1. A method of testing a feature of a microprocessor prior to the manufacture of the microprocessor, comprising: forming a model which represents a microprocessor having at least one feature called upon by control signals dispatched to the feature within the model, wherein the feature can operate according to several modes of operation, and wherein the feature is a subsystem or functional unit for increasing the performance of the microprocessor; interposing intruder logic between the control signals and the feature to restrict operation of the feature to only one of the several modes of operation; initiating a testing program to activate the control signals for producing a test result from the feature in accordance with the one of the several modes of operation being selected; and comparing the test result with a predetermined result to determine if the feature fails the testing program.