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Dec 30, 2009 CMOS vs TTL. TTL stands for Transistor-Transistor Logic.It is a classification of integrated circuits. The name is derived from the use of two Bipolar Junction Transistors or BJTs in the design of each logic gate. CMOS (Complementary Metal Oxide Semiconductor) is also another classification of ICs that uses Field Effect Transistors in the design.
The terms BIOS and CMOS both refer to essential parts of your computer's motherboard. They work together and they're both important, but they are not the same thing.
Let's take a look at what the BIOS and CMOS are, and how they're different.
BIOS
The BIOS (Basic Input/Output System) is firmware stored in a chip on your computer's motherboard. It is the first program that runs when you turn on your computer.
The BIOS performs the POST, which initializes and tests your computer's hardware. Then it locates and runs your boot loader, or loads your operating system directly.
The BIOS also provides a simple interface for configuring your computer's hardware. When you start your computer, you may see a message like 'Press F2 for setup.' This setup is your BIOS configuration interface. For more information about configuring your BIOS, see our guide to configuring your BIOS.
CMOS
When you make changes to your BIOS configuration, the settings are not stored on the BIOS chip itself. Instead, they are stored on a special memory chip, which is referred to as 'the CMOS.'CMOS stands for 'Complementary Metal-Oxide-Semiconductor.' It's the name of a manufacturing process used to create processors, RAM, and digital logic circuits, and is also the name for chips created using that process.
Like most RAM chips, the chip that stores your BIOS settings is manufactured using the CMOS process. It holds a small amount of data, usually 256 bytes. The information on the CMOS chip includes what types of disk drives are installed on your computer, the current date and time of your system clock, and your computer's boot sequence.
On some motherboards, the CMOS is a separate chip. However, on most modern motherboards, it is integrated with the RTC (real-time clock) on the southbridge.
Your BIOS memory is non-volatile: it retains its information even when your computer has no power because your computer needs to remember its BIOS settings even when it's turned off. That's why the CMOS has its own dedicated power source, which is the CMOS battery.
CMOS battery
The CMOS battery is a lithium-ion battery about the size of a coin. It can hold a charge for up to ten years before needing to be replaced. If your CMOS battery dies, your BIOS settings will reset to their defaults when your computer is turned off.
TipIf you turn on your computer and your settings have been reset or forgotten, your CMOS battery may be dead. For example, your hard drive may no longer detected correctly, or your system clock may be reset to a date in the past, such as January 1st, 1970. If so, you can replace the battery, configure the settings correctly, and the problem should not recur. For more information, see our CMOS battery replacement guide.
Some motherboards, including most modern ATX motherboards, will continue to provide power to the CMOS if the battery is replaced while the computer is actively powered.
WarningIf you want to replace your CMOS battery, make sure to consult your motherboard documentation for details and safety information. Visit your motherboard manufacturer's support website if you need to download a new copy of your manual.
Additional information
- See our BIOS and CMOS definitions for further information and related links.
CMOS (short for complementary metal-oxide-semiconductor) is the term usually used to describe the small amount of memory on a computer motherboard that stores the BIOS settings. Some of these BIOS settings include the system time and date as well as hardware settings.
A CMOS sensor is different — it's used by digital cameras to convert images into digital data.
Other Names for CMOS
CMOS (pronounced see-moss) is sometimes referred to as Real-Time Clock (RTC), CMOS RAM, Non-Volatile RAM (NVRAM), Non-Volatile BIOS memory, or complementary-symmetry metal-oxide-semiconductor (COS-MOS).
CMOS is also an abbreviation for other terms that are unrelated to what's talked about on this page, like cellular management operation system and comparison mean opinion score.
Clearing CMOS
Most talk of CMOS involves clearing CMOS, which means to reset the BIOS settings to their default levels. This is a really easy task that's a great troubleshooting step for many types of computer problems.
For example, maybe your computer is freezing up during the POST, in which case clearing the CMOS to reset the BIOS settings to factory default levels, might be the easiest solution.
Or maybe you need to clear CMOS to reset misconfigured BIOS settings to fix certain hardware-related error messages, such as Code 29 errors.
How BIOS and CMOS Work Together
The BIOS is a computer chip on the motherboard like CMOS except that its purpose is to communicate between the processor and other hardware components like the hard drive, USB ports, sound card, video card, and more. A computer without a BIOS wouldn't understand how these pieces of the computer work together.
The BIOS firmware is also what performs the Power On Self Test to test those pieces of hardware, and what ultimately runs the boot loader to launch the operating system.
CMOS is also a computer chip on the motherboard, or more specifically a RAM chip, which means it would normally lose the settings it's storing when the computer is shut down (just like how the contents of RAM are not maintained each time you restart your computer). However, the CMOS battery is used to provide constant power to the chip.
When the computer first boots up, BIOS pulls information from the CMOS chip to understand the hardware settings, time, and anything else that's stored in it. The chip typically stores as little as 256 bytes of information.
What Is a CMOS Battery?
The CMOS is usually powered by a coin-sized CR2032 cell battery, referred to as the CMOS battery.
Most CMOS batteries will last the lifetime of a motherboard, up to 10 years in most cases, but will sometimes need to be replaced.
Incorrect or slow system date and time, and loss of BIOS settings, are major signs of a dead or dying CMOS battery. Replacing them is as easy as swapping out the dead one for a new one.
More About CMOS & CMOS Batteries
While most motherboards have a spot for a CMOS battery, some smaller computers, like many tablets and laptops, have a small external compartment for the CMOS battery that connects to the motherboard via two small wires.
Some devices that use CMOS include microprocessors, microcontrollers, and static RAM (SRAM).
It's important to understand that CMOS and BIOS are not interchangeable terms for the same thing. While they work together for a specific function within the computer, they are two entirely different components.
When the computer is first starting up, there's an option to boot into BIOS or CMOS. Opening the CMOS setup is how you can change the settings it's storing, like the date and time and how the different computer components are first started up. You can also use CMOS setup to disable/enable some hardware devices.
CMOS chips are desirable for battery-powered devices like laptops because they use less power than other types of chips. Although they use both negative polarity circuits and positive polarity circuits (NMOS and PMOS), only one circuit type is powered on at a time.
The Mac equivalent to CMOS is PRAM, which stands for Parameter RAM. You can also reset your Mac's PRAM.