VAX-11/750 memory in 11/70 ?

Ethan Dicks ethan.dicks at
Wed Mar 11 12:10:51 CDT 2015

On Wed, Mar 11, 2015 at 11:39 AM, Johnny Billquist <bqt at> wrote:
>>> And if you absolutely want to use 1M cards in an 11/70, ping me
> 256K, not 256M, but anyway... :-)

Ah yes, as you say.  256K.

> There are several bits to it.
> The MK11 only have 18 address pins on the backplane, which means it cannot
> directly address larger boards.

Right.  That's why you have to add wires when upgrading an older
11/750.  Just putting in new boards isn't enough.

> In addition, there are 16 card select pins.  One for each slot.

Sure.... makes sense.

> At power on, the memory controller in the MK11 writes to
> all cards in parallel, in order to initialize the ECC bits on all cards.

Ah ha!  That's the not-obvious trick.

> This is not so hard. Essentially we're talking about a OR of 4 lines, and a
> 4-2 encoder.

Sure.  This part makes sense to reverse the "fan-out" for the per-slot select.

> Important additional detail is that you *really* want to make sure that when
> all cards are select together, that is equivalent to the lowest addressed
> cards.

This totally makes sense in light of the initialization phase.

> This means that the ECC for 3/4 of a 1M card will not be initialized.
> Once you've come this far, you need to write a short program that will
> initialize the ECC of the rest of your memory... You can turn off ECC
> checking in the MK11 by changing a CSR register... on the memory bus,
> and not on the Unibus. And the CSR address is in I/O space...

Thank you for a very good explanation of the pitfalls.  So I take it
that refresh isn't an issue then?

I'm not likely to try this since I do have enough 256K boards for my
11/70s (and I have plenty of places to stick 1M boards in VAXen), but
it's interesting to see what it takes.



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