VAX-11/750 memory in 11/70 ?

Paul Birkel pbirkel at
Wed Mar 11 10:53:57 CDT 2015

Perhaps the Chapters 9/10/11 on Cache, MMU and Unibus Map in
KA-K1170-MG-003 are what you're thinking of (somehow ... :->)?

On Wed, Mar 11, 2015 at 11:39 AM, Johnny Billquist <bqt at> wrote:

> On 2015-03-11 00:11, Ethan Dicks wrote:
>> On Tue, Mar 10, 2015 at 7:06 PM, Johnny Billquist <bqt at>
>> wrote:
>>> Which one of these, if any, could be used in an 11/70?
>>>> And if you absolutely want to use 1M cards in an 11/70, ping me, and
>>> I'll
>>> walk you through the howto. (Update have done this in the past. It is not
>>> trivial, but it is possible.)
>> I have a crate of 256MB RAM on my 11/70s but I'd love to learn what it
>> takes to use 1MB boards (which I have a few of from 11/750s and
>> 11/730s and 11/725s).  I might decide it's too much trouble, but I'd
>> to know what's involved.
> 256K, not 256M, but anyway... :-)
> There are several bits to it.
> The MK11 only have 18 address pins on the backplane, which means it cannot
> directly address larger boards. In addition, there are 16 card select pins.
> One for each slot. 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.
> The way you get 1M cards to work is that you need to tie together 4 card
> select lines, so that your 1M card would appear to be four 256K cards in
> the MK11.
> In addition, you also need to code back from the four select lines into
> the two additional address lines.
> This is not so hard. Essentially we're talking about a OR of 4 lines, and
> a 4-2 encoder.
> 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.
> So far for the hardware.
> After you've done this, you then also needs to fix a software problem.
> Like I said at the start, the MK11 initialize all the ECC of all the cards,
> at initialization time. And it does this in parallel for all cards. That
> means all card select lines are active together at initialization. And then
> the address lines runs through address 0-256K doing writes.
> This means that the ECC for 3/4 of a 1M card will not be initialized. The
> 1/4 that do get initialized, you want to be the low addresses, since those
> you need before you can really do anything more. Having memory errors for
> address 0 is really bad.
> Once you've come this far, you need to write a short program that will
> initialize the ECC of the rest of your memory. This can be done through
> software, but it requires some tricks. Normal writes of memory expects the
> ECC to already be correct, so you need to turn off ECC checking in the MK11
> before writing to the uninitialized memory. You can turn off ECC checking
> in the MK11 by changing a CSR register, where you can set the operation of
> the MK11.
> However, there is a problem here. The CSR is in the MK11 box. But the MK11
> box is on the memory bus, and not on the Unibus. And the CSR address is in
> I/O space.
> The trick for this is to use the MMU and the Unibus map to get access to
> the I/O space on the memory bus.
> How this actually is done is hidden deep inside one of the technical
> manuals of the 11/70, and in a rather weird form.
> I can't seem to find that manual online. It's something like "11/70 memory
> subsystem manunal" or something similar. I (Update) have a hardcopy of it
> somewhere.
> But the theory of it is not so hard. You access an address in the Unibus
> memory address space, using the MMU. The Unibus map then remaps this
> address into an address in the I/O space. All access of memory through the
> Unibus map will go to the memory bus.
>         Johnny

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