I am reading an old A+ certification book that is going over BIOS and CMOS.

Some background info (this may all very well be incorrect):

From what I understand, BIOS is a set of programs on the system ROM that the CPU uses to control certain hardware (keyboard, mouse, speakers, etc.) The Southbridge used to contain the various hardware controllers that would talk to the system ROM, but now the Southbridge is integrated into the CPU (?). A certain set of address space is reserved for the ROM, so that when the CPU encounters an address in that space, the Northbridge doesn't go to the RAM for instructions; instead the Southbridge looks to the ROM for the instructions.

However, you might want to configure some hardware-specific settings, which is done through the CMOS utility. Those settings are stored in the CMOS chip, which is volatile memory.

Where is the CMOS utility program stored? On the system ROM (same place as the BIOS)?

If so, why do people refer to "resetting the BIOS settings"? I figure, one would either be using the CMOS utility program that is stored in system ROM to change hardware-specific settings (thus, making changes to the CMOS chip), or one would flash the system ROM itself. I assume flashing the system ROM would physically alter its firmware, thus not allowing you to access any sort of CMOS utility program unless you specifically flash a new CMOS utility program onto the BIOS.

Is this simply a case of people using the terms "flashing", "resetting", etc., interchangeably as a way of speaking, when they technically mean something else?


According to this answer to a related question:

That is why the BIOS is reset when you remove the battery and re-attach it.

The BIOS itself wouldn't be reset, since it is firmware, right? Does the answer instead mean that settings, which are stored in the CMOS chip, related to the BIOS are reset?

  • Resetting usually means using a feature of the BIOS software to clear any saved settings, and going back to the default values for everything.
    – Zoredache
    Jul 22, 2014 at 19:58
  • @Zoredache, these would be the "fail safe settings" found in the CMOS utility, correct?
    – Josh Beam
    Jul 22, 2014 at 20:00
  • Probably, it all depends on the firmware. They all seem to label things differently.
    – Zoredache
    Jul 22, 2014 at 20:10

1 Answer 1


So you have to remember you have basically two separate components here that make up what most people call the BIOS.

There is the part that actually contains the software. This was an EEPROM in the past. Like all EEPROMs you could only change it a very limited number of times. These days this is typically flash memory. Re-flashing the BIOS usually means running a program that will replace the firmware in the EEPROM.

The second component was a bit of RAM in a chip that was very power efficient. CMOS (a type of semiconductor) RAM can run off a watch batter for a long time. This held all the configuration values associated with the BIOS software. Pulling the batter wipes out this RAM, or using the reset feature in the BIOS wipes out these values.

Often these two separate components can be, and usually are packaged within the same physical chip.

So to summarize, flashing means reloading/replacing the BIOS software. Resetting means reloading/replacing the configuration data.

  • 4
    The CMOS used to be actually part of the same chip (an Motorola MC146818) that contained the real-time clock, and 14 of the 64 bytes of memory in this chip were the clock registers. Today's hardware emulates an MC146818, but the memory may actually be an EEPROM, so doing things like "clearing the CMOS" may not be possible on some modern laptops (removing the battery will only reset the clock) - typically a "reset password" or similar jumper is provided to clear any power on password, though.
    – LawrenceC
    Jul 22, 2014 at 20:24

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