[Simh] VAX 8200

[Ethan Dicks]

On Thu, Mar 16, 2017 at 4:07 PM, Robert Armstrong <bob at jfcl.com> wrote:
>> Ethan Dicks (ethan.dicks at gmail.com) wrote:
>>I happen to have one (from when I worked for Jim Ebright).
>
>   I have a running 8350 as well, should you need a dump of the console media or something.

So far, so good... and I might have a DCL script to build a console
floppy from files on the system volume (I was in the habit of ensuring
I could do that from my days with the 11/730 and 11/725).

>   It is, I believe, the physically smallest BI bus VAX, and therefore interesting for those reason.

Yep.  One box for the CPU, memory and peripherals, and all inside a
42"-tall rack.  Just add disk.  Small footprint.

> It's the only such machine that I could fit in my garage :-)

With a VAXBI bus?  Yeah... there were equal (11/730) or smaller
(11/725) Unibus systems, but it was the smallest VAXBI machine outside
of the rare VAXstation 8000.

>   It's also the only multi-processor VAX system that I have and equally interesting for that reason too.

Sure.

> FWIW, I've always wondered if the limitation to two CPUs was purely for testing and support reasons - it doesn't seem that there's any reason why you could not plug in three or even more CPU cards.

What I remember from the 80s is that you _can_ plug in more than 2
cards (you'd need an ID plug on the backplane for additional CPUs,
IIRC) but you don't get any more performance from it.  Two CPUs rather
maxes out the VAXBI bus since all memory and I/O traffic are on the
one bus.  Also, from back when typical memory cards were 2MB and 4MB,
removing memory to add CPUs was also the path to a performance loss.
With the MS820-CA 16MB card (of which I have zero), that alleviates
this somewhat - 32MB of RAM in an 8200 is not a terrible thing and
then only takes up 2 slots.

I should say my 8200 has a DWBUA Unibus adapter, but I have 2 VAXBI
card cages in the BA-32 and the Unibus DD11-DK is in a BA-11 off to
the side.  One configuration had the DD11-DK _in_ the BA-32 next to a
single VAXBI backplane and there, you had little room for extra
anything.

-ethan