Re: Need help with PowerPC/Ada and realtime tasking

[eachus@spectre.mitre.org (Robert I. Eachus)]

In article <637048781wnr@diphi.demon.co.uk> JP Thornley <jpt@diphi.demon.co.uk> writes:

  > I wonder if this is where computer science and (my sort of) software 
  > engineering part company.  Or do you while away the hours when 
  > travelling by Boeing 777 wondering whether the flight control computers 
  > are about to be presented with an undecidable proposition?

   I wish I could take this as a joke.  In commercial avionics, it is
certainly a goal, and sometimes an achievable one, to design control
systems to stay out of those modes.  But in "high-performance" aircraft,
and in particular the Space Shuttle, it is not always possible to
come up with control laws which are not chaotic.  And placard or no,
the pilot will be in those regimes.

   In the case of the A320, I would say that the implicit denial by
the software design process that such things could happen was much
more at fault than the language chosen for implementations.  When the
software encountered situations not imagined by the specification
authors, it behaved in predictable, but fatal fashion.  I much prefer
the design philoshy that things can go wrong, and you do want sanity
checks in the software.

--

					Robert I. Eachus

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