|  | 7.1 | The Basics of IOS Versions
| 7.1.1 | Locating the Cisco IOS software
The default source for Cisco IOS software startup depends on the hardware platform, but most commonly the router looks to the boot system commands saved in NVRAM. Cisco IOS software does, however, allow you to use several alternatives. You can specify other sources for the router to look for software, or the router can use its own fallback sequence, as necessary, to load the software. The settings in the configuration register enable the following alternatives: You can specify global configuration-mode boot system commands to enter fallback sources for the router to use in sequence. Save these statements in NVRAM to use during the next startup with the command copy running-config startup-config. The router will use these commands as needed, in sequence, when it restarts. If NVRAM lacks boot system commands that the router can use, the system has its own fallback alternatives. It can use default Cisco IOS in Flash memory. If Flash memory is empty, the router can try its next TFTP alternative. The router uses the configuration register value to form a filename from which to boot a default system image stored on a network server.