Re: What is "real-time"?

[news.orst.edu!umn.edu!micro.cs.umn.edu!hansen@cs.orst.edu (David M. Hans]

Just to throw a wrench in things...

Many people will define a real-time system in terms of its execution speed,
e.g., "it must complete certain tasks before a deadline", or "an answer that is
late is the wrong answer."  These definitions aren't bad, but they leave out
(what I consider) a critical component: an answer that's _early_ is just as 
bad as one that's late.  Imagine a program sending data to a modem one characte
r at a time.  If the program loads the (single-character, in this example) 
transmit buffer _before_ transmission of the previous character has been
completed, the data is mangled and the program has failed.  Similarly, when
receiving data, the program must react quickly enough to snatch each 
character from the receive buffer before the next incoming character over-
writes it.  Both components are critical.

A better definition, I think (this is not original BTW), is that a real-time
system must _manage_ time as a resource.  Certainly, it must meet all its 
deadlines (I imagine that word has an interesting etymology), but also it
must delay taking action on a calculation if that calculation completes before
its answer is required.


					-=Dave