VHDL Programming

48 Chapter Three

is no ELSEclause, so no statements are executed in the IFstatement. In-

stead, control is transferred to the statement after the END IF.

Let’s look at another example where the ELSEclause is useful:

IF (day = sunday) THEN

weekend := TRUE;

ELSIF (day = saturday) THEN

weekend := TRUE;

ELSE

weekday := TRUE;

END IF;

In this example, there are two variables—weekendand weekday—that

are set depending on the value of a signal called day. Variable weekendis

set to TRUEwhenever dayis equal to saturdayor sunday. Otherwise, vari-

able weekdayis set to TRUE. The execution of the IFstatement starts by

checking to see if variable dayis equal to sunday. If this is true, then the

next statement is executed and control is transferred to the statement

following END IF. Otherwise, control is transferred to the ELSIFstatement

part, and dayis checked for saturday. If variable dayis equal to saturday,

then the next statement is executed and control is again transferred to the

statement following the END IFstatement. Finally, if dayis not equal to

sundayor saturday, then the ELSEstatement part is executed.

The IFstatement can have multiple ELSIFstatement parts, but only

one ELSEstatement part. More than one sequential statement can exist

between each statement part.

CASE Statements

The CASEstatement is used whenever a single expression value can be

used to select between a number of actions. Following is the BNF for the

CASEstatement:

case_statement ::=

CASE expression IS

case_statement_alternative

{case_statement_alternative}

END CASE;

case_statement_alternative ::=

WHEN choices =>