BTW, don't get me wrong about anything, your technical explanation about the deep details of how the System boots on an old Mac is fascinating. I've always been too intimidated by the nature of the Mac Toolbox to ever do any programming for it (beyond fooling around with a few "Hello World" level programs in MPW, which doesn't really count) so I only know the broad strokes.
(The Linux/BSD mailing list traffic and kernel documentation repositories are of course rife with complaints about how hard ancient Mac hardware is to deal with if you *do* have to bang on it at the register level. Just booting an old Mac from bare metal into an alternative OS without a Mac System file was the holy grail for the longest time.)
It does just really underscore how much difference there is between a 68k Mac and a PC. A PC is for all practical purposes a blank slate, particularly when discussing protected mode operating systems, which is why to this day x86 computers still preserve some fossils of compatibility with laughably ancient hardware. (Heck, even in real mode many if not most DOS programs ignored the BIOS I/O routines for the display hardware in order to get better performance, so video cards *still* need to either directly offer or use tricks to trap and emulate the original VGA register set and memory layout.) An old Mac is completely inside-out compared to that.
(The Linux/BSD mailing list traffic and kernel documentation repositories are of course rife with complaints about how hard ancient Mac hardware is to deal with if you *do* have to bang on it at the register level. Just booting an old Mac from bare metal into an alternative OS without a Mac System file was the holy grail for the longest time.)