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From: ballou@brahms.Berkeley.EDU.UUCP
Newsgroups: comp.os.minix
Subject: MINIX memory management/protection
Message-ID: <888@cartan.Berkeley.EDU>
Date: Wed, 4-Feb-87 18:12:05 EST
Article-I.D.: cartan.888
Posted: Wed Feb  4 18:12:05 1987
Date-Received: Sat, 7-Feb-87 09:17:39 EST
References: <252@hqda-ai.UUCP> <1169@steinmetz.steinmetz.UUCP> <511@bobkat.UUCP>
Sender: daemon@cartan.Berkeley.EDU
Reply-To: ballou@brahms.Berkeley.EDU (Kenneth R. Ballou)
Organization: Math Dept. UC Berkeley
Lines: 42
Keywords: shared swapped jobs

In article <511@bobkat.UUCP> m5d@bobkat.UUCP (Mike McNally (dlsh)) writes:
>In article <1169@steinmetz.steinmetz.UUCP> davidsen@kbsvax.UUCP (william E Davidsen) writes:
>>
>>If you allocate a full 64k to data, there is hardware protection: you
>>can't address more than that. This assures that any program which
>>doesn't deliberately set out to cause problems will not modify the
>>code. ...
>
>What about a program with a bug in it?  Like "strcpy(a, b)" when "a" is
>not quite what I meant?  It's real easy to make this kind of mistake;
>how many times while debugging a program on a VAX (or whatever) do you
>get SIGBUS or SIGSEGV?
[Omitted here:  a description of a phenomenon with which I'm sure we are
 all too painfully familiar -- how wild pointers can crash programs and
 machines.]

	I think you have missed a key point here which depends on the iAPX86
architecture.  Because addresses are constructed as SEGMENT:OFFSET, as long
as the compiler generates no code that would reload the segment registers
and as long as you do not use any assembly code in your program that does
this, then you are physically constrained to 64K bytes starting at absolute
location 16 * SEGMENT.  Since pointers are passed only as offsets, the
worst you can do is scribble over your 64K segment.

	However, this is a blatant lie, and it is possible you are
vindicated.  For, if auto variables are allocated on the stack, one
could still easily lose.  Since the stack must lie in the same segment
as the static data (otherwise, pointers must have segments associated
with them to distinguish between auto and static variables), it is
possible to scribble over the stack.  In doing so, one could alter a
return address and find oneself in another process, or perhaps the
kernel.  Also, equally likely is that one would try to execute data
and encounter an illegal opcode.  I believe (but I am not certain)
that this halts the 8086.

	So, in summary, it turns out you are right, in an indirect
sort of way.
--------
Kenneth R. Ballou			ARPA:  ballou@brahms.berkeley.edu
Department of Mathematics		UUCP:  ...!ucbvax!brahms!ballou
University of California
Berkeley, California  94720