CMOS

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 shows an example 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.

How to enter the BIOS or CMOS setup

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

Note: This document does not help if you cannot enter the BIOS or CMOS setup because of a password.

New computers

Computers manufactured in the last few years allow you to enter the BIOS setup using one of the five keys shown below during the boot process. Typically it is one of the first three keys.

• F1
• F2
• DEL
• ESC
• F10 *

* F10 is also used for the boot menu. If F10 opens a boot menu, your setup key is likely F2.

You will know when to press this key when you see a message similar to the example below 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 <F2> 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 on the keyboard to cause a stuck key error. Once you get this error, you should have the option to continue or enter setup.

Once you have successfully entered the CMOS setup, you should see a screen similar to the example below.

How do I change and save changes in CMOS setup?

Once in CMOS setup, the method for changing the settings often depends on the BIOS manufacturer. Typically you can change the settings by using the arrow keys to move to the option you want to change. Press Enter to select what to change and then use the arrow keys to select a new value. Some manufacturers may have you press the Page up and Page down keys to change the values.

Tip: All manufacturers list the steps on how to navigate on the bottom or right-hand side of the screen.

I cannot change the values to a setting I want to use.

If you are trying to change the clock settings, speed settings, or other setting and do not have an available option, it is because it is not supported by the motherboard. If you believe it should be supported, you may need a BIOS update.

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 key sequences to press as the computer is booting to enter setup.

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

ACER BIOS

If your computer cannot boot or you want to restore the BIOS back to its 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 are 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 or CMOS diskettes

Early 486, 386, and 286 computers required a floppy disk in order to enter the BIOS setup. These diskettes may be referred to as ICU, BBU, or SCU disks. Because these diskettes are unique to your computer manufacturer, you must obtain the diskettes from them. See the computer manufacturers list for contact information.

Early IBM computers

Some early IBM computers require you to press and hold both mouse buttons as the computer boots 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 causes 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 are doing this). Keep pressing the key until the computer stops booting and you have the option to enter setup.

BIOS

1. 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 picture below, 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.

The four main functions of a PC BIOS

• POST - Test the computer hardware and make sure no errors exist before loading the operating system. Additional information on the POST can be found on our POST and Beep Codes page.
• 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.