Sorry for the late reply, I just saw your message. I don't have a toy OS (not enough spare time), but I did work on one as part of my job.
I don't know of any documents, but the way you'd generally want to do it is wrap hardware- and board-specific parts of your code in a hardware-agnostic interface which is what you use to write higher-level code in the system. That way, getting the OS to run on another CPU or board is all about writing the relevant callbacks. You can get the general idea from Linux's mach-* and plat-* files. This isn't really OS-specific; I'm pointing at Linux because it will probably be easiest to get documentation about it, though my personal preference is elsewhere.
I don't know of any documents, but the way you'd generally want to do it is wrap hardware- and board-specific parts of your code in a hardware-agnostic interface which is what you use to write higher-level code in the system. That way, getting the OS to run on another CPU or board is all about writing the relevant callbacks. You can get the general idea from Linux's mach-* and plat-* files. This isn't really OS-specific; I'm pointing at Linux because it will probably be easiest to get documentation about it, though my personal preference is elsewhere.