6 Dec
2016
6 Dec
'16
4:13 p.m.
On Dec 6, 2016, at 2:56 PM, Fritz Mueller <fritzm
at fritzm.org> wrote:
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.
paul
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? :-)