21 Aug
2002
21 Aug
'02
11:21 a.m.
On Tue, 20 Aug 2002, Dennis Ritchie wrote:
Bilquist said (quoting Buitinck):
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.
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.
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(a)update.uu.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol