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From: jrv@siemens.UUCP
Newsgroups: net.micro.pc
Subject: Re: Re: Segment Registers -- 8086/80286
Message-ID: <23900028@siemens.UUCP>
Date: Thu, 26-Jun-86 08:40:00 EDT
Article-I.D.: siemens.23900028
Posted: Thu Jun 26 08:40:00 1986
Date-Received: Sat, 28-Jun-86 06:36:05 EDT
References: <1851@gitpyr.UUCP>
Lines: 38
Nf-ID: #R:gitpyr:-185100:siemens:23900028:000:1622
Nf-From: siemens!jrv    Jun 26 08:40:00 1986



>In article <1851@gitpyr.UUCP> kpk@gitpyr.UUCP (Kevin P. Kleinfelter) writes:
>>In article <494@ubvax.UUCP>, skip@ubvax.UUCP writes:
>>> ... and if you ever plan on writing programs for the i80286, don't
>>> "normalize" pointers.
>> ...
>>So how will one compare pointers?  If you can't normalize them, two pointers
>>could reference the same address, but not contain the same values. ...
>
>Not likely.  The 286 is a genuine segmented architecture (as opposed to the
>8086 which is an unsatisfactory approximation.)  The operating system
>controls where each segment is mapped.  Unless you have a strange
>operating system, you can assume that the contents of different segments are
>disjoint, so that if two pointers point into different segments, they're not
>pointing at the same thing.
>

The problem with this is that the "thing" you want to point to can not be
larger than the size of one segment. Sometimes one needs a big data item.
Using normalization and conversion to linear addressing for calculations
with pointers the segment boundaries can be overcome.

These processors were designed when 48K was a lot of memory on a
microprocessor system. Maybe the basic problem with the 8086 and 80286 is
that their segment size is smaller than what is needed for many of the
current programming problems.

Question:
	On other machine architectures/operating systems which use
	segmentation are the segment sizes larger? Do the high level
	languages allow you a way to work around this size limit?


Jim Vallino
Siemens Research and Technology Lab.
Princeton, NJ
{allegra,ihnp4,seismo,philabs}!princeton!siemens!jrv