a processor is by default will fetch an 'OS origin address' to 'Counter OS program' in any 'bootable device' as set in BIOS in 'flash ROM' on mainboard and copy all the related and paramount OS data to the RAM at boot up. so the only important for a complete OS is the :
2. related bootable devices (with original OS)
note : for certain conditions if different bootable devices with not the original OS installed it still can be boot up.
any other component is free to be change/swap/upgrade/downgrade as long as these two is secured