The architecture declaration describes the operation of the
component. There can be multiple architectures described for each
entity. However, for each instantiation of the entity, one of
the possibly several architectures must be selected.
In the above example, the declaration starts with the keyword
ARCHITECTURE followed by the name of the architecture (e.g.
behavior1) and the name of the entity with which the
architecture is associated. The keyword BEGIN marks the beginning of
the architecture body which may include concurrent signal assignment
statements and processes. Although not shown above, any
signals that are used internally in the architecture
description but are not found in the entity's ports are
declared before the BEGIN statement of the architecture body. The
keyword END marks the end of the architecture declaration.
