The Magical CMOS


Computers are, for many people, magical devices. You power them up, they store all kinds of data for you that take virtually no physical space, they play movies, music, whatever.

And what a lot of people find very intriguing is the following fact. Imagine you shut down you computer, unplug it and store it away. One year later, you take out your computer again, plug it i the way it was before and power it up.

The most magical thing happens… The computer STILL knows what date it is and the exact hour. How it that possible?!?
Witchcraft!!

Well.. No not really.

This is the secret behind the ever-correct-running-clock is in your CMOS.

CMOS stands for Complementary metal–oxide–semiconductor (fancy.. is it not?). This part of your motherboard will store all the system setup parameters including date and time.
So you can say that CMOS is a memory chip that contains small pieces of software. The special thing about this is that the changes you have done can only be saved as long as there is a current running through this chip.
Once the power is gone, your settings are as well. Only the factory defaults of the motherboard remain.

Ring a bell? This is the same as your RAM memory, it’s volatile.. Well.. Semi-volatile actually.

However this does not explain why the settings remain saved when you unplug the computer.
Indeed… But this will..
This shows the magical part that allows CMOS to remember all your settings:

A plain, simple battery.. Since CMOS uses very little power this little thing can keep the settings saved for years.

If you don’t believe me, unplug the computer, remove the battery and put it back in. Your settings will be gone ;-).

All hail the battery!

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