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FREQUENTLY ASKED QUESTIONS
CMOS and CMOS Batteries

Last updated: 1/27/03

Q.  Can you tell me what causes CMOS checksum errors?

A. A CMOS checksum is generated by adding all of the bytes (or bits) in the CMOS one after the other. That is, byte one is added to byte 2, byte 3 is added is to the sum of bytes 1 and 2, etc. The carry bits are dropped. The result (checksum) is stored in the CMOS. During the boot-up process or POST (Power-On Self Test) a checksum is generated by the BIOS from the CMOS and compared to the one saved the last time the CMOS Setup was run or the BIOS defaults were loaded.  If the two numbers don't agree it is an indication that the data in the CMOS has been corrupted (one or more bits in the CMOS changed when it/they weren't supposed to) and a checksum error is issued by the BIOS ("CMOS checksum invalid, " "CMOS invalid," and relate error, "CMOS battery low").  Causes include:

  • A bad battery.
  • A battery that has become discharged (the computer has been off a very long time).
  • A disconnected battery.
  • Insertion of an expansion board in such a manner (cock-eyed) as to short-out the bus (even if the computer is off, which it should be)
  • A power surge.
  • Lightning.
  • Static electricity.
  • Grounding the CMOS circuitry.
  • A bad motherboard.
  • A bad real-time clock.

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