VAX-11/750 memory in 11/70 ?

Perhaps the Chapters 9/10/11 on Cache, MMU and Unibus Map in KA-K1170-MG-003 are what you're thinking of (somehow ... :->)? http://bitsavers.trailing-edge.com/pdf/dec/pdp11/1170/ KA-K1170-MG-003_1170_Maintenance_Service_Guide_Apr88.pdf

On Wed, Mar 11, 2015 at 11:39 AM, Johnny Billquist <bqt at update.uu.se> wrote: On 2015-03-11 00:11, Ethan Dicks wrote: > > On Tue, Mar 10, 2015 at 7:06 PM, Johnny Billquist <bqt at update.uu.se> >> 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 > > >