Path: utzoo!attcan!uunet!mcsun!inria!irisa!news
From: leguerni@irisa.irisa.fr (Paul Le Guernic)
Newsgroups: comp.realtime
Subject: Re: What is "real-time" really?
Message-ID: <1990Feb27.185433.24783@irisa.fr>
Date: 27 Feb 90 18:54:33 GMT
References: <98692@linus.UUCP>
Sender: news@irisa.fr
Organization: IRISA, Rennes (FR)
Lines: 34

From article <98692@linus.UUCP>, by duncant@mbunix.mitre.org (Thomson):
> 
> 
> Someone asked me recently what real-time really means.  After some thought,
> I answered that real-time systems were simply those {in which the 
> timing constraints were so tight that speciall programming techniques
> were required.  Under this definition, a "real-time" system of today may
> become a non-real-time system whan hardware gets fast enough that I can 
> program it using any regular old operating system (say plain old UNIX)
> using high level language (no assembly language optimization) and not have
> to worry about my software failing to meet its deadlines.
> ~rk
> Anyone care to comment?  What is the best definition of real-time?
> 
> Duncan Thomson

Excuse me for the previous empty posting.

A real time system could be defined as a system which continuously interacts
with an environment characterized by dynamic properties, modifying its
behavior and being modyfied by it. In this meaning, real-time can not be
reduced to hardware performance : put an iPSC to control a train and
you will still have to meet real-time constraints (don't close the door
before anyone is entirely entered, don't crush the passengers with too many
G's, adapt the speed to the railroad configuration,...). So a real-time system
has to supply the user with the ability to compute time values; and it mainly
has to be deterministic !
Some languages have been defined in France (ESTEREL, LUSTRE and SIGNAL) to
meet this requirement; these synchronous languages assume that an event may 
handle more than one communication and, moreover, computing values is assumed
to spend no time. It is the way to get deterministic computing upon time.
Meeting of time constraints may then be proved depending upon properties of
the architecture used to implement the real-time application.
Paul Le Guernic