Also known 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 system settings for your computer. A CMOS is similar to the Apple Macintosh computer's PRAM (Parameter RAM, PRAM). To the up is an image of a CMOS battery on a computers motherboard and the most common CMOS battery you're likely to encounter with your computer.






To the up is some examples of other types of batteries that may be used in a computers to power the CMOS memory. As mentioned above the most common type of battery is the coin cell battery (lithium battery), usually about the size of a U.S. Nickel.

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

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How to enter the BIOS or CMOS setup.

Issue:

How to enter the BIOS or CMOS setup.

Solution:

Because of the wide variety of computer manufacturers and BIOS manufacturers over the evolution of computers, there have been numerous different ways to enter the BIOS or CMOS Setup. Below is a listing of the majority of these methods as well as other recommendations for entering the BIOS setup.

New computers

Thankfully, computers that have been manufactured in the last few years will allow you to enter the CMOS by pressing one of the below five keys during the boot. Usually it's one of the first three.

• F1
• F2
  
• DEL
• ESC
• F10

A user will know when to press this key when they see a message similar to the below example as the computer is booting. Some older computers may also display a flashing block to indicate when to press the F1 or F2 keys.

Press to enter BIOS setup

Tip: If your computer is a new computer and you are unsure of what key to press when the computer is booting, try pressing and holding one or more keys the keyboard. This will cause a stuck key error, which may allow you to enter the BIOS setup.

Older computers

Unlike the computers of today, older computers (before 1995) had numerous different methods of entering the BIOS setup. Below is a listing of general key sequences that may have had to be pressed as the computer was booting.

• CTRL + ALT + ESC
• CTRL + ALT + INS
• CTRL + ALT + ENTER
• CTRL + ALT + S
• PAGE UP KEY
• PAGE DOWN KEY

ACER BIOS

If your computer is unable to boot or you wish to restore the BIOS back to bootable settings and your computer uses an ACER BIOS, press and hold the F10 key as you turn on the computer. While continuing to hold the F10 key, you should hear two beeps indicating that the settings have been restored.

AMI BIOS

Older AMI BIOS could be restored back to bootable settings by pressing and holding the Insert key as the computer is booting.

BIOS / CMOS diskettes

Early 486, 386, and 286 computers may have required a floppy disk in order to enter the BIOS setup. These diskettes are known as ICU, BBU, and SCU disks. Because these diskettes are unique to your computer manufacturer, you must obtain the diskettes from them.

Early IBM computers

Some models of early IBM computers required that the user press and hold both mouse buttons as the computer was booting in order to enter the BIOS setup.

Other suggestions

Finally, if none of the above suggestions help get you into your CMOS setup you can cause a stuck key error, which will usually cause the CMOS setup prompt to appear and remain until you press a key to continue. To do this press and hold any key on the keyboard and do not let go (you may get several beeps as you're doing this). Keep holding the key until the computer stops booting and you're prompted with an option to enter setup or to press another key to continue booting.