(Note: In VHDL a comment is indicated by two dashes '--'. Also, by convention, reserved words are all in capitols.)

this document, and except for synthesis, only the IEEE standard 9 value logic system (std_logic) will be used. It is more robust than the bit or bit_vector types and is the logic system used exclusively at Sanders. Entities are also important in that it is possible to include passive processes within the entity. The impact of this is that it is possible to include setup and hold checking. This is only one possible use, but the one of greatest importance. 1.2 Architecture The architecture is the actual structure which makes up a given design. It is in here where we describe in some manner the actual behavior or function which defines the operation of the system. There are two methods used for writing VHDL, behavioral and structural models. 1.2.1 Behavioral Models A behavioral model is one which defines the behavior of a system, how a system acts. We can flesh out the nand gate above in a behavioral model as follows