Field notes on Mac OS 9.2, New World ROMs, and Apple Silicon

SheepShaver is a legendary Mac OS emulator (fast, compatible, beloved), but it cannot boot Mac OS 9.2. Iirc, you can coax it to run 9.2 through hacks, but you still won't get all the features. I really wanted SheepShaver + 9.2 back in the early 2000s. Twenty-five years on, it is still a nope. You can use QEMU (on Apple Silicon, UTM is the best practical option for 9.2.2 today), but that is not SheepShaver, and you can feel it when using it. What I still want is SheepShaver's particular trick running classic Mac OS: a thin emulation layer with a JIT. The JIT was only part of why it is fast, translating PowerPC instructions to native at runtime, but the bigger idea SheepShaver offered was what it declines to emulate. Rather than simulating a whole Macintosh, SheepShaver emulates the PowerPC CPU and patches the Mac ROM to route everything else (I/O, memory, threads) straight to the host. Guest memory is just a region of the host's address space at a fixed offset, so a memory access is…