In a safety-critical system each service has a specific level of safety criticality. Safety standards use classifications like Safety Integrity Levels (SIL), to describe the design requirements for the individual services of a system. Techniques like redundancy can be used to achieve a higher overall dependability than the used individual components provide. Using the notion of SIL, this can be called SIL arithmetic.In this paper we describe the concept of SIL arithmetic and point out how different safety standards provide hints for their support of using SIL arithmetic. We highlight the principal benefits of SIL arithmetic and provide simple examples. But the use of SIL arithmetic in a concrete system design can also have its pitfalls, which we also discuss in this paper. We specifically discuss these issues in the context of scheduling techniques for mixed-criticality systems, where resource shortages are to be handled by the scheduler.
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