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From: hs0l+@andrew.cmu.edu (Hugh Brinkley Sprunt)
Newsgroups: comp.software-eng,comp.realtime
Subject: Re: What makes a system "real-time?"
Message-ID: <MZDDenC00V42A0mV1E@andrew.cmu.edu>
Date: 18 Oct 89 22:35:31 GMT
References: <1989Oct18.151758.5227@planck.uucp>
Organization: Electrical and Comp. Engineering, Carnegie Mellon, Pittsburgh, PA
Lines: 35
In-Reply-To: <1989Oct18.151758.5227@planck.uucp>


Here is the definition I use for "real-time system":

    A real-time system is a system containing real-time tasks.
    A real-time task is a task that has timing constraints (e.g.,
    a deadline).  Timing constraints can be hard or soft.  A hard
    timing-constraint must always be met (otherwise, the system
    will fail).  A soft timing-constraint should be met if possible,
    but missing the constraint does not cause system failure.

An example of a hard timing-constraint would be the processing
of sensor data for an inertial navigation system.  If the processing
is not completed within its deadline, the error in the system's
position estimates will become too great for correct system
operation.

An example of a soft timing-constraint would be the processing
for an operator request to show the status of some device being
controlled.  Since the operator can tolerate a range of response
times for the request, the task may be assigned a soft-deadline.
It is good to meet the soft-deadline as often as possible.  However,
this timing constraint should not be met at the expense of missing
a hard timing-constraint.

The diffiult problem in designing a real-time system (as opposed
to a non-real-time system) is to "guarantee" that the system can
meet its hard timing-constraints under "all" conditions.  Once the
hard timing-constraints have been met, the next goal is to meet
the soft timing-constraints "as best as possible" without
endangering the system's hard timing-constraints.

Brinkley Sprunt
Electrical & Computer Engineering
Carnegie Mellon University
sprunt@k.gp.cs.cmu.edu, hs0l+@andrew.cmu.edu