What I miss most about the Commodore 64 and 128 is their memory maps where everything lives at a familiar known address. Exploring those memory maps was greatly aided by a number of books with names like "Mapping the Commodore 128", "Commodore 64 Internals", and Commodore's immensely detailed Programmers Reference Guides. That kind of in-depth information continues to bear fruit. People in the cbm forums are still developing and using new software and award-winning emulations of those old 8-bit micros. Contrast that with the deathly pall of silence enveloping Apple's soon-to-be vintage PowerPC Macintosh line. Unless something is done, it will soon be as if the powerpc Mac never existed.
I have decided to start exploring my more modern but inevitably obsolescent Apple eMac. Initially I just wanted to learn its cpu's powerpc assembly language as a way to avoid getting diverted into doing things the Intel way. I soon discovered that literature helpful to ppc beginners is very sparse, with gaps which require pre-existing knowledge of other modern cpus and assembly languages. Eventually I resorted to writing my own assembler which runs natively on New World ppc Macintosh firmware. I had to divert into teaching myself forth in order to do so.
I hope to follow on by doing for the Open Firmware boot monitor what others did for the 8-bit Commodore ROMs all those years ago. The available documentation of Macintosh hardware is even more appallingly sparse than it was for their cpus, and the inner Mac is proving a harder nut to crack. However, I hope eventually to provide some of that missing documentation, initially by "Mapping the eMac". I chose to stick with the eMac because it is a sealed unit with no expansion slots for diverting attention away from my immediate goal. The end result should be applicable to any G4 Macintosh.
I welcome all the help I can get. That is why I am here at superuser.com :-)