UNIX on PDP-11s

[Johnny Billquist]

Ethan Dicks <ethan.dicks at gmail.com> wrote:

> On Thu, Jun 4, 2009 at 11:42 AM, John Floren <slawmaster at gmail.com> wrote:
>> > Interesting, I didn't know a /23 could run V7--all I know for sure is
>> > that it works on /45s and /70s. Do you know any other machines it runs
>> > on? I'd like to try installing on my 11/73 (KDJ11-A) if possible.
> 
> I think it would come down to disk drivers.  If the kernel will run on
> an 11/70, there's no difference I can think of off the top of my head
> why it wouldn't run on a KDJ-11, unless there's something about the
> 22-bit Unibus mapping on the 11/70 vs no mapping on the Qbus.

Yikes! No, I would not expect it to work well at all.
The first, and most obvious problem is the bus. The Unibus is an 18-bit 
address bus, while the q-bus is 22-bits. So, an 11/70 have a Unibus map 
which is involved in all DMA from the Unibus. The q-bus don't have 
anything like that.
This obviously affects all device drivers doing DMA.

And then we have the obvious problem that the most likely disks used on 
V7 would have been massbus disks, which you have no correspondance to on 
a q-bus. But even with disks of a similar nature, such an RL, there are 
still differences between the controllers, so that you can't just run 
the version from a unibus on a q-bus machine without a little modification.

I don't know how much features V7 uses, but there are plenty of things 
in the 11/70 which the KDJ11 don't do. Stack limits are different. The 
MMU is different (although I don't think anyone uses the 11/70-specific 
features).

Now V7 is old enough to not make much use of the hardware, which still 
makes it easier to get it running on different machines (no split I/D 
space, no supervisor mode stuff). So, as always, it is possible, but you 
can't just drop the software in, and expect it to run.

> Disks, though... a 11/73 is likely to have an MSCP disk controller,
> but the 11/70 has quite a number of supported disk controllers.

True.

> I'd look at the 7th Ed. sources and the install process to see what
> 7th Ed. is expecting to load onto.

You need to rewrite things. There is no way you can get something 
written for the Unibus, and large memory, work on a q-bus without 
rewriting some stuff.

	Johnny

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