Here are the most common (needed) conf-reg settings and their effects on the router. These settings should work on any Cisco router above the 1000 series (unless indicated).
Register setting 0x2010—Configures the router to check NVRAM and boot directly into ROM monitor.
Register setting 0x8000—Configures the router (2500,4000,7x00) and loads the router into diagnostic mode. Setting 0xA042 is used on 4500 and 4700 series routers.
Register setting 0x2101—Loads the IOS from ROM, and the NVRAM configuration is loaded into RAM as the running configuration. This is useful for performing IOS upgrades or if the flash IOS image is corrupted. Not all routers have a ROM-based IOS to boot from, so this option only works on ...