In Chapters 5 and 6 we saw how to write entity declarations and architecture bodies that describe the structure of a system. Within an architecture body, we can write component instantiation statements that describe instances of an entity and connect signals, quantities and terminals to the ports of the instances. This simple approach to building a hierarchical design works well if we know in advance all the details of the entities we want to use. However, that is not always the case, especially in a large design project. In this chapter we introduce an alternative way of describing the hierarchical structure of a design that affords significantly more flexibility at the cost of a little more effort in managing the design.

16.1 Components

The first thing we need to do to describe an interconnection of subsystems in a design is to describe the different kinds of components used. We have seen how to do this by writing entity declarations for each of the subsystems. Each entity declaration is a separate design unit and has corresponding architecture bodies that describe implementations. An alternative approach is to write component declarations in the declarative part of an architecture body or package interface. We can then create instances of the components within the statement part of the architecture body.