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I have a quad core processor. AMD Phenom II x4 965 3.4 MHz Which is about a year old. It has never been overheated and has never really been stressed for any long periods of time. I don't think I've seen the usage get over 50% even when doing heavy computing. Its a sweet lil processor :)

When my Motherboard boots up it always says how many cores it loaded. Recently it is now only loading 3.

It states roughly "3x cores loaded (0,1,2)"

I am also monitoring my CPU temps with windows 7 gadgets and that too now only picks up 3 cores?

What would have caused a core to vanish all of a sudden? Do I need to update BIOS and what do I risk in doing that?

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If you've never seen usage go higher than 50% during heavy processing I'd take that as a sign something's wrong. At best you have a serious bottleneck somewhere. – Phoshi Jul 1 '11 at 21:49
late comment. The core just popped up. not sure how or how long itll stay. Not sure if something got updated or not. – sealz Jul 31 '11 at 14:11
up vote 4 down vote accepted

It sounds like the processor might be defective. You should contact AMD or your computer manufacturer to request a replacement. Many processors have manufacturer's warranties covering the product for one year or longer.

In case you would like to do further diagnostics, try downloading CPU-Z or a similar tool that can get some basic information about the processor and display it to you.

You could also try updating or re-flashing the BIOS. It's unlikely, but possible, that the BIOS has a bug that is preventing your processor from working correctly. Keep in mind that there is always a risk in updating the BIOS. If you lose power during a BIOS update, it might render your system unusable. Be sure to keep a charged battery in place if this is a laptop, and don't update BIOS during a time when the power supply is unreliable (like during a storm).

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Power outage because of storms? Where do you live exactly? :p – TFM Jul 1 '11 at 21:30
@TFM - Where do you live that the power has never once gone out due to weather? The desert? – Shinrai Jul 1 '11 at 21:46
Maybe he lives in a power plant... – surfasb Jul 1 '11 at 22:44
somewhere where your power lines are underground possibly. I've never had power issues during storms due to that. – Journeyman Geek Jul 2 '11 at 1:26
Of course, I didn't mean to be disrespectful. But worrying about power outages caused by storms during a BIOS update sounded a bit extreme. I mean, how often to you update a BIOS, and what's the chance of getting hit by a storm which causes a power outage (not all storms do that!) exactly at that moment? – TFM Jul 2 '11 at 5:39

Check Msconfig, Boot Tab > Advanced Options

Number of Processors should be Unchecked, along with everything else, if it is not, uncheck it, hit OK and reboot. Unchecked means use all cores and threads.

Some more info in my post here

enter image description here

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I have checked that too. Its unchecked and like everything else max is 3 – sealz Jul 2 '11 at 4:45
Is there anything in the bios that would help? – Moab Jul 2 '11 at 14:35
I am not to sure I haven't had the time to check it out but will soon – sealz Jul 2 '11 at 18:32

Today, I have had the same problem. I have exactly the same quad core processor [AMD Phenom II x4 965 3.4 MHz] and my OS is Windows7 (64).

Last night I updated the BIOS from BIOS 0902 to BIOS 2001.

Today I have found that I had been left with 3 cores instead of 4.

After some internet search for help, I suspected that the problem was in BIOS settings.

So I have done the following:

  1. restarted the PC.

  2. in "activation of cores" logo screen, I have pressed [DEL] and entered the [BIOS setup] screen.

  3. then I have chosen "Advanced".

  4. in that screen I have chosen "CPU Configuration".

  5. then I have seen the "CPU Core Activation" line. It was set to [Manual].

    Under it, was:

    2nd Core [On]

    3rd Core [Off]

    4th Core [On]

  6. so I have changed "3rd Core" from [Off] to [On],

  7. also changed "CPU Core Activation" from [Manual] to [Auto] (that action was not necessary),

  8. pressed [F10 Save and Exit],

  9. pressed again [F10 Save and Exit],

and that was all.

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Sounds reasonably, +1. Always reset BIOS to defaults after update. – gronostaj Oct 24 '14 at 10:50

Unless the AMD processor have some exceptionally smart redundancy built in I doubt there is something wrong with your processor. It is more likely that either Windows is not picking up the correct count or that your BIOS is failing.

Also remember that even if you are shutting down your computer there usually is power on the mainboard. None of that should be reaching the CPU but you can try to unplug your computer from the wall, hit the powerbutton (to dischagre any capacitors), plug your computer in the wall socket again and start up to see if something changes.

I also suggest starting up from a linux live cd, go to a terminal and write cat /proc/cpuinfo.

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