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From: daveb@geac.UUCP (Brown)
Newsgroups: comp.arch
Subject: Re: Big Programs Hurt Performance
Message-ID: <1502@geac.UUCP>
Date: Tue, 29-Sep-87 13:51:27 EDT
Article-I.D.: geac.1502
Posted: Tue Sep 29 13:51:27 1987
Date-Received: Wed, 30-Sep-87 03:20:28 EDT
References: <417@devvax.JPL.NASA.GOV> <12780006@acf4.UUCP> <2614@aw.sei.cmu.edu> <275@xyzzy.UUCP>
Reply-To: daveb@geac.UUCP (Dave Collier-Brown)
Organization: The little blue rock next to that twinkly star.
Lines: 38

In article <275@xyzzy.UUCP> meissner@nightmare.UUCP (Michael Meissner) writes:
>One of the things that is hard about position independent code is dealing
>with the initialization of pointers to static/extern data items, which is
>typically done at link time.  You have to have some way of getting the
>right value into the pointer before it is used.  

  Well, you delay some portion of linking 'til run-time.  If we assume that
externs are in the data segment, you link with a unique segment number
which at run-time turns out to mean "pages 23-29" to your paging
hardware. 
  The hard part is figuring out the data structures to pass to the
program loader (and to generate in the linker) so that gets done
cleanly and in finite time.

  Multics used a very simple scheme: an address had a segment number
(no, not the same as IBM-PC segments) and an offset.  When an address
was evaluated, the segment number was used as an index into an
on-board translate table which yielded a page address, and if it
wasn't there a page fault occurred.
    +-----+--------+
    | seg | offset | -------> address in page/page-sequence
    +-----+--------+
       \
        --------------------> index into page-tables

  If you can define how your address get interpreted, (ie, you're a
machine designer) you can set this up and migrate your customer base
to it via having a new linker, loader and "magic number" for the new
executables.  They won't run on *old* machines, of course...

 --daveb

ps: this *is* idealized, you understand.
-- 
 David Collier-Brown.                 {mnetor|yetti|utgpu}!geac!daveb
 Geac Computers International Inc.,   |  Computer Science loses its
 350 Steelcase Road,Markham, Ontario, |  memory (if not its mind)
 CANADA, L3R 1B3 (416) 475-0525 x3279 |  every 6 months.