The Designer's Guide to VHDL, 3rd Edition

Chapter 6. Subprograms

When we write complex behavioral models it is useful to divide the code into sections, each dealing with a relatively self-contained part of the behavior. VHDL provides a subprogram facility to let us do this. In this chapter, we look at the two kinds of subprograms: procedures and functions. The difference between the two is that a procedure encapsulates a collection of sequential statements that are executed for their effect, whereas a function encapsulates a collection of statements that compute a result. Thus a procedure is a generalization of a statement, whereas a function is a generalization of an expression.

Procedures

We start our discussion of subprograms with procedures. There are two aspects to using procedures ...