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BIOS CMOS Memory

The BIOS settings that you use to control how your PC works must be saved in non-volatile memory so that they are preserved even when the machine is off. This is as opposed to regular system memory, which is cleared each time you turn off the PC. A special type of memory is used to store this information, called CMOS memory, and a very small battery is used to trickle a small charge to it to make sure that the data it holds is always preserved. These memories are very small, typically 64 bytes, and the batteries that they use typically last for years. This non-volatile memory is sometimes called NVRAM.

CMOS stands for "Complementary Metal Oxide Semiconductor". This is one type of technology used to make semiconductors (integrated circuits) such as processors, chipset chips, DRAM, etc. CMOS has the advantage of requiring very little power, compared to some other semiconductor technologies. This is why it was chosen for this use, so that the amount of power required from the battery would be minimal, and the battery would be able to last a long time. This memory came to be called just "CMOS" since in the early days most parts of the computer did not use CMOS. Ironically, with today's processors having to do more and more and needing to do it with lower power consumption, they themselves are typically made entirely with CMOS technology. However, "CMOS" by itself usually still refers to the BIOS settings memory. Old habits die hard in the computer world.

The system uses something called a CMOS checksum as an error-detecting code. Each time you change the BIOS settings, the checksum is generated by adding together all the bytes in the CMOS memory and then storing the lowest byte of the sum. Then, each time the system booted, the system recomputes the checksum and compares it to the stored value. If they are different, then the system knows that the CMOS has been corrupted somehow and will warn you with an error, typically something like "CMOS