What happens if I clear CMOS when running the computer?

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grooge

Senior member
Dec 23, 2004
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Originally posted by: steimm
Might seem to be a stupid question, but what happens if I accidently would clear CMOS when the computer is up and running?
Some motherboard dont even start up when the clear CMOS jumper is in clear position. So, I guess that it will short something in your computer and you'll never have to flash the BIOS on that motherboard ever.

But, if you want to try it and tell us what happen, then you're welcome. I'm not going to risk it on my computer..

And by the way, it cannot be done accidently, unless somebody want to connect a switch on the jumper instead of having to fiddle with the jumper. And then, you could accidently use that switch instead on the others switch on the computer case...
 

dwb

Member
Oct 1, 2005
99
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I think that doing that would turn your hard drive into saurkraut, and possibly result in non-stop repeats of old Gilligan's Island episodes.

Don't do it.

dwb
 

Peter

Elite Member
Oct 15, 1999
9,640
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The non-boot is because some CMOS/RTC chips don't manage to get the system clock going unless it's been on battery power initially.
 

Greenman

Lifer
Oct 15, 1999
19,067
3,970
136
... I've always wondered what would happen if you slammed your transmission into Rev while going 55... Anyone want to try and find out?
Done it. Had a pickup with an engine and trans out of Shelby Mustang, shift linkage didn't work all that well. Was going form second to third at full throttle and hit reverse instead. Prolly doing 65 when it happened. Lots of noise, and a great deal of smoke from the rear tires, but no real damage.
 

Peter

Elite Member
Oct 15, 1999
9,640
1
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With a manual, clutch and drivetrain are going to suffer, but the gearbox might survive. An automatic box will be absolutely shredded.
 

Zap

Elite Member
Oct 13, 1999
22,377
2
81
Originally posted by: ericmast
easy answer! you'll trash the bios chip which can be replaced.
Does that really happen? Has someone tried this? I'm asking everyone, not just Eric.

My thinking would be that nothing at all would happen. I may even try this after I get a few more projects done. Too bad my Hi8 camcorder died otherwise I'd video tape it. Oh, the excitement of... nothing happening.
 

Jeff7

Lifer
Jan 4, 2001
41,599
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Some PCs will not power on with the CMOS Reset jumper closed. I imagine that there is a reason for this. Either it's one of those just-to-be-safe things, or else the full amount of power flowing through an ON PC might damage something if that jumper is closed.
 

sm625

Diamond Member
May 6, 2011
8,172
136
106
I've done it before and nothing happened. This was on a DFI athlon 800mhz.
 

Absolution75

Senior member
Dec 3, 2007
983
3
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I've turned on the computer with the jumpers set to cmos reset. Pretty similar issue. Nothing bad happened. Though I realized it almost immediately.
 
Feb 19, 2001
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You know there are CLEAR CMOS buttons on some motherboards now. I believe either my Gigabyte or Asus has it on the REAR panel.

It's obviously dumb to swap jumpers in the middle of running, but what if you hit the button in the back?
 

Kenmitch

Diamond Member
Oct 10, 1999
8,504
2,245
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That kind of stuff happens when the CMOS hits menopause... J/K I've done that before. Way back in the day there used to be information on how to do that on Shuttle's web site. Peter's explanation was pretty clear, but what I don't get is why someone didn't just try it. I was about ready to walk down to the garage and pick out a spare board to try it reading down the thread until I got to Peter's post. __________________
My eVga MB had a clear cmos button next to the power and reset buttons at the lower edge of the MB. I may have pushed it by accident a time or two on my overclocking adventures while the system was running :)

Needless to say the MB was fully functional until I sold it. As far as I know it's still working to this day!
 

marlinman

Member
Dec 10, 2006
160
1
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And by the way, it cannot be done accidently, unless somebody want to connect a switch on the jumper instead of having to fiddle with the jumper. And then, you could accidently use that switch instead on the others switch on the computer case...
A respected guy over at [H] recommends connecting the reset switch to these jumpers while testing the OC limits of a system...

I haven't done that - I'm just wondering what would happen if I did that.

I'm just thinking there is room for another product on the market...
Don't we have more than enough screwdrivers already!?!?!
 
