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From: Kilgallen@SpamCop.net (Larry Kilgallen)
Newsgroups: comp.lang.ada
Subject: Re: Type safety on wikipedia
Date: 6 Feb 2006 02:29:49 -0600
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In article <fildu1p2q1glsq3kmmpaesumabg0p5gjc0@4ax.com>, Dave Thompson <david.thompson1@worldnet.att.net> writes:

> On some (increasingly many) systems with virtual memory where it is
> easy (enough) to leave page (or even segment) 0 unmapped, it will give
> a clear error, usually even a recoverable signal (roughly like the
> exception in Ada). On some (other) systems, (virtual) 0 is accessible
> (and accessed) but is reserved and initialized with "useful" data,
> such as 4 bytes of zero which in the most common read cases (as a
> string, char, or int) gives a safe and possibly even useful result.

Well, there is only one virtual zero address within a given program
execution address space, and I seem to recall that at least one LISP
implementation used that for a crucial piece of data (that had to be
addressed as zero in that implementation).  Thus one could not depend
on the lack of physical memory at zero in any program built from both
LISP and Ada modules.