[pups] re: PDP-9

[Dennis Ritchie]

Bilquist said (quoting Buitinck):

 > > we all know that UNIX first ran on the PDP-7 and then on the PDP-11/20,
 > > but does anyone know anything about PDP-9 UNIX?  it\'s mentioned in \"The
 > > UNIX Time-Sharing System\" in the V7 manual:
 > > 
 > > \"The earliest [version of UNIX] (circa 1969-70) ran on the Digital
 > > Equipment Corporation PDP-7 and -9 computers.\"

 > Hmmm, I cannot exactly answer that, but the PDP-7 and PDP-9 were both
 > 18-bit machines, and somewhat compatible, I believe.
 > The whole line is (I believe):

 > PDP-4 -> PDP-7 -> PDP-9 -> PDP-15

 > So I guess that if you had it running on a PDP-7, you could probably
 > almost take the code unmodified and run it on the PDP-9.
 > The PDP-15 have a different bus (Unibus?) I believe, and thus,
 > peripherials are different from the predecessors.
 > This obviosuly affects the OS. :-)

The 7, 9, 15 were very compatible.  I think the -15
had some scheme for using an index register, which
the earlier ones didn't have, but it was otherwise
pretty much identical in IS architecture.

There was very little rewriting to try Unix out
on the -9 and -15; perhaps just some tweaks in
the disk device commands.  I don't think the
system actually ran on either for more than a few
hours.  Ken was just playing around.

The -15 may have had an electrically different
bus, but I'm reasonably sure it was not a Unibus.
All of them used IOT instructions, not memory-mapped
IO registers.

Both of the machines we tried were being used by other groups
and we couldn't squat on them as with the PDP-7.
I recall that  the -15's main job was controlling a
step-and-repeat camera that exposed LSI masks.

	Dennis