A method and apparatus for testing complex processor-based computer modules and their associated computer systems by allowing the normal initialization path between a memory component storing code utilized during initialization and the processor to be interrupted and test code from an external test system to be substituted for initialization code. Following initialization, a two-way communication link between the processor and the test system is created to allow interactive testing and status reporting. The testing method and apparatus maximizes the likelihood of precisely identifying defects on the module under test.