Chapter 2

VHDL Invariants

Features of VDHL

Not case sensitive (ex: 2.1)
Not sensitive to white space (spaces and tabs) (ex: 2.2)
Write comments following "--" (two dashes) (ex: 2.3)
Limit use of parenthesis (for better legibility) (ex: 2.4)
End statements with a semicolon ";"

If case and loop statements

Each if case and loop statements have specific rules in order to execute each command .

Every if statement has a corresponding then statement
Each if statement is terminated with an end if;
If you need an else if constructs, the correct format is elsif
Each case statements id terminated with an end case;
Each loop statement has a corresponding end loop; statement

Identifiers

An identifier is a name given to various items in VDHL. Examples include variable names, signal names, and port names. Some rules are below:

Should be self describing
Can be as long as needed (preferably shorter)
Can be a combination of numbers letters and underscores
Must start with a letter (A-Z)
Can not have two underscores in a row or end with an underscore

Reserved Words

The words located to the left have been assigned special meaning in the VDHL language and can not be used as identifiers. Missing are the standard operators such as AND, OR, and XOR.

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