Explanation/Reference:
Explanation:
The lowest four bits of the configuration register (bits 3, 2, 1, and 0) form the boot field. The boot field determines if the router boots manually, from ROM, or from Flash or the network. To change the boot field value and leave all other bits set to their default values, follow these guidelines:
If you set the configuration register boot field value to 0x0, you must boot the operating system

manually with the boot command.
If you set the configuration register boot field value to 0x1, the router boots using the default ROM

software.
If you set the configuration register boot field to any value from 0x2 to 0xF, the router uses the boot

field value to form a default boot filename for booting from a network server. For more information about the configuration register bit settings and default filenames, refer to the appropriate router hardware installation guide.
ReferencE.
http://www.cisco.com/c/en/us/td/docs/ios/12_2/configfun/command/reference/ffun_r/frf010.html