Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!psuvax1!psuvm!uh2
From: UH2@PSUVM.BITNET (Lee Sailer)
Newsgroups: comp.realtime
Subject: Re: What makes a system "real-time?" (Was: Real-Time references wanted)
Message-ID: <89291.133937UH2@PSUVM.BITNET>
Date: 18 Oct 89 17:39:37 GMT
References: <1989Oct18.151758.5227@planck.uucp>
Organization: Penn State University
Lines: 24

I've seen two different meanings assigned to "real-time."  In corporate data
precessing circles, it often just means something like "pretty fast" or
"as fast as a person needs it to be."  For example, a database might be
"real-time" is the user gets the results of a query within a few seconds of
hitting the <enter> key.

In cases where software is an integral part of some hardware, "real-time"
means that certain actions are guaranteed to complete within some known
amount of time.  For example, suppose that your operating system guaranteed
that a file creation action would never take longer than 1/100 second, or
that your program would be notified of a joystick motion within 1/1000 second
of when it occurred.

In this case, you could write software to do things like controlling the
flaps of your space shuttle (isn't everyone working on one?) without
worrying about whether the flap-controller might get swapped out at a bad
time.

It gets more complicated than this, because there can be multiple layers
of guarantee, and so on.

I like the previous responders generic defintion that a real-time system
is one where an operation must complete within a certain specified time or
else it is an error.