Double Buffer RK11-C

[Fritz Mueller]

> On Dec 6, 2016, at 7:51 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:
> 
> [data fetch] can't be off-loaded onto a separate interface unit, as it needs access to
> register contents held in the CPU.

Yeah, it’s pretty interesting!  My guess would be that it was a separate register/command oriented interface, sitting on the Unibus, and didn’t actually interface directly with the 11/20 CPU?  Such an interface could limit the instructions “fed” to the FPU to those accessing its internal registers, etc.  But who knows? :-)

I’ve gotten quite deep into the design of the FP11-B and associated KB11-A interfacing during my debug (which is how I noticed all the 11/20 refs in the docs, circuitry, and microcode), but I’m pretty ignorant of the 11/20 having never worked on one.

> I wouldn't be surprised if there's some microcode in the KB11 to support those memory operations.

Yes, there certainly is — quite a bit of it actually.  The are F/CLASS branches off all three of the A, B, and C forks.

	—FritzM.