Newsgroups: comp.os.minix
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!menudo.uh.edu!st12a
From: st12a@menudo.uh.edu (richard henderson~)
Subject: Re: Virtual addresses
Message-ID: <1991May16.210048.18717@menudo.uh.edu>
Organization: University of Houston
References: <53697@nigel.ee.udel.edu> <1991May16.171429.438@email.tuwien.ac.at>
Date: Thu, 16 May 1991 21:00:48 GMT

In article <1991May16.171429.438@email.tuwien.ac.at>
	hp@vmars.tuwien.ac.at (Peter Holzer) writes:

>I vote for #1. As far as I can see, MM does very little without aid by
>the kernel, so making separating it from the kernel makes things more
>complicated, not easier. (Maybe the 64k segment limit of the 8086 was
>the reason to separate it from kernel, I don't know the sizes offhand)
>Process control should go into the kernel, IMHO. (I guess, I will get a
>lot of flames for this)

It does violate the ring protection scheme, however, things might just
run a bit faster if the system task and the mm were merged; there would
be no waiting for the task switch.

>>One thing I'd like is the ability for processes to expand so
>>that when a process grows past it's chmem'd limit, it just gets
>>bigger. This could be done with swapping.
>
>This won't work because the Stack grows downward from the chmem'ed
>address. When stack and heap meet, you cannot just expand your
>data/stack segment and copy the stack to its new location without
>invalidating all pointers pointing into the stack area (e.g. argv). 

Not true.  Simply put the stack at the "top" of addressable memory for
each process and leave the memory between _brk and sp unmapped.  That
way, when the stack grows, a page fault is generated, and the os tacks
on more memory.  When brk(2) is called, more memory is tacked on from 
the bottom end.

At least, that is how it works on 80386 and 68020 and better.  I do not
know if a 68000 with MMU can generate faults instead of exceptions.
(Faults are restartable, exceptions are not).

This was something that I had been hoping for when I found out about the
386 patches.  By-the-by, have the people working on vm for minix gotten
that far?

----------
richard~
richard@stat.tamu.edu
st12a@menudo.uh.edu