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From: valentin@cbmvax.commodore.com (Valentin Pepelea)
Newsgroups: comp.realtime
Subject: Re: Realtime and fault-tolerance together?
Message-ID: <15596@cbmvax.commodore.com>
Date: 5 Nov 90 09:56:33 GMT
References: <720@dynasys.UUCP>
Reply-To: valentin@cbmvax.commodore.com (Valentin Pepelea)
Organization: Commodore, West Chester, PA
Lines: 37

In article <720@dynasys.UUCP> jessea@dynasys.UUCP (Jesse W. Asher) writes:
>
> I was wondering if any realtime operating systems also incorporated
> fault-tolerant considerations.  If so, how are they implemented
> software-wise?

Realtime operating systems typically provide for task exceptions handlers,
which are task specific functions that are called when an error occurrs during
a particular task's time slice. That task's function then decides on the
appropriate action to take, and corrects the fault under the same context
(at the same priority) that the fault occured. Of course, some exceptions such
as those generated by power faults, might require the same corrective action
no matter under what task's context it occurs.

> How do hardware based fault-tolerance systems interact with software based
> realtime?  It seems to me that they go hand in hand in many cases and that
> one would want to implement both in a system.

The typical hardware based fault tolerant system is one that uses several
units, and decides upon an action depending on what the majority of the
units vote to do. Democracy at work.

The typical hardware/software combined system is one where special circuitry
is used to detect a hardware fault to initiate a software recovery routine.
Although even in the case above some software might be necessary to control
the selection of a faulty unit, the saliant point here is that software is
the backbone of the fault tolerant system, and software will be used to
recover or circumvent the fault.

I'll let somebody more experienced give us some examples and juicy anecdotes.

Valentin
-- 
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