CMOS is an abbreviation for Complementary Metal Oxide Semiconductor. CMOS uses a complimentary arrangement of both NMOS and PMOS, negative and positive Metal Oxide transistor circuits. CMOS memory draws very little current (nano-Amps). Values stored in it can be maintained for very a long period of time while the computer is off with a rechargeable lithium battery. The CMOS memory is used to store system configuration information, such as disk drive parameters, memory configuration, and the date-time, some of which you enter in the CMOS Setup program, . It is used by the Basic Input Output System (BIOS), a program permanently stored in the flash memory on recent motherboards and in read-only memory (ROM) on older motherboards, to configure the computer. The CMOS Setup is part of the BIOS program. The CMOS memory is usually located with the real-time clock in the motherboard chipset or in a separate real-time clock chip. It is located in the chipset on most recent motherboards.

Can you tell me what causes CMOS checksum errors?
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.

How can the CMOS be cleared (e.g., a password is set in CMOS and is unkown or forgotten)?
There may be a CMOS discharge jumper near the battery. Most of them have three pins. Moving the jumper form one set of pins to the other will discharge the battery. Be sure the computer is off with the power cord pulled when you do this. Some motherboards can be damaged if power is applied with the jumper in the discharge position, but have not seen this to be the fact when I have done it in error. If you can't identify the jumper, pull the battery if it is removable and let the CMOS discharge for 10 or more minutes. Be careful when removing coin-shaped batteries (e.g., CR2032). Most of the battery holders I have seen for these batteries are quite fragile and easily broken. I ease these batteries out of the holder with the aid of a “tweaker” (small screw driver). If the battery isn’t removable, there may be a header on the motherboard for an external battery. Usually there are three (four-pin header with one pin missing) or four pins (you may have to break-off one pin or bend it over to install an external battery).

If the battery is near the keyboard jack, that header is often at the back of the motherboard and to the left of the keyboard jack when facing the front of the computer. It may be labeled with a + or pin 1. Many of them will have a jumper on them for jumpering the motherboard to use the battery on the motherboard or an external one. Usually, shorting the furthest aft pair of pins with a jumper (or moving the jumper to the pins that are not jumpered) discharge the CMOS. Removing the jumper may also work. I usually try to follow the traces from that header to determine which pins are which (or follow the traces, if they don’t disappear in a multi-layer board, from the battery to find the header). Shorting the battery may damage it and/or the CMOS, but it might work, and I have done it quite a few times successfully. “Shorting” it with a resistor (guess = 10 KOhms) might be a better idea. I have not done that. Some older computers have the battery and real-time clock built into a single unit. They are usually black and rectangular in shape. Dallas Semiconductor made many of them. They are supposed to last about ten years. If that unit is soldered to the motherboard and it dies, it usually spells the end of the motherboard (you may be able to pry off the top of the unit and “splice” a battery into it). I have not done that either. If the battery is cylindrical (usually blue), soldered to motherboard, and is leaking, it must be removed. The ooze will eat the traces on the motherboard and ruin it. Never attempt to unsolder one of these batteries (or any battery). They can explode. I carefully twist them off with piers and clean-up the remains with a small pair of diagonal cutters. Plugging a battery in backwards can damage the CMOS on some motherboards