The boot system command specify the name and location of the IOS image to load.
Router (config) # boot system flash [filename]
Tells the router to boot using the IOS which is located in flash memory.
Router (config) # boot system rom
Tells the router to look for IOS in ROM
Router (config) # boot system tftp [filename] [dirección_servidor]
Tells the router to boot the image has to load IOS from a TFTP server.