CMOS has been evolving since it was first created. At first the CMOS utilized RAM, which required a slow trickle of power to keep data (BIOS settings) stored and safe. This method required the use of a CMOS battery; typically a CR2032 battery was used. These batteries have a life of 2 to 10 years depending on the amount of electricity used, ambient temperature, and other stress factors. Later the technology evolved to a form of charging the CMOS battery while the system was powered on, to help provide a longer battery life. Further the CMOS technology has been improved to utilize EEPROM to store the BIOS settings. A battery however is still used to maintain the Real-time clock. So over time the CMOS has truly went from a volatile memory to a non-volatile memory type.
There is a lot of confusion on the difference in CMOS and BIOS.
CMOS (Complementary Metal-Oxide-Semiconductor) is a way of constructing integrated circuits. The technology is used today in microprocessors, microcontrollers, static RAM, and other digital circuits. CMOS utilizes transistors to store information in a volatile manner, but is sometimes thought of as non-volatile due to the common use of a battery to maintain the power to the chip, and recent changes to using EEPROM technology. In a personal computer, the CMOS is mainly used to store settings for the BIOS, and to