sexy PDP-11/44 panel

Don North ak6dn at
Fri Feb 24 14:26:09 CST 2006

Brad Parker wrote:
> Don North wrote:
>> The only problem with SPC slots is they are 18b UNIBUS only. This will 
>> work on 256KB 18b machines, but is a sticky problem on 4MB 22b machines, 
>> since you need to go thru the UNIBUS map to get to all of memory.
>> For an 11/44, you could do just a dual slot card that goes in a memory 
>> slot (which has 22b address available) and an SPC slot (to get the DMA 
>> lines) but this would be a hex card. To read/write memory, you would 
>> need to be a 22b DMA device, which I'm not sure the 11/44 can handle 
>> (ie, the CPU may always drive the upper 4 address lines and not provide 
>> for 22b DMA device). It may work, it may not. Certainly not what DEC 
>> designed for.
> You scared me there for a second.  There are no 22 bit unibus dma
> devices, right?  only 18 address bits on unibus, right?  even on an
> 11/44, right?

That is correct UNIBUS I/O is 18b only, even on all the VAXes that used 
UNIBUS. There is no such thing as 22b UNIBUS DMA; sorry to give that 
impression. I was just thinking out loud :-)

Besides UNIBUS proper (ie, the cable, or the in/out slots of a UNIBUS 
backplane) systems also implemented SPC slots (UNIBUS + other stuff) but 
these are 18b only as well.

There are also MUD (Modified UNIBUS Device) slots which are typically 
used by non-DMA memory cards. These are only 18b as well.

For 22b memory support, the 11/44 (and 11/24 iirc) implemented four EUD 
(Extended UNIBUS Device) slots that are specific for memory cards only 
(ie, the MS11-PB 1MB memory card). These provide the extra 4b from the 
UNIBUS map to allow a full 22b physcial memory address.

QBUS was originally 18b only as well, but was extended to Q22 to solve 
this problem in a much more reasonable way (the UNIBUS map was/is a big 
hack, but probably the only reasonable approach given the number of 
preexisting 18b interfaces at the time of the 11/70s design).

If Guy really can implement a 22b examine/deposit physical memory in a 
pure SPC slot kudos to him. The only way I can think of possibly doing 
it (thinking out loud again :-) ) is to steal a UNIBUS map entry and on 
each exam/deposit set it up correctly to map the 18b unibus address to 
the desired 22b address. I don't recall if the UNIBUS map is accessible 
from the UNIBUS itself (if it is, this could work; if not, then it won't).

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