The 6502 is a very simple processor, it has only two registers, the accumulator and the program counter. The accumulator is used to store the result of arithmetic and logical operations, and the program counter is used to keep track of the current location in the program. It also can address 64 kilobytes of memory, which is also used to address the ROM, RAM, and I/O devices. This makes it ideal for writing an emulator and learning how to use it. The Z80 and Intel 8080 are more complex and require a lot of concepts to understand that are irrelevant for this tutorial series.