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From: bruce@stride.UUCP
Newsgroups: comp.arch,comp.unix.wizards,comp.os.misc
Subject: Re: "fork" with no MMU (was Re: Shared libraries (Was: ...)
Message-ID: <691@stride.Stride.COM>
Date: Fri, 30-Oct-87 14:58:58 EST
Article-I.D.: stride.691
Posted: Fri Oct 30 14:58:58 1987
Date-Received: Sun, 1-Nov-87 09:56:32 EST
References: <8714@utzoo.UUCP> <286@usl> <73@usl-pc.UUCP> <881@sugar.UUCP> <31162@sun.uucp> <895@sugar.UUCP>
Reply-To: bruce@stride.stride.com.UUCP (Bruce Robertson)
Organization: Stride Micro, Reno, NV
Lines: 19
Xref: utgpu comp.arch:2591 comp.unix.wizards:4869 comp.os.misc:303


The Idris operating system works reasonably well without an MMU,
including the "fork" system call.  It uses a combination of memory and
swap space, implemented as a "swap cache".  The parent process is
logically swapped out, but if it is small enough it is actually
swapped to the swap cache.  I don't remember if Idris does, but you
could probably avoid swapping the text segment with this method.

This scheme works fairly well, AS LONG AS the child does the usual
thing and execs another process quickly.  This other process is loaded
elsewhere in memory, allowing the parent to come back into memory.  It
totally fails for things like "tip" and "cu", as written.  The Idris
version of cu actually has the child process exec another program to
handle the incoming stream of characters.
-- 

	Bruce Robertson
	bruce@stride.Stride.COM
	cbosgd!utah-cs!utah-gr!stride!bruce