several components of the same type needed in the circuit are referred to as the Instances of one and the same component . The first instance that represents the upper nor component is being defined using the following instantiation statement . The PORT MAP statement within the second line determines the connection of the component with the rest of the design.