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From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.arch
Subject: Re: Segmented Architectures ( formerly Re: 48-bit computers)
Message-ID: <6862@auspex.auspex.com>
Date: 27 Mar 91 23:09:12 GMT
References: <1991Mar21.164242.886@sj.nec.com> <efeustel.669650766@tiger1>
Organization: Auspex Systems, Santa Clara
Lines: 33

>C assumes that pointers are linear and monotonically increasing.

Well, many C programs do so.  The ANSI C standard makes an effort not to
demand that pointers refer to a linear address space with
monotonically-increasing addresses; pointers may be compared only for
equality, unless both pointers point into the same array,
pointer+integer is defined mainly in terms of array indexing, and
pointer-pointer is defined only if both pointers point into the same
array.

It may be that, at least at present, programs of that sort are
sufficiently common that you really *do* have to pretend the address
space is one single huge array even on machines where that model isn't
natural.  Maybe that'll change in the future; it'd certainly be nice if
it did.

>If you are saddled with small segments with N < 2**18 or so, you will soon
>come to hate segments because you have to continually map C addresses onto
>multiple segments in order to support the linear model.

And also have to deal with "near" and "far" pointers, and multiple
memory models, in programs that require more than a segment's worth of
code or data - at least in C.  What do other programming languages that
support pointer-style data types do? Do they also have to deal with
"near" and "far" pointers?  Or, in the memory models with more than a
segment's worth of data, do they just make pointers large?

>By using the same page table for every process, sharing of the operating
>system, code of shared libraries, etc. is enabled.

I assume you mean "simplified" rather than "enabled".  Sharing of the
operating system, code of shared libraries, etc. is certainly possible
on systems that have per-process page tables....