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On my pc (Dell Vostro 3750, Win 7 64bit SP1) I have two video cards installed:

  • Intel(R) HD Graphics Family
  • NVidia GeForce GT 525M

I can imagine that the Intel cards is a sort og "default" card or driver, so I don't care about that.

The problem is that I can manage the video properties through the control panel of both: it seems that both cards are working.

Could you give me some explanation?

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Sounds like Optimus –  Mehrdad Jan 5 '13 at 10:31
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2 Answers

up vote 3 down vote accepted

Both cards will not be running simultaneously. Laptops generally have an automatic switching mechanism. When you require the dedicated graphics card (for intense video work) it will automatically start using it. Otherwise the HD Graphics will be used instead to preserve battery life. This depends on the laptop you are using and the drivers, but that is what happens in most cases.

Some laptops won't even use the dedicated graphics card on battery to prevent rapid power drainage.

Also in regards to the initial question posted in the title, yes two cards can work simultaneously, this is normally on a desktop PC though. High End laptops can do it though. This can be done via Crossfire or Scalable Link Interface

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Thank you very much for your answer. Does this automatic switching lower my laptop performance? In short, what's the best practice about that? Maybe I have to set my laptop to use just one video card (because I always use it connected to main supply) –  Marco Panichi Jan 6 '13 at 9:37
    
Yes it can lower your performance, but probably not when plugged in. As it is done to preserve battery life most laptops try to use the internet GPU as often as possible when away from a power source. The internal graphics card will be used when you are doing mundane tasks such as browsing windows etc. Unless it does not recognize that it needs to be switched, which can happen on occasion you will not notice any difference. –  Sam Jan 6 '13 at 9:48
    
Thank you for the explanation –  Marco Panichi Jan 7 '13 at 7:00
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Well there's actually three or 4 possible scenarios

The first is the one in your laptop, where the system switches between video cards as needed for better performance or power usage.Nvidia + intel setups use a switching technology called optimus. AMD systems use hybrid crossfire.

The second is SLI or crossfire where two cards work together.

The third is having multiple independant cards - this would work in older versions of windows if all the graphics cards used the same drivers, but having hetrogenous cards has been supported since vista. This is useful if you needed to add outputs.

Finally there's a GPU virtualisation technology called virtu which does a form of graphics card switching on desktops. This has more than one option - using the discrete or integrated card for output, and does about the same thing as the first option.

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