Re: What is "real-time"?

[news.encore.com!mpalmer@uunet.uu.net (Mike Palmer)]

hansen@micro.cs.umn.edu (David M. Hansen) writes:

>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 i
s
>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 charact
er 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.

You definition is "interesting", but I'm not sure it's a definition that
instantly says to someone who does not understand real-time, "Oh yes, now
I see!". 

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