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From: glen@intelca.UUCP (Glen Shires)
Newsgroups: net.micro,net.arch
Subject: Re: Re: What if IBM used a 68000
Message-ID: <147@intelca.UUCP>
Date: Fri, 6-Dec-85 19:15:19 EST
Article-I.D.: intelca.147
Posted: Fri Dec  6 19:15:19 1985
Date-Received: Sun, 8-Dec-85 03:23:29 EST
References: <212@fas.ri.cmu.edu>, <702@petrus.UUCP> <6178@utzoo.UUCP>
Organization: Intel, Santa Clara, Ca.
Lines: 27
Xref: watmath net.micro:12988 net.arch:2230

> > 	besides, it is quite possible to write position ind. code
> > 	with a 68000.  (i.e. no MMU required)
> 
> Try writing the code for position-independent pointers some time; it's lots
> of fun, especially if you are never allowed to have a position-dependent
> value around even for an instant (except in pre-agreed places like the A
> registers).  It's easy to write position-independent code if that code and
> its data will *never* need to be moved once it starts running.  Writing code
> that can be moved at randomly-chosen times and continue to run is not
> so simple.
> -- 
> 				Henry Spencer @ U of Toronto Zoology
> 				{allegra,ihnp4,linus,decvax}!utzoo!henry

Precisely why Mac accesses all pointers (called "handles") through a table
requiring a double indirect access on all pointers.  Also, for position
independance during runtime, they limit some code pieces to 32K to allow
branches with 16-bit signed pointers.  And people ask why Mac is so slow!

Segments, especially big 4Gigabyte ones, allow simple runtime relocatability
without having to resort to such programming techniques.

-- 
^ ^    Glen Shires, Intel, Santa Clara, Ca.
O O     Usenet: {ucbvax!amd,pur-ee,hplabs}!intelca!glen
 >      ARPA:   "amd!intelca!glen"@BERKELEY
\-/    --- stay mellow