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Assertion Statement:

The assertion statement has three fields. The conditions in this statement is to be evaluated by a Boolean value i.e. either true or false. If it indicates the false then assertion violation takes place. When assertion violation takes place report clause is used to specify the message. In order to specify the severity level of the assertion violation, severity clause is used. Assertion statements are used in sequential and concurrent statements. Concurrent assertion shows the passive process. Normally in synthesis tools assertion is ignored. The Syntax is :

assert condition 
 report string 
 severity severity_level;
Example 1 :
assert (Test_data = '0')
	report InstancePath & ": test mode decided"
	severity Note_as_such;