VHDL Programming

82 Chapter Four

A typical enumerated type for a four-state simulation value system looks

like this:

TYPE fourval IS ( ‘X’, ‘ 0 ’, ‘ 1 ’, ‘Z’ );

This type contains four character literal values that each represent

a unique state in the four-state value system. The values represent the

following conditions:

‘X’—An unknown value

‘ 0 ’—A logical 0 or false value

‘ 1 ’—A logical 1 or true value

‘Z’—A tristate or open collector value

Character literals are needed for values ‘ 1 ’and ‘ 0 ’to separate these

values from the integer values 1 and 0. It would be an error to use the val-

ues 1 and 0 in an enumerated type, because these are integer values. The

characters Xand Zdo not need quotes around them because they do not

represent any other type, but the quotes were used for uniformity.

Another example of an enumerated type is shown here:

TYPE color IS ( red, yellow, blue, green, orange );

In this example, the type values are very abstract—that is, not repre-

senting physical values that a signal might attain. The type values in type

colorare also all identifiers. Each identifier represents a unique value of

the type; therefore, all identifiers of the type must be unique.

Each identifier in the type has a specific position in the type, determined

by the order in which the identifier appears in the type. The first identifier

has a position number of 0, the next a position number of 1, and so on.

(Chapter 5,“Subprograms and Packages”includes some examples using

position numbers of a type.)

A typical use for an enumerated type would be representing all of the

instructions for a microprocessor as an enumerated type. For instance, an

enumerated type for a very simple microprocessor could look like this:

TYPE instruction IS ( add, sub, lda, ldb, sta, stb, outa,

xfr );

The model that uses this type might look like this:

PACKAGE instr IS

TYPE instruction IS ( add, sub, lda, ldb, sta, stb,

outa, xfr );