Booting from ROM
If the IOS is configured to load from a TFTP server or from flash but an IOS can not be found in either place, the router will load the IOS from ROM.
You can also configure the router to load the IOS from ROM at the start.
If you modify the 16-bit configuration register in NVRAM using the config-register command, you can force your router to boot from ROM.
You must be in global configuration mode to configure the registers.