bootup process

Boot Sequence of a Cisco switch

The switch loads the boot loading software from NVRAM.
The boot loader:
Performs low-level CPU initialization.
 
Performs POST (power-on self test) for the CPU subsystem.
 
Initializes the flash file system on the system board.
 
Loads a default OS image into memory and boots the switch.
 
The OS runs using config.text, stored on the switch flash.

The boot loader can help recover from an OS crash:

provides access into the switch if OS has problems.
provides access to the files stored on flash before the OS is loaded.
use the boot loader command line to perform recovery operations.

To modify where to load IOS: startup configuration or configuration register.