Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!uwmcsd1!ig!agate!ucbvax!CORY.BERKELEY.EDU!dillon
From: dillon@CORY.BERKELEY.EDU (Matt Dillon)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Address Space on the Amiga (was Re: Need info on exceptions)
Message-ID: <8809010303.AA14718@cory.Berkeley.EDU>
Date: 1 Sep 88 03:03:36 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 38

:In article <8808311759.AA19249@cory.Berkeley.EDU> dillon@CORY.BERKELEY.EDU (Matt Dillon) writes:
:>	This does not mean that X must point to physical memory, just as long
:>as the MMU MAP is to a unique address, see?
:>	Thus, you can give processes access to each other's memory on a 
:>per-process basis, and even though this is restricted to the page size of
:>the MMU, it is still better than no protection at all.  Specifically, a 
:>task would be able to say "This is protected and nobody but me can access it".
:
:   Why not just have a separate virtual address space for each process's shared
:memory, and have the pointers contain which task's virtual address sapce it is

	You mean under a 'standard' VM model where each process has the same
address space (everybody starts at 0 model) and part of that is reserved for
shared memory?  Two reasons:

	(1) It introduces special cases (the set-aside address space is a 
	    special case)

	(2) For the Amiga at least, it is not a feasible solution with all
	    the message passing processes do... that is, it is not something
	    that can be added without rewriting just about the entire OS.

:   Of course, this won't work for the current 68000 architecture as it would
:ideally be supported at the ucode level.

	I figure a standard MMU will do the job, no?  Not saying I like the
method, just wondering if I missed something?  The VAX is the only machine I
know whos microcode knows about the MMU extensively.

:   For the current implementation, we could always make everyone use the
:MEMF_PUBLIC flag and too bad for the people who were sloppy. :-).. it's really
:the same kind of thing as happened with fast memory -- sloppy programs broke.

	No way.  MEMF_PUBLIC was never defined well and subsequently not used
consistantly (probably not even by the OS).  EVERYTHING will break!


						-Matt