Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!udel!rochester!pt.cs.cmu.edu!FAS.RI.CMU.EDU!schmitz
From: schmitz@FAS.RI.CMU.EDU (Donald Schmitz)
Newsgroups: comp.arch
Subject: Re: RISC != real-time control
Message-ID: <1534@pt.cs.cmu.edu>
Date: 26 Apr 88 16:36:29 GMT
References: <1521@pt.cs.cmu.edu>
Sender: netnews@pt.cs.cmu.edu
Organization: Carnegie-Mellon University, CS/RI
Lines: 18
Keywords: RISC, real-time

In article <1521@pt.cs.cmu.edu> koopman@A.GP.CS.CMU.EDU (Philip Koopman) writes:

>Real-time control programs often have a situation where only
>X microseconds are available to perform a task.  Therefore,
>the code to perform the task must be GUARANTEED to complete
>within X microseconds.  In real-time control, a late answer
>is a wrong answer.

This may be straying somewhat from the original point, but what sort of
applications really have such exact timing deadlines?  I have done a little
real-time motion control, using a CPU to implement a discrete position
control law for robot axes, and in general a few percent deviation in cycle
time has next to no effect.  As long as the deviation is small and well
distributed, ie.  delays of no more than 20% and occuring less than 10
sample periods in a row, I can't imagine a mechanical system reacting to the
error.

Don Schmitz (schmitz@fas.ri.cmu.edu)