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From: rracine@draper.com (Roger Racine)
Subject: Re: Space Station S/W in Ada -- No Tasking?
Date: 1998/05/05
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References: <354dadfd.2883074@news.mindspring.com>
Organization: Draper Laboratory
Newsgroups: comp.lang.ada
Date: 1998-05-05T00:00:00+00:00
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In article <354dadfd.2883074@news.mindspring.com> munck@Mill-Creek-Systems.com (Robert Munck) writes:
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>From: munck@Mill-Creek-Systems.com (Robert Munck)
>Newsgroups: comp.lang.ada
>Subject: Space Station S/W in Ada -- No Tasking?
>Date: Sun, 03 May 1998 18:04:15 GMT
>Organization: Mill Creek Systems LC
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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

The article is misleading; there is tasking being used for the ISS.  I was one 
of the people who convinced the Boeing management to allow it, and helped 
develop the tasking structure.  

Robert Dewar pointed out the development of the CIFO constructs for tasking 
within the Alsys compiler.  This was not used.  It was going to be used within 
the Space Station Freedom program, but was not allowed to be used within the 
re-designed computers in the International Space Station software (I have 
forgotten the reason).