Probability and computer safety

The operational reliability and safety of computer and computer-based systems cannot be estimated using probabilistic approaches, at least when the targeted level is industrial. It is commonly accepted that so-called reliability growth approaches are unhelpful. For hardware, it is not reasonable to give by analogy a reliability rate for a software component (indiscriminately regrouping software and functional application systems). 

The operational reliability and safety of computer systems, in an environmental context and given purpose, must be regarded as deterministic in terms of functionality. Indeed, if there is an interlocking gap , in a case not covered by functional specifications, each time the unforeseen input configuration occurs, the functional will deterministically lead to an unsafe system state, regardless of its hardware architecture and its safety level under the current standards (SIL). 

Moreover, in some cases, it is sufficient for this feared combination to occur only once for the functional to present multiple dangerous weaknesses. 

Test generators are tools that generate test sequences that conform to specifications. They ensure that the actual system conforms to the original specifications. This technique involves the actual system. It does not guarantee the completeness of the tests undertaken. The coverage is limited due to  

Consequently, the option chosen was to distinguish the functional software from all the basic hardware and software at the design stage, so that the industrial application of a formal method could be subsequently considered. 

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