Path: utzoo!attcan!uunet!mcvax!enea!kth!sics!aronsson
From: aronsson@sics.se (Lars Aronsson)
Newsgroups: comp.arch
Subject: Re: RISC != real-time control (actu
Message-ID: <1924@sics.se>
Date: 15 May 88 20:24:18 GMT
Reply-To: aronsson@sics.se (Lars Aronsson)
Distribution: comp
Organization: Swedish Institue of Computer Science, Stockholm
Lines: 42
Followup-To:

>>>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?...
>
>Sometimes microseconds can matter. Our most complicated real-time
>system runs a scanning interferometer and a laser. The interferometer

Enough! Obviously, real-time applications do exist. No more
interferometers in this news group, please.

A few years ago, there was a discussion on why you wouldn't use UNIX
for real-time applications. This was because of the virtual memory
system. Today, we have UNIX clones which allow you to lock a process
in main memory, just like the UNIX kernel. Since a virtual memory
system is but a cache mechanism for the disk, the following thoughts
come naturally to me:

Before I start: This might turn out to be Todays Dumb Suggestion.
Maybe my ideas are already implemented on lots of systems or totally
useless. Please, let me know!

As far as I know, RISC instruction caches are a gain only when the
processor runs through loops. What about the ability to declare
cache-resident functions (procedures/subroutines)? This might not be
the solution to real-time applications, but seems potentially useful
in many other cases.

Things normally managed by super-CISC instructions (decimal
arithmetics, string instructions and the like) in such machines, would
then be done with neat library functions declared as "register". The
CISC equivalent to this would be to allow users to define new machine
instructions at run-time.

Of course, you would have to decide on what to do on a context switch.
Maybe the the register functions should belong to a shared library and
be more or less permanently in the cache.

Perhaps, this kind of register functions would make the RISC vs CISC
debate fade a little.