UNIBUS PDP-11 memory expander card

Johnny Billquist bqt at update.uu.se
Wed Jan 28 16:13:41 CST 2015

On 2015-01-28 20:02, Noel Chiappa wrote:
> So, I'm in the process of trying to read those old MIT V6 Unix dump tapes
> (with a really big hand from Chuck, who did all the ugly work of actually
> reading the bits off the tapes, for which he has my undying gratitude!), and
> although I'm still trying to figure out what the format is, I did manage to
> retrieve, more or less, a copy of the assembler startup/support file (m47.s; a
> hacked version of m45.s), and it reveals that we were using an Able ENABLE
> card to take our 11/45 out to more than 256KB.
> I found a copy of brochure which very briefly describes/shows it here:
>    http://archive.org/stream/bitsavers_ablebrochuuctSummary_2920347/Able_Computer_Product_Summary_djvu.txt
> and although the picture didn't ring any bells for me, that must be it.
> It looks like it has a UNIBUS connector (which I take it is 'UNIBUS out'), and
> also some kind of 'over-the-back' connector which I assume is the bus to the
> memory, which sounds (from a brief mention I found somewhere else) like it was
> standard Extended UNIBUS memory in a separate backplane. (Although I could be
> wrong; maybe the ENABLE plugged into the EUB backplane, and the UNIBUS
> connector is 'UNIBUS in' from the CPU?)
> I was unable to locate any real documentation for it online (maybe my
> Google-fu just isn't strong enough), but if someone has any, or can point me
> at any, I'd be grateful.
> Any if anyone actually has one, in the flesh, that would be Really Cool!

I can only offer one slightly related datapoint here. I used to play 
with an 11/34, which had an Able ENABLE product of some sort (if my 
memory for names are to be trusted) to expand memory.

That thing brought an 11/34 up to 1M of memory, essentially working the 
same way an 11/24 would work.

However, the was one board that went into the CPU box, and then there 
was a cable or two out from that to a separate memory box that held the 
memory. So the memory cards themself was not in the CPU box, and I 
actually have a hard time understanding how it would have worked in an 
11/45 as well. The machines that had more than 256K of memory, and were 
Unibus based, and had the memory on the "unibus" actually had a special 
backplane for the memory cards, where otherwise unused signals were used 
to do the extra addressing bits. Which also means that all the memory 
cards had to sit in the same backplane as the CPU. They could not be put 
in any other Unibus backplane.

The only two machines that did this was the 11/24 and 11/44.

But apart from the problem of getting four more address lines, the rest 
is pretty simple. You will get a Unibus map, for all DMA, and you'll get 
a new MMU which implements all the bits in the PAR registers. And that's it.

So, it also implies that the original MMU have to be removed. And that 
is where you put the new card in.

But I don't know anything about the 11/45 extension this way. Like I 
said, I only played with an 11/34 that had this.


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