The best organized and most straightforward part of it is the ROM code. If somebody wanted to build a custom ROM, I think that would not be too difficult. I have been working on thoroughly understanding the boot sequence. The boot code is all there. The process manager is all there as far as I can tell. And the memory manager too. Making the memory manager reentrant was a big obstacle to making the macintosh operating system able to do preemptive multitasking. It would be fun to try to make a reentrant memory manager for System 7.

A nice feature of the old Mac OS was its modular construction. All the little pieces were put into resources and tucked into the system file. We can alter just one of those resources, put it back into a system file, and it should work.