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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.arch,comp.unix.wizards,comp.os.minix
Subject: Re: Free Software Foundation (was: Re: Mach, the new standard?)
Message-ID: <29933@sun.uucp>
Date: Sat, 3-Oct-87 23:49:22 EDT
Article-I.D.: sun.29933
Posted: Sat Oct  3 23:49:22 1987
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> I recently purchased the Bach book. The history he gives indicates
> that the probable reason Unix originally used swapping instead of
> paging, is because the early models of the PDP-11 that they originally
> implemented Unix on, did not support paging.

I would be very surprised if this were the case.  The PDP-11/20 that was the
first PDP-11 on which UNIX was implemented (the first *machine* it was
implemented on was a PDP-7), and it didn't have an MMU of any sort.  The
PDP-11/45, which was the second PDP-11 on which UNIX was implemented, did,
however, have sufficient support for paging.  See John Mashey's article,
discussing paging UNIX on a PDP-11/55 (which was an 11/45 with bipolar memory
hung off a fast memory bus).  That article also cites what is probably the
*real* reason UNIX didn't use paging; it added complexity and didn't buy you
much at all.

Some of the later PDP-11s that UNIX was ported to had an MMU but may or may not
have been able to support demand paging; the 11/45's MMU included a register
that, by recording changes made to registers by auto-increment and
auto-decrement addressing modes, permitted the OS to back up an instruction
that faulted in midstream.  Other PDP-11s, however, did not; a backup routine
that was sufficient to handle the case of faults taken by references past the
end of the stack was provided for those machines.  I don't think this routine
was sufficient to handle *all* the cases of faults taken in midstream, at least
not in a machine-independent manner.
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com