Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!aplcen!uunet!mcrware!ylee
From: ylee@mcrware.UUCP (Yeongleh Lee)
Newsgroups: comp.realtime
Subject: Re: What is "real-time" really?
Message-ID: <1556@mcrware.UUCP>
Date: 26 Feb 90 22:55:22 GMT
References: <98692@linus.UUCP>
Organization: Microware Systems Corp., Des Moines, Ia.
Lines: 33


     After read several excellent postings about  "What is real-time really" 
in this news group, I am not going to give best definition of real-time,
since every definition I saw in this news group is good in a certain point
of view. I just state the difference between realtime and non-realtime as
I can feel.

     Standard UNIX, designed as a time sharing system, is not considered
as a real time system. For example, the UNIX kernel is not preemptible.
A low priority task in a system call can not be preempted to allow a 
critical task to response to an external event. This will be serious
for those systems really need real time such as space shuttle, nuclear
energy plant, robot,.. etc. Even several milli seconds is important. 

     For program development, non-realtime system may be deemed good enough
since real time is not important. However, realtime programming and non-
realtime programming won't be the same.

     So one characteristic of real time system is "Priority-based preemptive
scheduling of tasks to ensure that the task of highest priority is executed
when it becomes runnable".

     Another characteristics of real time system is "Direct control of the
scheduler by user processes to guarantee the execution of the appropriate
task".

     Compare realtime systems such as OS-9, OS-9000, VRTX, and VxWorks  
v.s. UNIX, one can find there are differences between realtime and non-
realtime systems.


Yeongleh Lee
uunet!mcrware!ylee