Since the dawn of the discipline of software engineering, one of the persistent mantras has been modularization. This structured design and functional decomposition approach breaks software down into clearly defined functional units, or modules, with unambiguous interfaces. The proven value of this approach is high quality software.  In a modular, safety critical system it is necessary to  examine the way that the modules come together and interact , and confirm that  neither their  data  coupling nor their  control coupling  is defective .