Extending the four-variable model to bridge the system-software gap

System and software requirements are inextricably intertwined, yet the challenge of extracting software requirements from system requirements is often an exercise in frustration. We propose an extension of the four-variable model originally developed by Parnas and Madey that clarifies how system requirements can be allocated between hardware and software. This extension organizes the software so that it traces clearly and directly to both the system and hardware requirements. An attractive aspect of this paradigm is that it is consistent with object-oriented approaches and allows the system requirements to be organized to facilitate object-oriented software development