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From: munck@Mill-Creek-Systems.com (Robert Munck)
Subject: Space Station S/W in Ada -- No Tasking?
Date: 1998/05/03
Message-ID: <354dadfd.2883074@news.mindspring.com>#1/1
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Date: 1998-05-03T18:05:12+00:00
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A paragraph in Popular Science notes that the software for
the International Space Station is being written in Ada,
about 3M lines worth.  However, it goes on to say:

   "To make troubleshooting easier, the software that runs
   the trio of computer networks aboard the space station is
   written to operate in synchronous, or serial, fashion 
   rather than the faster but more complex asynchronous."

Does this mean that they're not using tasking, but rather the
old "crystal clock" architecture where you organize your
processing into major and minor cycles, disable interrupts, and
poll for events "just in time" at various places in the cycles?

In my experience, large systems built that way tended to be
complete disasters: nightmares to debug ("troubleshoot!"),
horror shows to maintain and enhance.  They often had
interdependencies that were handled purely by the positions
of pieces of code in the cycles and the processing times of
the other (unrelated) functions between those positions. 
Adding a tiny fix in one place could break code half a major
cycle and 1 million lines of code away from it.

Could we possibly be using this approach for a life-critical
system that will run in an incompletely-understood
environment, be subject to extensive and rapid change, and
have a lifetime of decades?

Bob Munck
Mill Creek Systems LC