whilst reading my 11/45 processor manual the other night, I came across the statement that, if Unibus A and B are separated, Unibus B can be used for inter-processor links, so long as one of the connected devices is a Unibus controller.

Tom Uban wrote:

> The 11/45 processor handbook talks about the utility of a multiprocessor > system, considering the dual UNIBUS architecture and fast/slow memory > scheme.

> I know that many discussions have gone by about a multiprocessor 11/70, > but did DEC ever make (internally?), produce, or sell a multiprocessor > 11/45 system?

> Did anyone in put a pair (or more) of 11/45's together on their own?

I had an 11/45 with a 11/20 front end, but it gets tricky. You can only separate the two Unibuses (unbusi?) if you have fastbus memory. The controllers were dual ported, one to unibus B and the other a direct path to the processor. Unibus A was always used by the processor, and had the bus arbitration logic. Unibus B had no such logic, and was used for DMA transfers from peripherals to the fastbus memory.

If you separate the buses (just remove a jumper), and run a second processor on Unibus B there is a problem. The second processor and its peripherals have full access to the fastbus memory (only), but the peripherals on the 11/45 had no access.

In my case using an 11/20 (which doesn't have memory management) the fastbus memory has to be strapped into the first 56Kb of memory. The DMA devices on the 11/45 couldn't have access to this memory, so I had to write a special bootstrap loader that buffered data in normal memory, then transferred it to the fastbus segment.

A different hardware solution was the 'Unibus Window', where you could transparently map chunks of memory (or peripherals) between to unibus machines.