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From: gerten@uklirb.UUCP (Rainer Gerten)
Newsgroups: comp.arch
Subject: Re: IBM PC prehistory
Summary: Using base registers is the same as segment registers
Message-ID: <8019@uklirb.UUCP>
Date: 17 Jan 90 13:48:15 GMT
References: <1990Jan16.055625.8255@esegue.segue.boston.ma.us>
Reply-To: gerten@uklirb.UUCP (Rainer Gerten)
Followup-To: comp.arch
Organization: University of Kaiserslautern, W-Germany
Lines: 38

In article <1990Jan16.055625.8255> (John R. Levine) writes:
>Coding a program so that it can be moved while it's running is a lot harder
>than a relocating loader.  Either you have to tag every single pointer that
>the program uses, which is impractical in C, or else you need a coding style
>that reserves some registers as base pointers and make all memory references
>relative to the base pointers.  
 ^^^^^^^^^^^^^^^^^^^^

This sounds quite like segment register at the 8086 and co. For the
use in a non-secured enviroment the approach of "base registers" for
getting relocatable code is'nt that bad, if the segments are big 
enough. Strutured programming in mind, it is too a good way, to cut
the code in small pieces (based upon separate modules like modula).
Therefore, the need for big code pieces is'nt that urgent. The problem
in the intel architecture is not only the architecture at all, it's mainly
the design of the compilers (look at C, there you have 6 (?) memory-models).
Why are the compiler not clever engough, to decide the right kind of
addressing (like for example Turbo-Pascal with their units). 
I know, it's a problem for a C-compiler to decide, wether a function
is used only inside a module or outside too. But this problem is based
on the language design ! It true, the architecture of the 8086 and its
decendents is not that good for C, but in languages like modula or
ada this is no problem. 
A the side of data structures, it is a big problem for handling big
arrays in the style, most C compiler do it: assigning a huge amount
of linear memory and walking through it with "pointer++". But how
often are these arrays needed (in structured programming, you use
more often STRUCTURED data like b-trees and so on), so why are some
people crying (thats not up to you, John) ? Developing more sophisticated
languages and compilers, it is not that much the question of the
architecture, if there are problems for doing this or that.
With the above, I don't protect the architecture of segmented registers
nor flame on linear addressing, but I like to point out, that a combination
of looking at both would be more sophisticated.

Rainer Gerten
University of Kaiserslautern
mail: gerten@uklirb.informatik.uni-kl.de