Updated: 07/06/2021 by Computer Hope

CMOS may refer to any of the following:

CMOS battery

1. Alternatively known as an RTC (real-time clock), NVRAM (non-volatile random-access memory), or CMOS RAM, CMOS is short for complementary metal-oxide semiconductor. CMOS is an onboard, battery-powered semiconductor chip inside computers that stores information. This information ranges from the system time and date to your computer's hardware settings. The picture shows an example of the most common CMOS coin cell battery (Panasonic CR 2032 3V) used to power the CMOS memory.

History of CMOS

The Motorola 146818 chip was the first RTC and CMOS RAM chip to be used in early IBM computers, capable of storing 64 bytes of data. Since the system clock used 14 bytes of RAM (random-access memory), this left an additional 50 bytes for storing system settings. Today, most computers have moved the settings from CMOS and integrated them into the southbridge or Super I/O chips.

How long does the CMOS battery last?

The standard lifetime of a CMOS battery is around 10 Years. However, this can vary depending on the use and environment where the computer resides.

Which devices use CMOS?

How would I know if my CMOS battery is failing?

If the CMOS battery fails, the computer cannot maintain the correct time or date on the computer after it's turned off. For example, after turning your computer on, you may notice the time changed to 12:00 P.M., and the date is reset to January 1, 1990. This error indicates the CMOS battery has failed.

Another indication of CMOS battery failure is if one of the following messages are shown as the computer boots.

  • CMOS Read Error
  • CMOS Checksum Error
  • CMOS Battery Failure
  • System battery voltage is low

Apple computer users

PRAM is the Apple Macintosh equivalent of CMOS.

How do you pronounce CMOS?

CMOS is pronounced as one word that sounds like see-moss.

2. With a camera, see our CMOS sensor definition.

3. CMOS is an abbreviation for The Chicago Manual of Style style guide.

BIOS, Computer abbreviations, Hardware terms, MOS, Motherboard terms, PRAM, Quartz crystal, RTC