|  | 7.1 | The Basics of IOS Versions
| 7.1.2 | Configuration register values
The order in which the router looks for system bootstrap information depends on the boot field setting in the configuration register. You can change the default configuration register setting with the global configuration-mode command config-register. Use a hexadecimal number as the argument for this command. In this example, the configuration register is set so that the router will examine the startup file in NVRAM for boot system options. The configuration register is a 16-bit register in NVRAM. The lowest 4 bits of the configuration register (bits 3, 2, 1, and 0) form the boot field. To change the boot field and leave all other bits set at their original values (initially the config register contains 0x010x), follow these guidelines: Set the configuration register value to 0x100 if you need to enter the ROM monitor (primarily a programmer's environment). From ROM monitor, boot the operating system manually by using the b command at the ROM monitor prompt. (This value sets the boot field bits to 0-0-0-0.) Set the configuration register to 0x101 to configure the system to boot automatically from ROM. (This value sets the boot field bits to 0-0-0-1.) Set the configuration register to any value from 0x102 to 0x10F to configure the system to use the boot system commands in NVRAM. This is the default. (These values set the boot field bits to 0-0-1-0 through 1-1-1-1.) To check the boot field setting, and to verify the config-register command, use the show version command.