Abstract: The present invention includes a method and apparatus for performing intrusive testing upon a microprocessor to ensure the absence of errors or defects in performance-enhancing features and related control circuitry which affect the results of software instructions being executed by the microprocessor. In the intrusive testing method, an intrusive testing unit capable of generating logic signals is coupled between a computer system motherboard and a microprocessor. The microprocessor is then caused to execute a test software program. As the microprocessor executes the test software program, the intrusive testing unit generates logic signals (alternate control signals) at regular intervals. Control signals generated by system hardware on the motherboard in response to program execution are logically combined with alternate control signals generated by the intrusive testing unit. Resulting output control signals are applied to the control signal pins of the microprocessor.