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Difference between BIOS and CMOS?

Posted on 2001-11-14
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Last Modified: 2010-05-18

What is the difference between the BIOS and the CMOS? And is the CMOS also a battery for the BIOS or not. Also, in some computers I can't find the battery. I assume I have to look for something other then the ones that look like a watch battery.
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Question by:ekrisch
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jhance earned 200 total points
ID: 6631058
This is like comparing Microsoft Windows with your hard drive.

BIOS - Basic Input Output System is a PROGRAM supplied with your computer and stored semi-permanently in a device called a EPROM or FLASH MEMORY on the motherboard of your computer.  This is what you see when you first turn the computer on.  It's tasks are to self-test the system, initialize the devices, and load a boot block from one of your storage devices.  After that, it's pretty much out of the picture.

CMOS - in the context you are using it is really "Battery Backed-up CMOS RAM" and is a place, also on your computer motherboard where settings for the hardware of your computer (usually used by the BIOS) are stored.  This type of RAM system retains its settings even when power of OFF so that your computer will still remember how it was setup the last time it was on.

Most motherboards have a "coin" type battery which is a silver disk shaped lithium cell.  But many other configurations are possible.  Some even use what is called a "Dallas" chip which is a complete CMOS RAM chip with the battery embedded in the case of the device.  These are made by Dallas Semiconductor and are a real pain to replace...
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