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From: tim@ism780c.UUCP (Tim Smith)
Newsgroups: net.micro,net.arch
Subject: Re: Re: What if IBM used a 68000
Message-ID: <133@ism780c.UUCP>
Date: Tue, 10-Dec-85 21:30:48 EST
Article-I.D.: ism780c.133
Posted: Tue Dec 10 21:30:48 1985
Date-Received: Sun, 15-Dec-85 06:41:03 EST
References: <212@fas.ri.cmu.edu> <702@petrus.UUCP> <6178@utzoo.UUCP> <147@intelca.UUCP>
Reply-To: tim@ism780c.UUCP (Tim Smith)
Organization: Interactive Systems Corp., Santa Monica, CA
Lines: 34
Xref: watmath net.micro:13076 net.arch:2294

Henry Spencer == "<"
"<" == Henry Spencer @ U of Toronto Zoology
">" == Glen Shires, Intel, Santa Clara, Ca.
"!" == someone else

! besides, it is quite possible to write position ind. code
! with a 68000.  (i.e. no MMU required)

< 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.

One system that does this sort of thing is the Macintosh ( anyone know
if the Amiga and the 520ST do this also? ).  They are able to get away
with it because memory only gets shuffled on calls to the memory manager.
Relocatable blocks are accessed through a "handle", which is a pointer to
a fixed pointer to the block.

> 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!

The Mac is slow because the resource manager is slow.  Also, the Finder
does not deal with directories in an efficient manner.  When a program
actually gets running, the Mac comes out OK.  I doubt that 32k code
segments have anything to do with it.

Also, unlike certain other systems that enforce segmentation, you can
have data bigger than 64k.
-- 
Tim Smith       sdcrdcf!ism780c!tim || ima!ism780!tim || ihnp4!cithep!tim
			  ^
			  ^-- Not ISM780C, ignore the header!