Out of 12 GB, only 8 GB of RAM is usable in Windows 7 Home edition 64-bit

Solution 1:

Two likely possibilities

  1. Windows 7 Home Basic can only address up to 8GB of RAM regardless of 64 bit. If you're on Home Basic, you can't address your additional 4GB.
  2. You have a graphics chip that shares the RAM, in which case it will allocate part of it to graphics. You can check this with the following steps:

Windows Vista and Windows 7

Press the Windows key, type Control Panel, and then press Enter. Under the Appearance and Personalization, click Adjust screen resolution. On the middle of the right-hand side of the screen, click the Advanced settings. A new window should appear with your video adapter's properties; similar to the image shown below. Your video card's memory is under the Adapter tab in the [Shared] Video Memory: section (shown below).

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