UNIBUS PDP-11 memory expander card

Johnny Billquist bqt at update.uu.se
Thu Jan 29 07:26:07 CST 2015

On 2015-01-29 02:55, John Wilson wrote:
> On Thu, Jan 29, 2015 at 01:36:07AM +0100, Johnny Billquist wrote:
>> I am rather sure the cpu was left.  But you really need additional bits in
>> the par as well as actually addressing the extended memory.  I probably
>> should try to find the documentation for exact details here then.  I might
>> be able to track down the machine, but I haven't seen it in almost 20
>> years.
> This is hurting my head.  To stick a completely outboard 22-bit MMU onto
> any Unibus machine with an 18-bit MMU, you'd need to sniff all PAR writes
> and add four bits to all PAR reads (so hopefully those accesses would appear
> fully on the Unibus even though the PARs are internal to the CPU), and then
> jump in the way of all accesses from the CPU to main memory (which might be
> OK if you cleared out the rest of the SU so it's just the CPU and the MMU
> expander and RAM, and all peripherals are beyond the expander and go through
> its Unibus map), but even then w/o the three MSBs of the VAs the MMU
> expander wouldn't know which PAR to use (and all it's seeing on the Unibus
> is the 18-big PA, which isn't necessarily unique).

The Unibus DMA is the easy problem, so we can leave that.
The interesting part is the page address table. How on earth did they do 
it? I honestly don't know at this point, but I am getting curious.
I probably did know 20 years ago, when I had the machine. I think I even 
might have had the documentation.

> I have a hazy memory that someone sold an F11-based CPU card that was a
> drop-in replacement for the 11/34a CPU and included all the memory (so
> there's no need to confuse things with Unibus DMA), and gave you something
> which looked like an 11/34a but acted like an 11/24 (so, no odd address
> traps for one thing).  Could that be what this was?  I don't remember who
> the vendor was.  Who made the PEP70?  Maybe them?

Well, Able did no such thing.

I tried to jog my memory a bit more last night, and managed to recall 
that the product name was something like Able/34. So I searched around a 
bit more today.

I did find a couple of brochures on bitsavers that mentions it. My 
memory was almost right. The product was called Enable/34, and it was 
apparently usable both on the 11/34, 11/45 and 11/60.



You even have a picture of the board in the latter one.

Also, it would appear they had two products. The Enable/34 and the 
Megabox. But some text seems to suggest that the Megabox just included 
the Enable/34 and a box with memory installed.


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