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From: chris@umcp-cs.UUCP
Newsgroups: net.arch
Subject: Re: Why not virtual files?
Message-ID: <6953@umcp-cs.UUCP>
Date: Mon, 7-May-84 18:24:28 EDT
Article-I.D.: umcp-cs.6953
Posted: Mon May  7 18:24:28 1984
Date-Received: Fri, 11-May-84 07:31:51 EDT
References: <6845@umcp-cs.UUCP>, <1893@rlgvax.UUCP> <939@hou5d.UUCP>
Organization: Univ. of Maryland, Computer Science Dept.
Lines: 17

Actually, you don't need a special system call to map a file into
virtual memory.  All you need is ``read'' and ``write'', and a way
to protect memory areas.  If ``read'' just marks each page as
fill on demand (similar to the way ``vread'' works but with the
bugs fixed), and you then take away write permission for yourself
on those pages, and catch addressing faults, you can set things up
so that anytime you modify such a page, your routine takes over,
redoes the modify with permission turned on, write()s the page back
wherever it belongs, and then turns permissions off again.

See, no extra system calls beyond the ones already in System V.
(I think - I haven't read the System V manuals, but they do have
memory protection stuff, don't they?)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci (301) 454-7690
UUCP:	{seismo,allegra,brl-bmd}!umcp-cs!chris
CSNet:	chris@umcp-cs		ARPA:	chris@maryland