We implemented filesystems as a supervisor process so that it
could support file access from both a UNIX supervisor process
and an RSX/11 supervisor process in the MERT operating system.
See Figure 1 in the MERT paper in the July/August 1978 BSTJ.
On 6/29/2020 2:11 AM, Paul Ruizendaal wrote:
Date: Wed, 24 Jun 2020 14:31:34 -0400 (EDT)
From: norman(a)oclsc.org (Norman Wilson)
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
Norman Wilson Toronto ON
I am still digesting all the inputs (thanks, all!)
The above post made me realise that the delineation of what is a FSS/VFS or not, is not
I did a little bit of reading, and the concept of an ACP arrived with RSX11D in May 1973,
but only matured in RSX11M in November 1974. As I understand it, originally in RSX11 file
system code was closely tied to the low-level device driver for each device. ACP’s
separated the file system code from the device driver itself, and became separate
In essence there were two switches: one switch into abstract devices, implemented in ACP
code and one kernel switch to deal with hardware interfacing. The first is indeed like a
file system switch (although still tied to specific devices).
Looking at this stuff made me realise that my retro machine of choice (the TI990) went
through a similar evolution. In the early seventies it had a sort of abstract device
switch that linked to individual ‘device service routines’ (drivers). Initially, these
modelled batch oriented ‘logical units’ that tied to files at the job control level. Later
(late 70’s), the ‘open’ command would carry a file name and the file system was delegated
to the device service routine. Still later (say 1983) this was used for networked disks.
As several people have observed in this topic, indeed there appears to be a close
relationship between a device switch and a file system switch.