4.4 enum and HLA Enumerated Data Types

In a previous section discussing constants and constant expressions, you saw the following example:

const          TapeDAT          :=     0;
const          Tape8mm          :=     TapeDAT + 1;
const          TapeQIC80        :=     Tape8mm + 1;
const          TapeTravan       :=     TapeQIC80 + 1;
const          TapeDLT          :=     TapeTravan + 1;

This example demonstrates how to use constant expressions to develop a set of constants that contain unique, consecutive values. There are, however, a couple of problems with this approach. First, it involves a lot of typing (and extra reading when reviewing this program). Second, it's very easy to make a mistake when creating long lists of unique constants and reuse or skip some values. The HLA enum type provides a better way to create a list of constants with unique values.

enum is an HLA type declaration that lets you associate a list of names with a new type. HLA associates a unique value with each name (that is, it enumerates the list). The enum keyword typically appears in the type section, and you use it as follows:

type
     enumTypeID:          enum { comma_separated_list_of_names };

The symbol enumTypeID becomes a new type whose values are specified by a list of names. As a concrete example, consider the data type TapeDrives and a corresponding variable declaration of type TapeDrives:

type
     TapeDrives: enum{ TapeDAT, Tape8mm, TapeQIC80, TapeTravan, TapeDLT};

static
     BackupUnit:        TapeDrives := TapeDAT;

     .
     .
     .

     mov( BackupUnit, al );
     if( al = Tape8mm ) then

          ...

     endif;

     // etc.

By default, HLA reserves 1 byte of storage for enumerated data types. So the BackupUnit variable will consume 1 byte of memory, and you would typically use an 8-bit register to access it.^[50] As for the constants, HLA associates consecutive uns8 constant values starting at 0 with each of the enumerated identifiers. In the TapeDrives example, the tape drive identifiers would have the values TapeDAT=0, Tape8mm=1, TapeQIC80=2, TapeTravan=3, and TapeDLT=4. You may use these constants exactly as though you had defined them with these values in a const section.

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^[50] HLA provides a mechanism by which you can specify that enumerated data types consume 2 or 4 bytes of memory. See the HLA documentation for more details.