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From: mheaney@ni.net (Matthew Heaney)
Subject: Re: Ada and Automotive Industry
Date: 1996/11/11
Message-ID: <mheaney-ya023280001111960843400001@news.ni.net>#1/1
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references: <55ea3g$m1j@newsbf02.news.aol.com> <3280DA96.15FB@hso.link.com>
 <EACHUS.96Nov7203951@spectre.mitre.org> <bb7cc$ae31.10d@marius>
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organization: Estormza Software
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newsgroups: comp.lang.ada,comp.realtime
Date: 1996-11-11T00:00:00+00:00
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In article <bb7cc$ae31.10d@marius>, ken@nrtt.demon.co.uk wrote:

>>    A HUGE amount of wasted effort because people couldn't be bothered
>> to read the reference manual.  What the Ada 83 RM said, and pretty
>> plainly at that, was that a critical task would run exactly (within
>> the limits of accuracy of the physical clock, etc.) when scheduled,
>> unless there was an equal or higher priority task using every
>> available processor.  There was even an AI, published as a
>> ramification, titled "Preemptive scheduling is required" (AI-32).
>> Can it get any clearer?  
>
>Well, yes, it can. It's got little to do with pre-emption (in fact, preemption
>is often a bad thing in real-time systems). It's got to do with "delay" vs
>"delay until". Delay creeps the time forwards so that time events get
>jittered and periodic events drift.

Ada 83 didn't have "delay until," but Ada 95 does.  But in Ada 83, there
was no drift either:

Periodic_Processing_Without_Drift:
declare
   use Calendar;
   Next_Time : Time := Clock + Interval;
begin
   loop
      delay Next_Time - Clock;
      <do some work>
      Next_Time := Next_Time + Interval;
   end loop;
end Periodic_Processing_Without_Drift;

This example comes straight out of RM83 Sec 9.5, and is repeated in every
single Ada book I have ever read.  There is no drift.  So what's the
problem?

>
>I think we should have a little respect for the Boeing point-of-view.
>It doesn't help them to be continuously pushed by academics to
>adopting unsound techniques. They have to live with the consequences of
>screwing up big time.

Which academics?  I'm not an academic, but I guess I am one of those who
argue for Ada. :-)  And which "unsound techniques," exactly?

Where do these wild academics teach?  Is there a conspiracy of some kind,
to undermine software safety by pushing for these unsound techniques?  Gee,
I'm sure glad you've let the rest of us know about it.  Thanks!

--------------------------------------------------------------------
Matthew Heaney
Software Development Consultant
mheaney@ni.net
(818) 985-1271