Abstract: Apparatus, and a related method, for testing complex integrated systems, such as avionics systems. A unified approach to the testing architecture provides for a selection of on-line and off-line tests to be run on a complex system, at selected hierarchical levels. Built-in test logic is provided on each component or chip to be tested, and additional built-in test logic is provided at a module level, where a module includes multiple chips or components of various types. Module testing is, in turn, controlled in part by a maintenance processor responsible for multiple modules, and the maintenance processor is controlled in part by a maintenance executive. The testing architecture can be configured to provide a desired mix of performance testing, processor testing and physical testing of components.