MEM11 Status Update

Noel Chiappa jnc at
Tue Jan 20 10:24:06 CST 2015

    > From: Jay West

    > My hope is to drop one of these boards in my 11/45. The only issue is
    > I'm afraid even though it will give the /45 maximum memory

Well, there is that board (by Able?) that allowed a /45 to have up to 2MB of
main memory. I know they exist[ed], we had one on our 11/45 at MIT 'BITD'.

It didn't require any mods, it was a simple plugin - although my memory
doesn't recall how the bus from the board to the memory worked, whether it
used an over-the-back connector, or if the expansion board, and memory,
plugged into a custom backplane (I have this vague memory that it could use
Extended UNIBUS memory, a la 11/44, so maybe it was a custom backplane).

I do recall that it had a whole separate set of mapping registers, to map the
entire 'outbound' (from the CPU) UNIBUS to the larger memory, and also a
UNIBUS map for the 'inbound' UNIBUS (from the devices) to map into the memory
- i.e., it did not have access to the CPU mode/space signals, so it didn't
have K/S/I PARs. One had to set up the mapping registers in the 11 as well.

We used a static mapping where User I and D went to fixed chunks of address
space in the 'outbound' UNIBUS, and the expansion MMU then mapped those
chunks (variably) into the larger main memory. IIRC, we just changed the
definition of the address of the User PAR registers (but kept the User PDRs),
and the UNIBUS map registers (although maybe those were at the same locations
as those in the 11/70), and then re-compiled the affected kernel modules.
(The assembler startup required a bunch of changes, too.) I should have all
that code on my backup tapes...

That might not have supported I+D for K, S and I (since together they need
128KB*3 bytes, more than a complete UNIBUS address space), but it worked OK
for the early Unix we were running.

    > I think I have too many pdp11 racks and it could be time to thin the
    > herd before too long. Or maybe I just need to get more space. Yah,
    > that's the ticket!

Please put me on the list for an -11 stuff you get rid of... ;-)


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