I was thinking about classic Mac OS and the transition to PowerPC. If I understand correctly, there was residual 68k code in Mac OS 9.
I hope this question doesn’t sound too ridiculous.
Does modern MacOS (or up to what version) still have any residual code from the PowerPC days? Left over universal binaries perhaps?
The classic MacOS had residual 68K code because that code had been written in 68K assembly and, for whatever reason (if I had to guess, because it was highly optimized code, other code depended on the quirks of the particular implementation/optimization, and the people who had originally written it, e.g. Andy Hertzfeld, were long gone), not yet rewritten for PPC. That's one of the reasons the 68K emulator was embedded so deep into the classic Mac OS...
OS X (and the NeXT operating systems) is written in higher-level languages (just like Linux or NT-based Windows). When you compile for x86 or x64 or ARM or whatever target you pick, that's... what you get. And then you can combine binaries for different architectures into so-called universal binaries. (And then you have an emulator that's... much less integrated into anything... than the 68K emulator was. Which is why Rosetta/Rosetta 2 is less flexible than the 68K emulator was)
And, while Microsoft may have had good reason to loosen their emphasis on portability in the late 1990s/early 2000s when every non-x86 NT platform died, I think NeXT/Apple have learned the opposite lessons over decades - having a portable code base, having your code secretly compiled for other architectures in a secret lab, having the ability to add more architectures easily, etc gives you options that you may badly need. (Honestly, we know OS X _always_ ran on x86... and when do we think they started compiling the full thing for iPhone/iPad ARM chips? I'd guess in the early 2010s, if not earlier.
Certainly I would expect all the ARM/tvOS-based Apple TVs have always run full Mac OS X in a lab deep inside Apple somewhere but it probably started much earlier.)
