The version is probably the least important information as you want your OS to run on any version of the processor.
I mean that you have to understand the way that the processor works, the registers it contains, and all the instructions that it obeys. For example, with an x86 processor you need to understand the state it is in when it starts up and how to move it from real mode to protected mode and then (possibly) to 64-bit long mode. To do this you will need to understand how the processor uses the segment registers to address memory and how it translates virtual addressed to physical addresses via the paging mechanism. That's just for starters.
The processors are very well documented by the manufactures (the Intel Programmer's Manual, which is a free download explaining every aspect of how the processor works, comes as a 5-volume manual), the support chips less so. And some peripherals are more difficult to control than others - text display, keyboard input, hard disk support are easy; graphic display more difficult, and USB devices difficult even for the most expert programmers.
Basically, OS programming is about the most difficult development work that you can undertake. But if you are already skilled in assembly language and a higher-level language such as C it can be a very rewarding hobby.