[pups] re: PDP-9

[bqt@update.uu.se (Johnny Billquist)]

On Tue, 20 Aug 2002, Dennis Ritchie wrote:

> Bilquist said (quoting Buitinck):
> 
>  > 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.

About as I suspected then.
Interesting to hear that the grade of compatibility was that high. I've
neved had the chance to play with any 18-bitters.

> 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.

The Unibus do not require a memory mapped I/O model, though. And it does
have 18 address and data bits. (Two data bits are used for parity on a
PDP-11.)
The DEC-2020 also used a Unibus.

	Johnny

Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt at update.uu.se           ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol