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Explain clearing cmos settings

Last response: in Motherboards
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June 17, 2011 10:34:04 PM

Hello,
does clearing bios bring back original bios, in case flashing caused a bricked motherboard?
a b V Motherboard
June 17, 2011 11:34:56 PM

The CMOS memory is where the BIOS stores its settings. Clearing the CMOS memory sets all the CMOS memory locations to FFH this is detected by the BIOS which then loads its default values. Clearing the CMOS does not affect the BIOS program in any way. You should always clear the CMOS after you upgrade the BIOS as the updated BIOS can use different memory locations in the CMOS memory and the different (wrong) data can cause unpredictable operation or even no operation at all.
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a b V Motherboard
June 18, 2011 3:16:30 PM

If you have 'bricked' your motherboard maybe because a biosflash was interrupted for example, then you need to either replace your bios chip or find a bookshelf with a motherboard-sized space on it
Moto
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a b V Motherboard
June 21, 2011 12:47:26 AM

Not quite so fast, moto! Quite a few motherboards these days have two BIOS chips, and you can only flash the first one. If you have a failure during the flashing process, then you can boot from the second BIOS (the manual will tell you how to do that) and then repeat the flashing process with a good BIOS.

As for your direct question, pjmelect has it right - the clearing process only clears the small amount of CMOS memory which holds the parameters which control what the BIOS does (like what settings to use for RAM, which drive to boot from, etc). It won't fix your BIOS - sorry!
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a b V Motherboard
June 21, 2011 6:46:09 AM

^Live and learn hehe, I've not encountered any 2 chip boards yet though
Moto
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June 28, 2011 5:11:22 AM

pjmelect said:
The CMOS memory is where the BIOS stores its settings. Clearing the CMOS memory sets all the CMOS memory locations to FFH this is detected by the BIOS which then loads its default values. Clearing the CMOS does not affect the BIOS program in any way. You should always clear the CMOS after you upgrade the BIOS as the updated BIOS can use different memory locations in the CMOS memory and the different (wrong) data can cause unpredictable operation or even no operation at all.


Thank you for that involved explanation. What is FFH? Your explanation is extremely important to me, but I don't understand it yet. Does flashing replace original BIOS program or put a new program in another area of cmos memory or something else?

Will clearing cmos settings enable bootup to use original cnmos/bios settings? If so, why would one clear cmos/bios if flashing worked?

one person told me flashing produces a set of cmos/bios settings in a different memory area than the original (enabling bootup to use new bios/cmos settings) and clearing cmos enables bios to return to original cmos settings useable on bootup. Is that true?
Marv100
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a b V Motherboard
June 28, 2011 7:08:04 AM

Flashing a new BIOS erases and overwrites the original BIOS which is stored in the BIOS flash memory. This is not the same as the CMOS memory which is battery backed. When the CMOS memory is cleared all of the memory locations are set to a logic “one” and as the CMOS memory is eight bits wide this is 255 decimal or to a programmer FFH.
When you flash a new BIOS it may use different memory locations in the CMOS memory to the old version which can cause unpredictable operation or even no operation at all. Clearing the CMOS memory causes the BIOS program to set all of its settings to its default values.
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June 28, 2011 9:09:27 AM

CMOS is not even a part of the BIOS-chip. It is located elsewhere.
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June 29, 2011 1:43:41 AM

pjmelect said:
Flashing a new BIOS erases and overwrites the original BIOS which is stored in the BIOS flash memory. This is not the same as the CMOS memory which is battery backed. When the CMOS memory is cleared all of the memory locations are set to a logic “one” and as the CMOS memory is eight bits wide this is 255 decimal or to a programmer FFH.
When you flash a new BIOS it may use different memory locations in the CMOS memory to the old version which can cause unpredictable operation or even no operation at all. Clearing the CMOS memory causes the BIOS program to set all of its settings to its default values.


Thank you very much, this seems to be much clearer to me. As I understand your explanation flashing not only overwrites BIOS memory, but also may write to cmos. If the writing to cmos changes the value of those 256 bits, screwup may occur. In addition, clearing the cmos will nullify any writing flashing did to the cmos because it will all be set to logic one, and change bios back to default, (original settings), as they were before flashing. Did I understand correctly?
Thanks again for all that patience.
Marv100
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a b V Motherboard
June 29, 2011 2:08:34 AM

Flashing the BIOS does NOT write to the CMOS memory which is on a totally different chip. The BIOS uses the CMOS memory to store its configuration data. The CMOS memory is eight bits wide and depending on the manufacturer can be 64 bytes or more long.
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