Short for Basic Input/Output System, the BIOS, ROM BIOS, or System BIOS is a chip located on all motherboards that contain instructions and setup for how your system should boot and how it operates. In the below picture, is an example of what a BIOS chip may look like on your computer motherboard. In this example, this is a picture of an early AMIBIOS, a type of BIOS manufactured by the AMI. Another good example of a BIOS manufacturer is Phoenix.
The BIOS includes instructions on how to load basic computer hardware and includes a test referred to as a POST (Power On Self Test) that helps verify the computer meets requirements to boot up properly. If the computer does not pass the POST, you will receive a combination of beeps indicating what is malfunctioning within the computer.
In most PCs, the BIOS has 4 main functions:
- POST - Test the computer hardware and make sure no errors exist before loading the operating system.
- Bootstrap Loader - Locate the operating system. If a capable operating system is located, the BIOS will pass control to it.
- BIOS drivers - Low level drivers that give the computer basic operational control over your computer's hardware.
- BIOS or CMOS Setup - Configuration program that allows you to configure hardware settings including system settings such as computer passwords, time, and date.
Alternatively referred to as a Real-Time Clock (RTC), Non-Volatile RAM (NVRAM) or CMOS RAM, CMOS is short for Complementary Metal-Oxide Semiconductor. CMOS is an on-board semiconductor chip powered by a CMOS battery inside computers that stores information such as the system time and date and the system hardware settings for your computer. The picture below, Shows one of the most common CMOS coin cell battery used in a computer to power the CMOS memory.
A Motorola 146818 chip was the first RTC and CMOS RAM chip to be used in early IBM computers. The chip was capable of storing a total of 64 bytes of data. Since the system clock used 14 bytes of RAM, this left an additional 50 bytes of space that was available for IBM to store system settings. Today, most computers have moved the settings from a separate chip and incorporated 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 that the computer resides. When the battery fails the system settings and the date and time will not be saved when the computer is turned off until the battery has been replaced.
What is difference between BIOS and CMOS ?
Often the BIOS and CMOS can be confused because instructions may either indicate to enter the BIOS Setup or the CMOS Setup. Although the setup for BIOS and CMOS is the same, the BIOS and CMOS on the motherboard are not.
If you have already read the above BIOS and CMOS definition links you should now know that the BIOS and CMOS are two different components on the motherboard. The BIOS on the motherboard contains the instructions on how the computer boots and is only modified or updated with BIOS updates, the CMOS is powered by a CMOS battery and contains your system settings and is modified and changed by entering the CMOS Setup.
Although the setup is often referred to as the BIOS and CMOS setup, we suggest you only refer to the setup as "CMOS Setup" as it is more appropriate. Computer Hope often refers to the setup as BIOS and CMOS Setup to help users who are looking for one instead of the other.