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From: davidsen@steinmetz.steinmetz.UUCP (William E. Davidsen Jr)
Newsgroups: comp.os.minix
Subject: Re: File system/virtual memory integration
Message-ID: <1265@steinmetz.steinmetz.UUCP>
Date: Thu, 5-Mar-87 12:25:41 EST
Article-I.D.: steinmet.1265
Posted: Thu Mar  5 12:25:41 1987
Date-Received: Sun, 8-Mar-87 08:54:51 EST
References: <1276@husc6.UUCP> <1057@ius2.cs.cmu.edu> <7730@utzoo.UUCP>
Reply-To: davidsen@kbsvax.steinmetz.UUCP (William E. Davidsen Jr)
Organization: General Electric CRD, Schenectady, NY
Lines: 33
Keywords: file system,virtual memory,Multics

In article <7730@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>> I was wondering how difficult it might be to 'integrate' virtual memory
>> with the file system process.  I'm told that the Multics operating system
>> mapped a file into the virtual address space when it was opened.

VMS also allows this, called mapped sections. I used it to improve
the performance of our mail system.
>
>Multics did indeed do that.  The problem is that the ability to do tricks
>like that is *very* dependent on good memory-management hardware.  That's
>why Unix doesn't do it.
     ^^^^^^^^^^^^^^^^^^  I believe that mapped files were in the
BSD plans but never implemented. I know I saw it in some initial
documentation of BSD< perhaps someone will check and post.

>                         An 8088- or 8086-based machine has no hope of
>doing it.  Nor does a bare 68000 as in the ST.  The somewhat better MMU
>in the 80286 just might make it possible in a sleazy and inefficient way
>on an AT.  You really need paging to do it right, and it better be a well-
>designed paging box at that.

The 80286 generates a memory fault when accessing a page not
mapped, which could also mean 'not in physical memory'. It could
set pages write protect, and when a WP fault occurs, change the
permissions on the page and set a dirty bit.

>"We must choose: the stars or	Henry Spencer @ U of Toronto Zoology

-- 
bill davidsen			sixhub \
      ihnp4!seismo!rochester!steinmetz ->  crdos1!davidsen
				chinet /
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