Solved CMOS Clarification

April 16, 2011 at 19:17:58
Specs: Windows Vista, AMD Phenom x4 2.3GHz
I've thoroughly researched CMOS and BIOS but I haven't found an answer yet. Here is what I need clarified;

The CMOS is a semi-conductor chip that stores hardware configuration information and can be located on the BIOS chip or elsewhere on the motherboard. It uses static ram (does not need to be refreshed) and is sometimes referred to as NVRAM (non-volatile). It is powered by a lithium coin cell battery, which powers the RTC as well. But if the CMOS uses SRAM and is non-volatile memory, then why does it need a power source? Since it is non-volatile it should "remember" its information, even without a power source.

This might seem like a minor point but it's driving me bonkers. Any feedback would be appreciated.

Windows XP Pro SP2
AMD Athlon 64 3200+ 2.0GHZ Venice Core
Gigabyte GA-K8N-SLI Motherboard
1GB Geil DDR400 Dual Channel
ATI Radeon X800 GTO Fireblade Edition 256MB DDR PCIE


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#1
April 16, 2011 at 20:41:43
That does seem to be a paradox. As near as I can tell cmos memory is non volatile because of the battery. When the computer is running the cmos circuitry is powered by the PSU. When the computer is off it retains its memory by running off the battery.

Obama's a 2012'er. That explains EVERYTHING..


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#2
April 16, 2011 at 20:43:15
✔ Best Answer
The CMOS data is a small part of the BIOS data , but it's not all of it.
The BIOS chip is a combo chip, not just the CMOS part - it has both non-volatile and volatile data. There is non-volatile data, the majority of the data, that does not need power to retain it, in the (e.g. 4 mbyte) EEPROM part of the chip (Electrically Erasable Programmable Read Only Memory) and volatile data in the tiny (e.g. 64 byte) CMOS part of the chip that requires at least a minimal voltage and a minimal current from a battery, rechargeable or not rechargeable, to retain at least the current Time and Date, and any other User set settings that are not defaults, when the computer is not running or when all other power to the mboard has been removed..
When you clear the CMOS, e.g. by moving a jumper then moving it back on a desktop mboard, or when you remove the battery for a short time then re-install it, you are only clearing the data for the current Time and Date, and any User set settings that are not defaults.
Some of what you see in the bios Setup is determined automatically by the bios for particular ram and cpu the mboard has installed in it and the drives connected to it every time you boot the computer - it does not require a battery and the user can't change the settings of most of that data.

http://computer.howstuffworks.com/b...
Excerpt:
The first thing the BIOS does is check the information stored in a tiny (64 bytes) amount of RAM located on a complementary metal oxide semiconductor (CMOS) chip. The CMOS Setup provides detailed information particular to your system and can be altered as your system changes. The BIOS uses this information to modify or supplement its default programming as needed.
Most of what you see in Setup is on the EEPROM part of the bios chip. Most of the data on the EEPROM part of the chip is not visible in Setup at all.
The EEPROM part of the chip is what you change data on when you flash the bios to a different version, although a small part of that data is for the Boot Block portion, which is not normally changed when you update or downgrade the bios version.

In the case of modern laptops and netbooks (about a dozen years old or newer) , they have a second chip that user data is stored on, including the password info, that cannot be cleared by removing the power to the chip. Clearing the CMOS or removing the CMOS battery then re-installing it does NOT clear the user data on that second chip.


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#3
April 17, 2011 at 04:28:03
Dave has it. It is called non-volatile because of the battery, to differentiate it from main memory which loses it contents as soon as power is removed.

The main thing about CMOS memory is that it has a very low power requirement. The CMOS battery typically powers the CMOS memory for five years or more.. CMOS memory is also very slow compared to other type of memory but with only a few bytes to contend with, that is not a problem.

Stuart


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#4
April 17, 2011 at 22:49:07
Thanks for the feedback. It makes sense that it requires power BECAUSE it's non-volatile. I was just looking at it from the wrong perspective.

Incidentally, I learned something else too. I was also wondering why my BIOS settings didn't change at all when my CMOS battery died, only the RTC. It was apparently because I never changed any of the default settings. Reading the explanation cleared that up too.

Thanks again.

Windows XP Pro SP2
AMD Athlon 64 3200+ 2.0GHZ Venice Core
Gigabyte GA-K8N-SLI Motherboard
1GB Geil DDR400 Dual Channel
ATI Radeon X800 GTO Fireblade Edition 256MB DDR PCIE


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#5
April 17, 2011 at 23:47:35
I think there's a misunderstanding in your OP about the difference between static and dynamic memory. Both require power to retain their contents but, in addition, dynamic memory has to be accessed at regular intervals to retain its data - this is what is referred to as "refreshing".

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#6
May 18, 2011 at 08:51:30
It does NOT make sense that it needs a battery if it is nonvolatile. It would make sense if it is volatile, then it would need a battery or power source to maintain its information. What I have determined is: in modern computers, it still needs the battery to maintain the time (RTC).

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#7
May 18, 2011 at 11:18:53
It does NOT make sense that it needs a battery if it is nonvolatile.

It is the battery that makes it non-volatile to differentiate it from main memory which loses it contents when the mains power is removed. It makes a lot of sense.

The only other memory that retains its contents when power is removed is the BIOS and that is called Read Only Memory.

Stuart


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#8
May 18, 2011 at 21:11:18
You're applying 'nonvolatile' too precisely. You turn off the computer but cmos retains its data. That's as far as you need to go. Don't over-think it.

Ding dong the witch is dead. . . .


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