If Then Statement (AHDL)

The If Then Statement lists a series of behavioral statements to be activated after the positive evaluation of one or more Boolean expressions.

The following example shows an If Then Statement:

IF a[] == b[] THEN
        c[8..1] = H "77";
        addr[3..1] = f[3..1].q;
        f[].d = addr[] + 1;
ELSIF g3 $ g4 THEN
        f[].d = addr[];
ELSE 
        d = VCC;
END IF;

The If Then Statement has the following characteristics:

• The keywords IF and THEN enclose the Boolean expression to be evaluated and are followed by one or more behavioral statements, each of which ends with a semicolon (;).
• The keywords ELSIF and THEN enclose any additional Boolean expressions to be evaluated, and are also followed by one or more behavioral statements. These optional statements can be repeated.
• The behavioral statement(s) following the keyword THEN are activated for the first expression that evaluates to true.
• The keyword ELSE followed by one or more behavioral statements is similar to the WHEN OTHERS default alternative of the Case Statement. If none of the previously evaluated Boolean equations is true, the behavioral statement(s) following ELSE are activated. In the example shown above, if neither expression evaluates to true, the equation d = VCC is activated. The ELSE clause is also optional.
• Expressions following IF and ELSIF keywords (in the example shown above, a[] == b[] and g3 $ g4) are evaluated concurrently.
• The keywords END IF and a semicolon (;) end the If Then Statement.
• An If Then Statement may generate logic that is too complex for the Compiler. If an If Then Statement contains complex expressions, the inversion of each expression is likely to be even more complex.