Bug-for-bug compatibility [was RE: SimH DECtape vs. Tops-10 [was RE: Writing emulators [Was: Re: VCF PNW 2018: Pictures!]]]
dab at froghouse.org
Wed Feb 28 12:10:32 CST 2018
> Imagine our chagrin when days of trying to correct the
> problem led to the conclusion that the diagnostic was incorrect.
I may have a situation like this in working on my FPGA PDP-10. The
Processor Reference Manuals seem quite clear that the rotate
instructions take E mod 256. One of the manuals I've found even adds
that they never move more than 255 positions. And yet the diagnostics I
have clearly want ROT AC,-256 to move 256 positions to the right, not
0. Not having a real PDP-10 to compare against, I don't know which is
Doing it mod 256 would be easier; I had to add an extra rotor to my
barrel shifter to handle the -256 case to make the diagnostic pass.
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