[SystemSafety] Separating critical software modules from non-critical software modules

[Stachour, Paul D CCS]

Yes, I believe it makes sense to separate safety-critical
and non-critical software and treat them separately.

For just one example, let us suppose that:

1) There is a product whose software size is 125 (by some measure).
2) That software is organized into some set of modules.
3) Those modules identified as safety-critical (SC) are of size 15
(about 10% of the total). Non-critical (NC) is about 90%.
4)  The modules are packaged into 15 separate tasks.
5)  The tasks are separated by some mechanism.
6)  All of the safety-critical modules are in 3 of the tasks.
7)  The safety-critical variables in the safety-critical
modules are protected by some mechanism.

Then it is appropriate to have different amounts of assurance
applied to the two different sets of modules. For example,

1)  COTS or reused code used in NC is reviewed to a less
strict set of criteria than SC.
2)  Code which is SC is tested (for example, code-coverage
is required) to a greater degree than NC.

However, not all is different.  For example,
1)  The same coding standards are applied to all new code.

There can thus be a signficant savings in effort needed,
with a corresponding reduction in calendar-time resuting
in a faster time-to-market without adverse effect
on safety.


Paul D. Stachour
Software Quality Assurance
Detector Electronics Corporation
A UTC Fire & Security Company
6901 West 110th Street, Bloomington, MN 55438 USA
952-941-5665, x8409
Paul.Stachour at det-tronics.com
www.det-tronics.com


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