VAX-11/750 memory in 11/70 ?

On 2015-03-11 00:11, Ethan Dicks wrote: 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