I've been told, can't prove it, though, that the NEC version of the Z-80 didn't have that "bug" (short for undocumented feature, as in "one man's feature is another man's creature") that in certain instances the flags were incorrectly set by a block load instruction or some such. It's little things like that that can throw folks off. That, actually, is why I favor a simulator. I've just never seen a proper simulator for either processor in an "open" environment, i.e. where only the processor is simulated. That makes it entirely hardware independent. Some folks would believe, however, that since it's not possible to build a system that's hardware independent, it's not valid to simulate one. Do we really want to build hardware for the sake of this comparison? Writing a bare-bones simulator would be straightforward enough. It's really just a big switch statement. The beauty is that you can include/exclude undocumented features as you see fit. The gotcha is that it's easy to go down a road which has no relevance to reality, i.e. if the processor doesn't work like that, even though it should, then simulating it like that is not valid. Dick -----Original Message----- From: Allison J Parent <allisonp(a)world.std.com> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Saturday, April 17, 1999 9:20 AM Subject: Re: z80 timing... 6502 timing t