28 Feb
2018
28 Feb
'18
10:10 a.m.
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
right.
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.