Strange third party board in PDP-11/45

Noel Chiappa jnc at
Fri Jul 27 08:06:09 CDT 2018

    > As to what _else_ it is doing, and why it has the cable to the main
    > card... I think that it must intercept MSYN from the processor and only
    > let it pass if there's no hit in the cache.
    > (To explain why it would need to do that... normally with the MS11,
    > there's a static partitioning between FastBus memory and UNIBUS A
    > memory. So when the CPU goes to do a memory cycle, it can put the
    > address out on both the UNIBUS and FastBus, with the certainty that it
    > will only get a reply on one.

So, my guess was wrong there. I had _assumed_ that when doing a memory
operation, the CPU started a cycle on both busses, to minimize the delay on
the UNIBUS cycle if the MS11 on the FastBus didn't have that location. (I had
previously checked, and there is an 'I have that location' signal which the
MS11 sends back, making the concept of a cache possible.)

Well, no. In the "KB11-A CPU Maintenance Manual" (DEC-11-HKBB-D), section
7.7.1 ("Bus Control Introduction"), it says:

  If the address applies to a Fastbus device, that device will respond in time
  to inhibit the UNIBUS MSYN signal

(It turns out the KB11-A is very clever; it has to delay for a bit after
asserting the address, etc, signals on the UNIBUS, for de-skew, before it can
assert MSYN, and during that delay, in parallel, it checks to see if the MS11
has that location.)

So my theory about what that double-card does (prevent an MSYN sent to main
memory on a cache hit) is apparently wrong.

Which leaves the question of what that card _does_ do...

The cache _does_ need to have the A and B UNIBUS tied together, so that it can
snoop UNIBUS memory cycles (which are on the A UNIBUS) via the B UNIBUS (which
is what it has access to on the backplane), _but_ it doesn't need a special
card, with i) active logic, and b) a cable back to the main board, to do that
- a regular M9200 will do that.

The cable back to the main card, and the logic on the two dual-width cards,
mean it wants to interfere somehow in the connection between the A and B
UNIBUS. But if not the MSYN thing, what else could it be?  I have pondered
that question, but I can't think of anything.

It can't be doing anything with interrupts or DMA, I would think, so no grant
interception. So what else could it be? Anyone have any ideas?


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