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dawp

Lifer
Jul 2, 2005
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If I remember correctly from my years as a bench tech repairing boards for a now defunct computer maker, at least on the boards I worked on, the cmos clear jumper is tied to ground when you set the clear jumper, it just isn't tied to battery when in run mode.
 

deimos3428

Senior member
Mar 6, 2009
697
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You can't clear the CMOS while the computer is on. "Clearing the CMOS" involves removing power so the contents of the normally battery-backed CMOS RAM are lost. However, you also should not try to do so. I personally found the first response to this post by Paul to be a good read in explaining why:
http://www.eggheadcafe.com/software/aspnet/34255587/cmos.aspx

Clearing the CMOS itself will not fry your BIOS; they are two distinct components. The BIOS is a separate chip on the motherboard. The CMOS RAM is contained within the southbridge on modern PCs.
http://www.differencebetween.com/difference-between-cmos-and-vs-bios/

That said fiddling with jumpers and other metallic parts while the system is on could well fry motherboard components, including but not limited to the BIOS chip.
 

Elmarco38

Junior Member
Nov 26, 2018
1
1
6
My wife just did it. I have an aging system I’ve been too damn lazy to repair or replace so the issue of the mobo rebotting endlessly unless I reset the cmos is necessary everytime i boot the system. So I installed a small tactile switch just for this purpose. I press the switch prior to booting to clear the cmos. It boots up fine after pressing F2. My wife did not realize the pc was on and pressed the tactile switch and boom everything powered off immediately. We pressed it once more rebooted and had to press F2 like normal then it did successfully reboot but it took a really long time to do it. It really hosed up something but it successfully came back and we are streaming Netflix as we speak.
 
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VirtualLarry

No Lifer
Aug 25, 2001
54,558
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My wife just did it. I have an aging system I’ve been too damn lazy to repair or replace so the issue of the mobo rebotting endlessly unless I reset the cmos is necessary everytime i boot the system. So I installed a small tactile switch just for this purpose. I press the switch prior to booting to clear the cmos. It boots up fine after pressing F2. My wife did not realize the pc was on and pressed the tactile switch and boom everything powered off immediately. We pressed it once more rebooted and had to press F2 like normal then it did successfully reboot but it took a really long time to do it. It really hosed up something but it successfully came back and we are streaming Netflix as we speak.
Pretty sure my old KT400 MSI mobo, had a warning in the manual not to do that, it said that you could damage the board, by clearing the CMOS and applying power while it was in the "CLEAR" jumper state.

Edit: About the problem with the board - maybe you just need a new CMOS battery?
 

Theogstoner

Junior Member
Mar 20, 2019
1
0
6
Well there is surely a risk to CLRCMOS without removing all the cables, but that is something that I never do. I have always cleared CMOS with power cable connected to PSU but of course with the computer turned off.

And I sure do understand that it is no good idea to clear CMOS while it is running, but I was just curious if someone knows what would happen. I haven't tried it yet, but what can really happen?

- Crash boom bang - everything goes black? All the stuff inside the computer destroyed for all future use?
- Crash boom bang - everything goes black, but it would work after a cold boot again but w/ risk for BIOS is crashed?
- Nothing at all until you reboot and BIOS is destroyed?
- Nothing at all until you reboot and BIOS-settings is back to original?
- Nothing at all?
So i did this while installing a new processor and now i cant clr my cmos to get my display back so would not do this while the computer is on believe i have to get a new motherboard but will let you know
 

shiblysi

Junior Member
Jul 27, 2019
2
0
6
Restart happens. My pc was stuck in error code something so after getting the same error several times my patience was lost and I pressed clear CMOS button of my motherboard and it just restarted with default settings.
 

Yeroon

Member
Mar 19, 2017
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I pulled the cmos battery out of a G45 board the other day while it was running. I knew it was a bad battery and thought I could swap it, and the psu power would keep it running. Nope. Hard shutdown. Put the other battery in and while the bios was obviously back to defaults, nothing wrong at all.

As for putting a trans in rev on the highway, I've accidentally done that too. 4L60e - got a tire squawk, then coast since the engine shut off. Trans went another 80K km before towing finally did that one in. Not that 4L60e's are known for longevity either.

Yes, I know this is a triple necro'd thread.
 

thilanliyan

Lifer
Jun 21, 2005
11,659
1,762
126
Was wondering the same thing. I have the reset switch wired to the clear cmos jumper as this is a custom case and it's a pain in the butt to remove the panel and find the buried jumper if I need to reset the BIOS.

Just worried about an accidental press of the reset switch while it's running. Maybe I should put some cover over it to prevent accidentally pressing it.

Have to say, the Asus Z87-Expert motherboard I had previously handled failed overclocks much more elegantly than this MSI X570 board I'm using now. The Asus board would basically try several boots before it resets to default without my intervention. The MSI board seems to just keep trying to boot, requiring a CMOS clear.

I know, necro of a necro... :D
 

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