Background - Using Event-B for Critical Device Software Systems - page 24

Fig. 2.7 The V model of safety-critical system development

tional requirements and safety-integrity requirements. The safety integrity require-
ments are calculated individually for each of the functions previously identified.
Having done this, one may concentrate on providing the high levels of assurance
on the safety-critical aspects. We intend using the safety life-cycle model as a basis,
with a view to ascertaining its suitability to support the production of formal mod-
els with high integrity. Our contention is that we treat carefully the non-functional
requirements and to put forward a selection of viewpoints and methods highlighting
further the safety concepts, which are often subtle, then the life-cycle model can
be effective [ 103 ]. A safe system can be characterised as one in which risks from
hazards have been minimised throughout a system life. The process of providing
hazard analyses and risk assessments are thus crucial activities to ensure the safety
of a system.
In Fig. 2.7 , Preliminary System Safety Analysis (PSSA) and System Safety Anal-
ysis (SSA) are the collection of various techniques like FTA, HAZOP, FMEA, etc.
The aim of all these techniques is to identify failures and derive the safety require-
ments, which prevent from the occurrence of the hazard. FTA focuses on the dif-
ferent components of a system, while HAZOP focus on the flow between compo-
nents. There are also a number of other techniques, which are used in the PSSA for
analysing failures, an overview can be found in [ 87 ].

2.7 Standard Design Methodologies

A design is a meaningful engineering representation of a higher-level interpretation
of a system, which is actually a part of an implementation in a source code. Design