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From: ddb@starfire.UUCP (David Dyer-Bennet)
Newsgroups: net.micro.pc
Subject: Re: Segment Registers -- 8086/80286
Message-ID: <256@starfire.UUCP>
Date: Thu, 24-Jul-86 00:12:58 EDT
Article-I.D.: starfire.256
Posted: Thu Jul 24 00:12:58 1986
Date-Received: Fri, 25-Jul-86 00:11:08 EDT
References: <165@sci.UUCP> <146@apr.UUCP> <494@ubvax.UUCP> <1851@gitpyr.UUCP> <140@ima.UUCP> <592@looking.UUCP>
Organization: Terrabit Software
Lines: 36

> >>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.
> 
> Ummm, if you ask DOS to allocate memory for you, it returns a *segment* 
> address, and you are to assume zero offset (see DOS function calls 48 and
> 49).  Now, unless you are allocating in 64K-sized chunks, the segments
> won't likely be disjoint.  I am no great fan of DOS, but I'm not sure I'd
> call it *strange* in net.micro.pc...
> 
> --
> \tom haapanen					looking glass software ltd.
> syncro@looking.UUCP				waterloo, ontario, canada
> watmath!looking!syncro				(519) 884-7473

If you're running DOS, the 286 is running in real mode and its differences
are irrelevant.

If you're running Xenix V2, then it's in virtual mode.  Segments have a
length associated with them, so again you have non-overlapping segments
(many smaller than 64k).  There's no hardware to prevent you from having
segments pointing at the same place, but it doesn't happen naturally to
a user program.

		-- David Dyer-Bennet
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