Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!usc!rutgers!cbmvax!jesup
From: jesup@cbmvax.commodore.com (Randell Jesup)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Memory protection for AmigaDOS
Message-ID: <12514@cbmvax.commodore.com>
Date: 10 Jun 90 06:34:51 GMT
References: <4250@darkstar.ucsc.edu>
Reply-To: jesup@cbmvax (Randell Jesup)
Distribution: na
Organization: Commodore, West Chester, PA
Lines: 28

In article <4250@darkstar.ucsc.edu> davids@slugmail.UCSC.EDU (Dave Schreiber) writes:
>Although this part of the paper is talking about each process having
>its own virtual memory map, it seems reasonable to assume that it
>(especially the last sentence) would be the same if the whole system
>ran off of one address map.

	Sure, you could have shared pages and private pages, even under
a one-memory-space setup.

>So, what have I missed here?  Is there something else in the Amiga's
>OS that makes memory protection difficult to do?  If compatibility is
>an issue, why not have the OS convert the memory of the message while
>its being passed to its destination (i.e. process A sends a message to
>process B.  Exec makes the memory of the message readable and writable
>by both process A and B, then passes it to process B.  When B Reply()'s
>to the message, remove B's access to it, then pass the reply signal
>back to A)?

	Now you've reached the problem.  a) such a "conversion" is expensive
at best, impossible (or nearly so) at worst.  b) Usually the message includes
pointers to other areas of memory, and you must deal with them also - but
you have no way of knowing the format of most messages.  There are more
problems, but most people here know them pretty well by now.  :-)

-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com  BIX: rjesup  
Common phrase heard at Amiga Devcon '89: "It's in there!"