Re: COSMAC variants
I didn't comment on this before because I'm rather new to the 1802 and others are MUCH more qualified, but...
From my reading of the datasheets I concluded also that the "C" versions were rated only to 6 ish volts and the limited speed associated with that.
As for the "X" I have seen references to that (Wikipedia, maybe?) but I'm not certain.
Can't comment on this -- I have two "ACE" chips not yet used.