UNIBUS PDP-11 memory expander card

John Wilson wilson at dbit.com
Wed Jan 28 19:55:09 CST 2015

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

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).

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?

John Wilson
D Bit

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