Next, the switch loads the boot loader software. The boot loader is a small program stored in ROM and is run immediately after POST successfully completes.
The boot loader performs low-level CPU initialisation. It initializes the CPU registers, which control where physical memory is mapped, the quantity of memory, and its speed.
The boot loader initializes the flash file system on the system board.
Finally, the boot loader locates and loads a default IOS operating system software image into memory and hands control of the switch over to the IOS.