Configuration Register.

Values:
Default Value :  0x2102.


0x2102 through 0x210F : Try each boot system command in the 
                                       startup config file,if none work load the 
                                       first valid IOS file in Flash.

0x2101 : Always Boot the first valid IOS file in Flash.
               Last Hex 1 indicates bypass looking in NVRAM for 
               any boot system commands, instead Boot first IOS  
               found in Flash.   

0x2100 : Always boot to ROMmon.

0x2142 boots from flash without using NVRAM contents good for password recovery.


All config registers above and 0x2142 Boots into ROM(mini-ios) if initial boot fails


To enter ROMmon and reset password:
CTRL+BREAK
confreg 0x2142  (at ROMmon prompt) .
reset (command).
startup config file in NVRAM is ignored.
create new passwords.
config-register 0x2102 (set back to default value)
reload (command).

Parts:
Flash : IOS.
NVRAM:Startup file.
ROM : POST,bootstrap, ROMmon, and on older  routers rxboot microcode.
RAM.

ROMmon runs mini-ios  when IOS in Flash fails to load called RXBOOT or bootloader.



show version  > to view config register setting