ijor wrote:But I wasn't talking about that. What I meant is that an FPGA is not an ASIC. If you implement, say, a NOR on an FPGA, you are not actually producing a real NOR gate. You will use a table based LUT that, at the transistor level, it's completely different than a real NOR gate. So some might argue that an FPGA is some sort of hardware simulator. You might say that you don't care, the functional behavior is exactly the same. But that's exactly my point, what really matters is accuracy. And you can get, in theory, the same accuracy using plain software if you want.
I agree that the only way to make an exact replica, is to get the original chip masks, go to the foundry and get it done again. There is no other way, not even in an ASIC as modern digital technologies are plain different from those of the 80's. We do not do NMOS logic, we do not do hand layouts, we have on chip scan tests, and a lot of other different things.
So it is fair to say that when you use a different technology you will end up emulating part of the original behaviour in a different fashion. But the distinct point different from software emulation is clock cycle accuracy and electric equivalence. My aim when I do sound synthesizers is to have a core that could actually replace the original part on an original PCB. Could you do that with software? In some cases you can, in others you cannot. Not just because of pure CPU power needed, but also for the need of unusual frequencies either for video or sound. Yamaha chips are usually operated at funny frequencies like 55kHz, which are not the standard 44.1 or 48kHz of today's audio. Converting from one sample rate to another is not a trivial matter and introduces lag and artifacts. Ghosts'n Goblins arcade board has a funny vertical frequency too, not quite 60Hz. To name two examples.
The opposite case are FPGA cores that are really just emulating the system in the sense that they do not follow original timings at all, be it for lack of information or for lack of consciousness. I hear you have made an accurate Atari ST core. Impressive! Is that the one in the MiST repository?