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
.The main hardware component of the system BIOS is the system BIOS ROM itself. This is normally located in an electrically-erasable read-only memory (EEPROM) chip, which allows it to be updated through software control. This is commonly called a flash BIOS.

The BIOS ROM is located in a socket on the motherboard and is relatively easy to locate because it is usually labelled with the name of the BIOS manufacturer. Most times this is Award, American Megatrends (AMI) or Phoenix. There is also often a version number on the chip, although the actual BIOS version within the chip may be different from what is labelled, because of the ability to flash the BIOS mentioned above.

Under normal circumstances, the BIOS ROM is permanent and there is normally no reason to need to deal with it in any way. If for some reason the BIOS ROM were to become corrupted due to an aborted flash update, for example, you might find your PC left in a state where it could not be booted. In this situation, you might have to physically replace the BIOS ROM, but this is a very rare happening.

The special type of memory used to store this information is called CMOS memory. 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. 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 Checksum Error".

There are many different types of batteries used to power the CMOS; mostly, these have changed over time as technology has evolved. These batteries are discussed here. You will not normally have to deal with the CMOS memory directly; it holds the settings you enter in the BIOS setup program. Over time, it is possible you will have CMOS problems; for example, you may find that the machine may begin to forget its settings when it is booted. These are usually signs of trouble with the motherboard battery.

In addition to the standard CMOS memory used to hold system settings, Plug and Play BIOSes use an additional non-volatile memory to hold extended system configuration data (ESCD). This is used to record the resource configurations of system devices when Plug and Play is used.