Application of Formal Methods for Establishing Regulatory Requirements for Safety-Critical Software of Real-Time Control Systems

Formal methods are usually used for computer system specification, production and verification. In this paper, a new direction for the use of formal methods is proposed, namely formalization of the regulatory requirements for software of safety-critical real-time control systems. Formal regulatory requirements, as opposed to formal specifications for a concrete system, have a generic nature, are applied for a wide range of safety-critical control systems, and are the basis for a licensing process. The use of formal regulatory requirements could help to eliminate ambiguity or misunderstanding of informal definitions, to allow rigorous assessment of satisfaction with respect to requirements and finally to increase the safety level of a system. As a formal method for describing the regulatory requirements, the Z notation is proposed. To demonstrate the proposed approach, requirements for protection against common mode software failures and requirements for protection against unauthorized access are considered. Z schemas for these requirements are presented.