A methodology for building safer software based critical computing systems

It is increasingly common to use programmable computers in applications where their failure could be life threatening and could result in extensive damage. For example, computers now have safety-critical functions in both military and civilian aircraft, in nuclear plants, and in medical devices. The safety aspects of computer-based systems as increasingly important as the use of software escalate because of its convenience and flexibility. Incorrect requirements have been identified as a major cause of software accidents and it appears that current software safety standards do not place a proportionate emphasis upon this causal factor. This paper reviews existing software safety standards, guidelines and other software safety documents and also examines the limitations, practical problems and issues associated with the use of current software safety standards. In this paper, a Methodology is proposed for modeling software safety based on the current software safety standards, their merits and limitations. The tasks in this proposed methodology pertains to System and software hazard analyses; Identification of software safety-critical requirements, safety-constraints based design, software safety implementation and software safety critical testing. This paper also presents a set of safety metrics that can be used to evaluate the maturity of hazard analysis processes and its interaction with the software development process. The proposed methodology was applied to a laboratory prototype safety-critical Railroad Crossing Control System (RCCS). The results showed that all safety critical operations are safe and risk free and capable of handling the contingency situations.