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From: bitting-douglas@cs.yale.edu (Douglas Bitting)
Newsgroups: comp.sys.mac.misc,comp.windows.ms
Subject: Re: Mac Vs. Windows? (sorry)
Message-ID: <1991Jun12.055711.21457@cs.yale.edu>
Date: 12 Jun 91 05:57:11 GMT
References: <0E010021.e0mxxc@gla-aux.uucp> <1991Jun4.154854.19649@dbase.A-T.COM> <56nD02AQ08cd01@JUTS.ccc.amdahl.com>
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In article <56nD02AQ08cd01@JUTS.ccc.amdahl.com> kls30@DUTS.ccc.amdahl.com (Kent L. Shephard) writes:
>>In article Something'er'other, someone wrote:
>>>2) The Motorola (and for that matter, any linear-addressing processor) is
>>>faster than segmented memory.
>
>Excuse me but this is not the case.  If you have an OS like Unix, System 7
>or any OS that provide virtual memory you are back to -(drum roll please)
>a segmented OS.  Pages are just large segments.

Paged != Segmented.  Pages are, by definition, fixed in size.  Segments are, by
definition, variable in size.  To have fixed page segments defeats the purpose
of the hardware support given to segmentation.  This aside, a paged system is
much more efficient than a segmented system.  This is because there is not
nearly the amount of overhead to keep track of on a paged System.  For example,
if one had a "demand segmenting" system vs a "demand paging" system (two
possible ways to implement VM given different hardware support), the complexity
of the OS would be vastly different.  In the demand segmenting system, not only
owuld the OS have to worry about swapping in a segment, but it would also have
to worry about "creating" the right amount of space tin order to fit the
segment in.  Most of the time, a perfect fit cannot be made so space is wasted
in memory and overall performance dips.  On a demand paging system, if a page
needs to be swapped in, it is just a matter of swapping out a page to create
the necessary space... much simpler and more efficient.

Also, (this is in reply to the quote of the quote of the...) not all Macintosh
Models are linearly addressed (this is especially true with the introduction of
VM in System 7).  On some models with the PMMU (the IIci comes to mind), memory
is not linear even *WITHOUT* virtual memory.  The PMMU makes it look that way
to software, but it really isn't the case...

>
>--
>/*  -The opinions expressed are my own, not my employers.    */
>/*      For I can only express my own opinions.              */
>/*                                                           */
>/*   Kent L. Shephard  : email - kls30@DUTS.ccc.amdahl.com   */


--Doug
-- 
Doug Bitting             | "And we know that in all things God works
PO Box 3043 Yale Station |  for the good of those who love him..."
New Haven, CT 06520      |                       --Romans 8:28
bitting@cs.yale.edu      +------------------------------------------