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The BIOS settings that you use to control how your PC works must be saved in non-volatile memory so that they are preserved when he machine is off. This is a opposed to regular system memory, which is cleared each time you turn off the PC. a special type of memory is used to store this information, called CMOS memory, and a very small battery is used to trickle a small charge to it to make sure that the data it holds is always preserved. These memories are very small, typically 64 bytes, and the batteries that they use typically last for years.

CMOS

CMOS stands for "Complimentary Metal Oxide Semiconductor". This is one type of technology used to make semiconductrs (integrated circuits) such as processors, chipset chips, DRAM, etc. CMOS has the advantage of requiring very little power, compared to some other semiconductor technologies. This is why it was chosen for this use, so that the amount of power required from the battery would be minimal, and the battery would be able to last a long time. This memory came to be called just "CMOS" since in the early days most parts of the computer did not use CMOS. Ironically, with today’s processors having to do more and more and needing to do it with lower power consumption, they themselves are typically made entirely with CMOS technology. However, "CMOS" by itself usually still refers to the BIOS settings memory.

In addition to the standard CMOS memory used to hold system settings, Plug and Play BIOSes use an additional non-volatile memory to hold extended system configuration data (ESCD). This is used to record the resource configurations of system devices when Plug-and-Play is used.

BIOS

The BIOS stands for Basic Input/Output System and is the chip which holds the basic instructions for booting your system. This firmware is ingrained into non-volatile flash memory and is the default software you will see when a computer boots up.

BIOS is a very important program, its quality and modernness determine the features and capabilities of your machine to a larger degree. Many manufacturers of motherboards are able to expand the capabilities of their boards, or fix problems with them, by making changes to the BIOS and either giving free downloads or selling upgrades, just as software application houses like Microsoft do.

The BIOS program in your system is programmed into a read-only memory (ROM). ROMs are, of course, not rewriteable the way RAM is; that is why they are called ‘read-only’. This presents a problem when you want to update your BIOS. In "the old days" when you wanted to update your BIOS, the manufacturer sent you a new BIOS chip; you opened the box, pulled out the old chip, and put the new one in. Needless to say, this is a pain. Fortunately, technology came to the rescue through the invention of the flash BIOS. Some machine still require physicall upgrade but by using the flash BIOS, most newer machines can upgrade using special software without having to open the case at all.

Many motherboards have a special "safety feature" to prevent accidental (or malicious) changes to the flash BIOS – a jumper that must be changed before performing a flash BIOS upgrade. While this is a security feature, it also obviates one of the great advantages of the flash BIOS, namely not having to open the case. The motherboard manual will tell you weather you have a jumper or not. With the increasing commonality of viruses that can change flash BIOS code, this may soon be a feature on every motherboard.

The major disadvantage to using this flash process to upgrade your BIOS. While the BIOS is actually being flashed, it is in a very vulnerable state. If you are really unlucky and something very bad happens in the middle of the upgrade (for example, a power outage), it is possible to end up with a corrupted BIOS chip. You can also end up with a corrupted system if your boot the wrong flash BIOS image into the chip. These days manufacturers have created software which will check the image file against the model of the motherboard and will throw and error if there is a mismatch.

SOOOOOO… What is the difference?

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/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 comptuer 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, it is suggested that you only refer to the setup as ‘CMOS Setup’ as it is more appropriate.

How to acccess and modify CMOS configuration

The following videos complete the BIOS and CMOS series and show in detail how to enter the boot configuration, modify CMOS settings and navigate through the firmware. For contast the videos show varying OEM firmware and explains how they are visually and functionaly different.

Summary

• BIOS - Basic Input/Output System
• CMOS – Complimentary Metal Oxide Semiconductor
• Bios is a firmware chip that holds the instructions to control low level hardware.
• Unlike BIOS, the CMOS is a read/write chip, which means users can save settings to the chip.
• BIOS uses Read-only Memory (ROM)
• CMOS uses Random Access Memory (RAM)
• The BIOS and CMOS chips work hand-in-hand. BIOS has the basic instructions and CMOS has the instructions which users can change. For example the system time, boot priority/order, CPU clock speed, or the CPU multiplier.
• BIOS instructions are flashed to the chip where as CMOS settings need to be saved in memory. This is why the CMOS has a battery. The bettery gives power to the system to allow the information to be saved and stored to the memory.
• The locations of the chips can be anywhere on the motherboard, however generally they are on the bottom right hand side. Usually the CMOS chip will be located close to the CMOS battery. Next to the battery will be a reset jumper which allows resetting the CMOS to factory default settings.
• While the BIOS settings are constant the CMOS setting are volatile. This means if the reset jumper is removed or if the system looses power by the battery being removed the CMOS settings will be lost.
• While original equipment manufacturers (OEM) have differing firmware there are many simularities in the CMOS setup screens.