The original 68k (used in the original 1984 Mac) was funky: it had a fully 32-bit instruction set and 32-bit registers, but 16-bit internal and external data b

The original 68k (used in the original 1984 Mac) was funky: it had a fully 32-bit instruction set and 32-bit registers, but 16-bit internal and external data buses and a 16-bit ALU. And a 24-bit address bus, because reasons? Later variants of the 68k family were fully 32-bit internally, but exactly what was exposed externally varied by processor and Mac model. And 24-bit addressing remained a thing in the Macintosh system software up until System 7-- this capped RAM at 8 MB even if the CPU could handle more. But Apple never really had the same sort of 16-bit/32-bit transition that Microsoft and Intel did. The 68k-to-PPC shift is kind of analogous (and happened at around the same time as Microsoft was going fully 32-bit), but backwards compatibility there was purely based around software emulation-- same basic approach as Apple's PPC-to-x86 and x64-to-ARM transitions, as opposed to Intel's "just make the new processor a superset of the old one" approach.