IF construct

• A sequential statement which executes one branch from a set of branches dependent upon the conditions, which are tested in sequence.

Syntax

[Label:] if Condition then
SequentialStatements...
[elsif Condition then
SequentialStatements...]
... {any number of elsif parts}
[else
SequentialStatements...]
end if [Label];

NOTE: Be careful about the spelling of elsif and end if.

Synthesis

• Assignments within if statements generally synthesize to multiplexers.

• Incomplete assignments, where outputs remain unchanged for certain input conditions, synthesize to transparent latches in unclocked processes, and to flip-flops in clocked processes.

• In some circumstances, nested if statements synthesize to multiple logic levels. This can be avoided by using a case statement instead.

• A set of elsif branches can be used to impart priority to the conditions tested first.

• To decode a value without giving priority to certain conditions, use a case statement instead.

Example

IF (xELSIF (x=y AND w='0') THEN
temp := "11110000";
ELSE temp:=(OTHERS =>'0');

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