VAX-11/750 memory in 11/70 ?
ethan.dicks at gmail.com
Wed Mar 11 12:10:51 CDT 2015
On Wed, Mar 11, 2015 at 11:39 AM, Johnny Billquist <bqt at update.uu.se> 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
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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