[HECnet] PDXVAX - 40MB Memory - VAXstation 4000/VLC?

[Johnny Billquist]

Pontus Pihlgren wrote:
Makes you wish you had a real PDP-11/74 around... :-)
Hmm, we have several 11/70 you know. Could we build one? Or would we have to rebuild the whole cache/memory tingies?

Hehe. Yes, we have three PDP-11/70 machines.
But sadly, no. It is not an easy task. The MMU needs modifications to implement the cache bypass bit in the PDR, which a normal PDP-11/70 don't have. Then we need a modification to the core instruction set of the machine, as the ASRB instruction in an 11/74 always bypass the cache unconditionally.
And then we need a different type of memory boxes, that accept cables from up to four CPUs into the memory box, do the arbitration between requests from several CPUs, and also support a read-modify-write cycle interlocked.
And then we need a new piece of hardware called the IIST (Interprocessor Interrupt and Sanity Timer), which is what the CPUs use to interrupt each other, boot each other, and also make sure that a CPU isn't hung.

So, while possible in theory, it would require quite a lot of engineering to actually produce an 11/74 from a few 11/70 machines.

But all the pieces are documented, and the documents are available on the internet...

	Johnny