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From: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Newsgroups: net.arch
Subject: Re: Why not virtual files?
Message-ID: <1047@sdcrdcf.UUCP>
Date: Fri, 27-Apr-84 11:31:13 EDT
Article-I.D.: sdcrdcf.1047
Posted: Fri Apr 27 11:31:13 1984
Date-Received: Mon, 30-Apr-84 05:24:13 EDT
References: <333@oliveb.UUCP>
Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer)
Organization: System Development Corporation, Santa Monica
Lines: 20

While it's based on address spaces of only about a megabyte (2^18*36 bit),
TENEX running on DEC PDP-10s and 20s has a system call called PMAP which
declares that a set of pages in a file are to be mapped into a set of memory
pages.  Following this call, the mapped pages are accessed by memory
references (including all normal demand paging effects directly to the
mapped file) for both reads and writ.  In fact, executing a program is based
on the same mechanism--do a PMAP of the file with the program to memory.
PMAP has a mode which simplifies sharing programs--a file can be mapped
read-only, with writes going into a delta file (the latter is a common case,
so large systems like interlisp generally map two files on start up--a
makesys file [usually a major version] and a sysout file [a saved user delta
file].
-- 
Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,sdccsu3,trw-unix}!sdcrdcf!darrelj
VANBUER@USC-ECL.ARPA