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In this paper we introduce a detailed process for software requirements safety analysis. The approach for identification of safety faults in this process is based on fault tree analysis. This process consists of four main steps, 1 ) software requirements fault tree generation, 2 ) software requirements fault tree verification and validation, 3) software safety requirements generation and safety fault mitigation, and 4 ) software requirements safety ver@cation and validation. We have defined a set of rules that can automatically generate the software requirements fault tree given the software requirements specified in Statemate, and the Software fault tree top events identified in the system safety analysis. Statemate CASE tool is used in this process for specification of software requirements, and execution and analysis of the software requirements to verifr and validate safety. Essential tree analysis is introduced as a method for dependency analysis of the software fault tree top event. The outcome of essential tree analysis is captured in an essential tree which iwe introduce as a graphical representation of the dependencies and their interrelations.

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There are many software development processes defined in the literature that address the particular needs for the type of software being developed [Pressman 921. However, for software in safety-critical systems most of the existing processes do not address safe software development and delivery. We propose a software development process for developing safe software, consisting of four basic activities; forward engineering, safety analysis, reliability analysis, and program management.