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From: davids@ucscb.UCSC.EDU (Dave Schreiber)
Newsgroups: comp.sys.amiga.tech
Subject: Memory protection for AmigaDOS
Summary: Why not (here we go again?) ?
Message-ID: <4250@darkstar.ucsc.edu>
Date: 9 Jun 90 19:37:05 GMT
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Reply-To: davids@slugmail.UCSC.EDU (Dave Schreiber)
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[And just when you though this topic had been discussed to death...]

A while back (I'm sure you all remember), there was a discussion
regarding putting memory protection into the Amiga OS.  As I remember,
the two big problems were:
 1.  Compatibility
 2.  Loss of performance, due to the loss of the ability of programs
     to access memory allocated by other programs.

Since then I've been reading _Unix Papers for Unix Developers and Power
Users_ by the Waite Group.  In it there is a paper regarding porting
Unix to 68030 systems; basically an intro to the '030, its features,
considerations, etc.  On pages 444-445, the paper talks about shared
memory:
  "Shared-memory segments are a way in which Unix allows two processes
   to communicate.  ...one process creates an area of memory and allows
   it to be shared by other programs.  Another process can then request
   that this shared memory segment be 'attached' to its logical address
   map.  Data that is written into the shared segment by one process
   can be read from the segment by another process.
        Shared-memory segments can be easily implemented using the
   MC68030 MMU. [...] Each process can have either read/write or read
   only access to the segment."

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.

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)?

Note that I'm not taking into consideration the fact that most Amiga's
don't have MMUs; i.e. that memory protection might not be practical
from the financial point of view of Commodore (although I'd be willing
to pay extra for a crash-proof version of Kickstart), but there's
always the future...


-- 
Dave Schreiber                        The blue leprechaun
at davids@slugmail.ucsc.edu (prefered but flakey)
or (not both) davids@ucscb.ucsc.edu   "Coffee, Darling?"