NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/sys/amiga/programmer/7714

Internet Message Format | 1996-08-05 | 2.6 KB

Path: ami-cg.GraySage.Edmonton.AB.CA!cg
Newsgroups: comp.sys.amiga.misc,comp.sys.amiga.programmer
From: [e-mail] (Chris Gray)
Subject: Re: toy operating systems, like AmigaOS
References: <[e-mail]> <4kplgo$[e-mail]> <4ksu6q$[e-mail]> <[e-mail]> <[e-mail]>
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Date: Sat, 20 Apr 96 09:57:07 MST
Organization: Not an Organization

In article <[e-mail]> [e-mail] writes:

> for the developer, lack of memory protection is a pain. but how many
> Unix clones can handle realtime tasks as good as amiga can? Linux can't.
> So your Toy Operating System beats many bigger OSes just because it is
> smaller. If all OS kernels had the same feature list, we would need only
> one kernel. But different applications need different feature lists, and
> different kernels. and there is still use for a small single user multi
> tasking OS without memory protection. You must not see the missing
> feature, you must see the advantages of a simple, performant design.
> add memory protection to Amiga OS and you remove its main advantage.

Adding memory protection to an OS does not prevent it from being a real
time OS. The messing around with MMU tables takes only a constant (small)
amount of work - some work when a process is created, or when it allocates
large chunks more of memory. On process switches, only a very small amount
of effort is needed, on most CPUs. Virtual memory, where some memory
gets paged out to disk, *can* prevent real-time performance, but only
if you use more memory than you physically have.

The problem with memory protection for the Amiga is that having it will
break most software, and having it increases the cost of message passing,
even after software is fixed to work with it. My guess is that the
increased cost would vary from 10% to 100%, depending on the application.
Things at the high cost end would be things like terminal programs, which
use a message per character sent or received on the serial port (which
is bad programming even now, but I image some do it). The cost to programs
like ray-tracers, compilers, editors, etc. would be quite small. The
average cost is sufficiently small that it would be completely unnoticed
if combined with a switch to a PPC CPU.

I *want* memory protection, both as a programmer and as a user.

--
Chris Gray   [e-mail]