Difference between System Bootstrap and Cisco IOS

I'm new to the Cisco stuff and I'm just playing around with routers such, and am curious what's the difference between the System Bootstrap and the Cisco IOS?

router#show ver
Cisco Internetwork Operating System Software
IOS (tm) C2600 Software (C2600-IK9S-M), Version 12.2(31), RELEASE SOFTWARE (fc2)

Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2005 by cisco Systems, Inc.
Compiled Thu 11-Aug-05 17:24 by tinhuang
Image text-base: 0x8000808C, data-base: 0x8128D9F4

ROM: System Bootstrap, Version 12.2(6r),  RELEASE SOFTWARE (fc1)
ROM: C2600 Software (C2600-IK9S-M), Version 12.2(31), RELEASE SOFTWARE (fc2)

Should they also always be on the same main version number?  Also I know Cisco provides the IOS's to download and upgrade,but how about the System Bootstrap?

Thanks.
kdavidmAsked:
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ECNSSMTCommented:
The IOS is the full Cisco Operating system that permits the Cisco device to perform all of its routing functions.  
The System BootStrap in ROM: contains the minimum code to bring up the Cisco device and permit you to operate minimumly; this should at least permit you to configure the 2600 to download a usable Cisco IOS in the event that the IOS in NVRAM is trashed.  

Regards,
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ECNSSMTCommented:
oh, Cisco IOS, OS, CatOS are updated on a regular basis and I also assume that the ROMs are also.  I usually don't pay attention to those version numbers unless there are issues.  

The ROMs are soldered on; BTW so they can't be changed.

Regards,
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Don JohnstonInstructorCommented:
The Bootstrap is code in ROM that instructs the router on how to boot.

The bootstrap code in Cisco ROM instructs the router to first check the boot field of the config-register. If it's 0x0, ROM monitor is loaded, If it's 0x1, the mini-IOS is loaded. If the boot field is anything else, the config file in NVRAM is examined for any commands that start out "boot system". If there are any, the remaining part of that command is parsed and the IOS is loaded from the specified location.

Absent any boot system commands, the first file in flash is loaded.

If there is no file in flash, a netboot is attempted.

As ECNSSMT points out, the IOS is the operating system.

-Don
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Don JohnstonInstructorCommented:
Also, not all ROM's are soldered. I've upgraded Boot ROM on a couple different routers. In fact, the last time I did it was to a 2500 and it was free from Cisco. I haven't looked into the 2600's though.

-Don
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Don JohnstonInstructorCommented:
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ECNSSMTCommented:
I don't doubt that there are Cisco devices with removable parts including ROMs.  Considering how many versions of OSes Cisco has for their devices, it would be hard enough not to imagine that they have hardware teams constantly revising or improving their hardware.

Regards,
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