Path: utzoo!attcan!uunet!lll-winken!ncis.tis.llnl.gov!helios.ee.lbl.gov!pasteur!ucbvax!hoptoad!tim
From: tim@hoptoad.uucp (Tim Maroney)
Newsgroups: comp.sys.mac.programmer
Subject: Re: System 7.0 Q & A -- memory protection (none)
Message-ID: <7320@hoptoad.uucp>
Date: 16 May 89 17:10:25 GMT
References: <1838@internal.Apple.COM>
Reply-To: tim@hoptoad.UUCP (Tim Maroney)
Organization: Eclectic Software, San Francisco
Lines: 34

In article <1838@internal.Apple.COM> lsr@Apple.COM (Larry Rosenstein) writes:
>I don't think the probability will change at all under 7.0.  There is one 
>large address space (up to 14 megabytes) and everything lives there.  
>There is no protection at all.

In that case, I think the probability of one application trashing
another actually increases in System 7.0.  To be useful, virtual memory
requires that process sizes be able to increase.  Given one large
address space, the only way to do this on the Mac is by discontiguous
heaps -- that is, when the application heap runs out of memory, a new
chunk is added at a discontiguous location in the global address
space.  So after a few applications grow, their heaps are thoroughly
intertwined.  One of the more common program errors consists of
assuming that heap block is larger than it actually is and writing past
the end.  In 7.0, that has a good chance of writing into another
application's heap instead of simply corrupting one's own heap.
Very messy, and it seems like a giant step backwards.

>The comment about low memory was simply repreating what was stated at the 
>Developer's Conference.  If protection was done between application heaps 
>(which it isn't), it would only be a red herring since the system heap and 
>low memory is shared.  The thought was that it was better to come up with 
>a total solution in some future system, rather than give users a false 
>sense of confidence.

Why wait?  Do you want developers to have to upgrade once for System 7.0
and once for System 8.0?  Talk about your pissed-off users!
-- 
Tim Maroney, Consultant, Eclectic Software, sun!hoptoad!tim
"Now hear a plain fact: Swedenborg has not written one new truth: Now hear
  another: he has written all the old falshoods.
 And now hear the reason.  He conversed with Angels who are all religious, &
  conversed not with Devils who all hate religion..."
    - Blake, "The Marriage of Heaven and Hell"