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From: norman@oclsc.org (Norman Wilson)
Subject: Re: [TUHS] VFS prior to 1984
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Reaching outside of UNIX, RSX/11 used external
supervisor-mode processes called ACPs (ancillary
control processes) to implement file systems.
I don't know exactly how they were plugged in,
but I do know they were pluggable, so their
interface must have constituted a file-system
switch of some sort.  RSX dates back into the
1970s.

At some point in the latter part of the 1980s,
Ralph Stamerjohn (a name instantly recognizable
in the 16-bit DEC software world) gave a DECUS
talk about implementing a remote file system
through ACPs: a stub ACP on the client exporting
RPCs over the network, a real one at the server
end.  I remember chatting with him about how
that did and didn't resemble the way pjw had
done it; interesting architectural comparison.

Norman Wilson
Toronto ON