TABLE OF CONTENTS The concept of type is very important when describing data in a VHDL model. The type of a data object defines the set of values that the object can assume, as well as the set of operations that can be performed on those values. A scalar type consists of single, indivisible values. In this chapter we look at the basic scalar types provided by VHDL and see how they can be used to define data objects that model the internal state of a module.
An object is a named item in a VHDL model that has a value of a specified type. There are four classes of objects: constants, variables, signals and files. In this chapter, we look at constants and variables; signals are described fully in Chapter 5, and files in Chapter 18. Constants and variables are objects in which data can be stored for use in a model. The difference between them is that the value of a constant cannot be changed after it is created, whereas a variable's value can be changed as many times as necessary using variable assignment statements.
Both constants and variables need to be declared before they can be used in a model. A declaration simply introduces the name of the object, defines its type and may give it an initial value. The syntax rule for a constant declaration is
constant_declaration <span class="unicode">?</span> <b class="bold">constant</b> identifier {, <span class="unicode"> </span> } : subtype_indication...
