Nonvolatile BIOS memory refers to the memory on a personal computer motherboard containing BIOS settings and sometimes the code used to initialize the computer and load the operating system. The nonvolatile memory was historically called CMOS RAM or just CMOS because it traditionally used a low-power CMOS memory chip (the Motorola MC146818, or one of its higher-capacity clones), which was powered by a small battery when the system power was off. The term remains in wide use in this context, but has become a misnomer. The nonvolatile BIOS memory in modern computers is generally an EEPROM or Flash memory chip. There is still a backup battery involved, but its role is not to maintain the data stored in the RAM, only to keep the RTC chip operational.

CMOS mismatch

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CMOS mismatch errors typically occur if the computer's power-on self-test (POST) program:

1. Finds a device that is not recorded in the CMOS.
2. Does not find a device that is recorded in the CMOS.
3. Finds a device that has different settings than those recorded for it in CMOS.
4. Detects a CMOS checksum error. [http://www.computerhope.com/issues/ch000385.htm

Resetting the CMOS settings

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To break into a BIOS setup when the machine refuses cooperation, occasionally a drastic move is required. In older computers with battery-backed RAM, removal of the battery and short circuiting the battery input terminals for a while did the job; in some more modern machines this move only resets the RTC. Some motherboards offer a CMOS-reset jumper. In yet other cases, the EEPROM chip has to be desoldered and the data in it manually edited using a programmer. Sometimes it is enough to ground the CLK or DTA line of the I2C bus of the EEPROM at the right moment during boot, this requires some precise soldering on SMD parts. When the machine lets you boot but does not want to let you into the BIOS setup, the CMOS checksum can be damaged using direct port writes using debug.exe, corrupting some bytes of the checksum-protected area of the CMOS RAM; at the next boot, the computer typically resets its setting to factory defaults.

External links

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• CMOS memory map

BIOS | Motherboard | Non-volatile memory