ROM
Read Only Memory stores the initial bootstrap program that initializes the router's basic hardware components.
ROM is not modified during normal operations, but it can be upgraded with special plug-in chips.
The content of ROM is maintained even when the router is rebooted.
In the event of a boot failure you can recover the system by booting into ROM monitor mode which has a small version of the IOS.
You know that you are in ROM Monitor mode when the prompt is just a greater than sign (>).