The 6502 is a CPU, not a full machine - and the OS really is about interacting with the whole machine. You need to learn about a specific 6502-based system - th

The 6502 is a CPU, not a full machine - and the OS really is about interacting with the whole machine. You need to learn about a specific 6502-based system - the C64, the Apple II, the NES... Moreover, usually when you’re writing a 6502 program, you’re writing a program targeted towards an individual machine, with no OS in the way - the program you’re writing is written against a particular machine; it effectively is the OS. So the answer is: learn how to write programs for a 6502-based machine first. Then, make your program an OS. Your ability to make it anything like a modern OS is going to be limited, though.