> it's not unheard of to emulate a legacy/antique CPU on a newer one to make use of proven code.
It's probably less to do with using proven code and more to do with maintaining perfect bug-for-bug compatibility. You can't even reimplement the bugs you haven't discovered yet. Also, writing an interpreter for most microcode is fairly methodical. Depending on what level of detail you need to emulate (clock timings, interrupts etc) it could be quicker and simpler than digging up an old fossil of a developer who knows enough about the original software to port it for you.
It's probably less to do with using proven code and more to do with maintaining perfect bug-for-bug compatibility. You can't even reimplement the bugs you haven't discovered yet. Also, writing an interpreter for most microcode is fairly methodical. Depending on what level of detail you need to emulate (clock timings, interrupts etc) it could be quicker and simpler than digging up an old fossil of a developer who knows enough about the original software to port it for you.