Double Buffer RK11-C

Paul Koning paulkoning at
Tue Dec 6 15:13:43 CST 2016

> On Dec 6, 2016, at 2:56 PM, Fritz Mueller <fritzm at> wrote:
>> On Dec 6, 2016, at 7:51 AM, Noel Chiappa <jnc at> 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 don't know anything of a DEC product along those lines, but a college classmate of mine (Bill Black, Lawrence Univ. class of 1975) built a floating point coprocessor for our PDP11/20 that was a Unibus peripheral.  I helped with the software interface.  The device had 4 registers, two for source and two for second source and result.  They appeared at several different bus addresses; you'd select the operation to perform based on which address you used.  The device would start when the 4 source words had been loaded, then a read cycle of the result register would simply be held off until the operation was done (since it would complete well within the SSYNC timeout).

The implementation took, if I remember right, one hex-sized wire wrap board.


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