Specifications

Language Structure
VHDL Reference Manual 2-5
Architecture
The architecture is the actual description of the design. If you think
of an entity as a functional block symbol on a schematic, then an
architecture describes what's inside the block. An architecture can
contain both concurrent and sequential statements, which are
described below. Note that VHDL allows you to have more than one
architecture for the same entity. For example, you might have an
architecture for synthesis and a gate-level (netlist) architecture. If you
have more than one architecture for an entity, use configuration
declarations to determine which architecture to use for synthesis or
simulation.
An architecture consists of two pieces: the architecture declaration
section and the architecture body. Consider the following example:
architecture behavioral of ent is
signal c_internal: small_int;
begin
c_internal <= a0 + b0;
c0 <= c_internal;
c1 <= c_internal + a1 + b1;
end behavioral;
The declaration section of the architecture is the area between the
keyword architecture and the keyword begin. Here you may declare
objects that are local to the architecture. After the declaration section
comes the architecture body, which is where you specify the behavior
of the architecture.
Configuration
Configuration declarations may be used to associate particular design
entities to component instances (unique references to lower-level
components) in a hierarchical design, or to associate a particular
architecture to an entity. As their name implies, configuration
declarations are used to provide configuration management and
project organization for a large design.