When the first 64 bit Macs shipped none of the software was 64 bit clean. Even if the kernel source code had been 64 bit clean, it would have broken support for

When the first 64 bit Macs shipped none of the software was 64 bit clean. Even if the kernel source code had been 64 bit clean, it would have broken support for all the the 3rd party 32 bit drivers that existed. So the kernel switched itself in 32 bit virtual mode, though some parts of the pmap layer understood 64 bit page tables in order to manage the 64 bit physical address space. While the kernel could not see all of memory, it did not need to, it could map any apertures it needed in by manipulating its own page tables. At that point it is pretty trivial to setup a 64 bit address space for a process. The kernel could not directly map all of virtual address range of the process at once, but it already had the pmap and aperture management code to deal with that. That decoupled 64 bit user space bringup from the 64 bit kernel port. The first release that supported 64 bit processes did not have 64 bit UI frameworks, just the C library and some low level posix functionality, but it allowed scientific code that benefited from more memory to run.