We present a design approach which allows us to formally specify a real-life processor as composed out of its basic architectural (formally specified) components. The methodology provides means to rely upon hierarchical refinements and modular structuring of the specifications as a discipline to control the behaviour of complex units in terms of the behaviour of their components. In particular this enables us to prove interesting dynamic properties about the processor in terms of properties of its basic architectural components. We have developed the method to accomplish a reverse engineering project for the VLSI implemented microprocessor zCPU, the controller of the successful APE100 massively parallel machine.