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<p>FWIW, I've always been sceptical about 61508. Do I detect a large
dose of irony in the word "finessed"? I'd be more inclined to call
it "fudged". In my view you cannot meaningfully speak of software
reliability unless you have rigorous input-source and output-sink
modelling (and many thanks, BTW, to Michael J, who kindly pointed
out my omission of the output side in a previous posting).</p>
<p>Having had long and frustrating experience of random C shops,
populated with what I call the "lumpenengineeriat", I have strong
views in this matter. Things that one sees all the time are:</p>
<p>(1) Underassessment of system criticality,</p>
<p>(2) A "tick the boxes" mentality that ignores fitness-for-purpose
of the practices upon which the box-ticking was putatively
justified,</p>
<p>(3) Dire ignorance of and hostility towards soundly-based
techniques,</p>
<p>(4) Haphazard use of CASE tools, <br>
</p>
<p>(5) Absence of intellectual discipline among engineers - s/w
engineering does not require great intellectual puissance but it
needs intellectual discipline by the train-load.</p>
<p>TBH, I've given up on expecting rapid progress in this field, if
only because the standardisation itself has become, IMO, pretty
dysfunctional. Everywhere there are tribal frictions where there
ought to be constructive synthesis. You see this particularly in
programming language standards, where it sometimes seems as if the
language itself has engendered an in-group, inward-looking
culture. How else could one be told that a technically sensible
improvement was "not in the spirit" of a language? Call me a
pessimist but I honestly believe we need to get specialists to
look at this sort of thing from a sociolinguistic perspective.</p>
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<p>Olwen</p>
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<div class="moz-cite-prefix">On 24/09/2020 12:35, Peter Bernard
Ladkin wrote:<br>
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<blockquote type="cite"
cite="mid:7231ca9b-c8c6-80ab-e77a-6b628a807b6a@causalis.com">
<pre class="moz-quote-pre" wrap="">
On 2020-09-24 11:59 , Martyn Thomas wrote:
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<pre class="moz-quote-pre" wrap="">IEC 61508 ignore the probability that a hardware design is incorrect (we used to talk about
'systematic errors' to distinguish them from random failures', but I haven't seen that expression
used recently).
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Yes, 61508 ignores probabilities attached to systematic failure, of either HW or SW. Deliberately.
It is a controversial subject, as we see here in miniature.
It is finessed by talking about objects (HW, SW or both) having a "systematic capability", which is
a non-numerical quality saying how appropriate it is for that object to be used to implement a
safety function with a given SIL. So for example an object with SC3 can be used to implement a SIL3
safety function and an object with SC2 not.
It goes along with a list of techniques and analysis types. Basically, you can claim a higher SC if
you have used more of those techniques during development, and lower if fewer.
The general idea is sort of right. If you have used SPARK thoroughly in your software development,
used a validated SPARK-to-C compiler and a verified C compiler, and you have done it all right, then
I think we can have more reliance on the machine code produced than if we farmed the specs out to
some random C shop and took what they delivered without oversight or further analysis.
However, I am sceptical about the way SC is handled in 61508, which is check-boxy: "have you used
FM?" Yes/no. "Have you used semi-FM?" Yes/no. Not how or why or to do what. I am wary, but it is
central to 61508.
PBL
Prof. Peter Bernard Ladkin, Bielefeld, Germany
Styelfy Bleibgsnd
Tel+msg +49 (0)521 880 7319 <a class="moz-txt-link-abbreviated" href="http://www.rvs-bi.de">www.rvs-bi.de</a>
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