Processor interacts with RAM mostly but at starting ROM takes over the charge. Processor → ROM (POST, Setup, System)
Function of Power-On Self-Test (POST) is to scan for hardware changes and see added or removed components and mentioning those I/O devices details functioning well. If any component is not functioning then it returns an error which can be of two types – fatal and non-fatal. Fatal errors are major and booting can’t continue without rectifying whereas non-fatal can be neglected for booting process to continue. Setup decides boot priority and hierarchy of boot devices from which the operating system is to be loaded. The main step in System is run the small program called bootstrap loader or boot loader stored on ROM which loads data and programs.
Then processor cuts interaction with ROM and starts interaction with RAM. Operating System takes 96 MB of RAM space. From RAM 8 MB is taken by default at starting. Master Boot Record (MBR) stores system files into RAM. As soon as MBR stores OS files into RAM, RAM is interacted with Processor. Read More on Booting from Wikipedia