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From: duncant@mbunix.mitre.org (Thomson)
Newsgroups: comp.realtime
Subject: Re: What is "real-time" really?
Message-ID: <98987@linus.UUCP>
Date: 26 Feb 90 02:25:25 GMT
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I read with interest the responses to my question "What is 'real-time' 
really?".  The definitions seem to contain various combinations of the
following criteria:

1. Real-time systems must provide answers by some specified time.

2. The time response of the system must be "deterministic", i.e. one must
   be able to be certain that functions will be performed within specified
   time limits.

I have a couple of problems with these definitions. 

First, it is not enough to say that a real-time system is one which must
provide answers by a specific time.  Consider a company's payroll system,
which absolutely must compute the pay of each employee by the 29th of the 
month (otherwise all hell breaks loose).  This system has "hard" timing
requirements, but I doubt that many people would want to consider it a 
real-time system.  There are plenty of other examples, with time scales
ranging from very short to very very long.  Where does one draw the 
"real-time" line?

Second, I don't like the "deterministic" answer too much either, mainly 
because I doubt that there are many "real-time" systems out there which
are really deterministic.  One of the net responses raised the issue of
"proving" that the system can meet its time constraints.  Given the current
state of the art, I doubt that there are any real real-time systems which
have actually be formally proven to be able to meet their timing requirements.
Or are there?

I still stick to my definition (until I get convinced of the error of my
ways by some of you experts out there) that "real-time" means "timing 
constraints so tight that you have to use special programming techniques
to meet them."  (I also have a definition for AI: Problems so hard that
you can't solve them using conventional programming techniques - but that's
another discussion.)

At least I seem to have generated some action on this sleepy interest group!

Duncan Thomson