Path: utzoo!attcan!uunet!lll-winken!lll-ncis!helios.ee.lbl.gov!pasteur!agate!ucbvax!hplabs!hp-pcd!hpcvca!charles
From: charles@hpcvca.HP.COM (Charles Brown)
Newsgroups: comp.sys.amiga.tech
Subject: Re: IRQ virus (and a personal note to Steve)
Message-ID: <1410014@hpcvca.HP.COM>
Date: 4 Jan 89 18:48:22 GMT
References: <10788@s.ms.uky.edu>
Organization: Hewlett-Packard Co., Corvallis, Oregon
Lines: 45

>> Tell it to NeXT. The NeXT box can multitask and DSP too.
>>  -- Sean Casey

> Peter didn't say real-time Unix is impossible, he said virtual memory
> and real-time don't get along.
> 					-=] Ford [=-

This is (almost) true, but most people reading it will get the wrong
impression.

The "almost" is because we really should say "demand paged VM" rather
than "virtual memory".  There is NOTHING which prevents virtual memory
without demand paging to be used in real-time.  This would allow
several programs (for instance, one real-time and several
non-real-time) to co-exist in RAM and all think that they are located
at the same fixed origin in memory.  Virtual memory without demand
paging also allows you to easily write protect major portions of RAM.
Thus the operating system and drivers can be (at least in theory) made
untouchable by viruses.

From what I understand of Hewlett-Packard's implementation of
Real-Time Un*x (yes, HP has real-time also.) the real-time program
must be locked into physical RAM and be given high priority.  It is
still running under a virtual memory operating system and may coexist
with several programs which get swapped as needed.  Its just that the
real-time code never gets swapped.  No specialized CPU is required.

So Amiga could be modified to use VM and even demand paging and still
have provisions for real time.

I expect to get flamed for this.  There are several people who read
this notes group who are violently opposed to demand paging for
performance reasons.  However, they simply do not know what they are
talking about.  Its really very simple.  There are two cases:
  1.  All processes will fit into physical RAM.
     Not demand paged:  Runs fast.
     Demand paged:  No paging.  Runs just as fast.
  2.  Processes will NOT fit into physical RAM.
     Not demand paged:  WILL NOT RUN.  Either fails to load or crashes.
     Demand paged:  Pages.  Runs slow, but at least it still runs.
So under the same circumstances, demand paged systems do not run slower
than non-demand paged systems.
--
	Charles Brown		charles%hpcvca@hplabs.hp.com
	Not representing my employer.