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I'm running Windows 8.1 on an HP DX5150 (small form factor), AMD Athlon 4400 2.20Ghz. I have four 1GB sticks of DDR RAM installed, but Windows says that only 3.06GB is "available" (although it says 4.00GB is installed).

I've swapped out each of the 1GB sticks with a 512MB stick (so 3.50GB installed), and have switched around which DIMM I put each stick in to, but still Windows says 3.06GB is available. BIOS says 4GB RAM (and 3.5GB RAM when the 512MB stick is switched in), so BIOS is reading it.

I've run Memtest86, which only recognizes 3.06GB; Memtest86 says no errors on the 3.06GB, so it's not really helpful. Msconfig advanced boot options does not have 'max memory' enabled so there is no Windows-set cap. And I've enabled memory remapping.

Any ideas of what the problem is?

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    First question... Are you running windows 8.1 64-bit or 32-bit? Also are you using onboard graphics or discrete (PCI-e) graphics? – ianc1215 Nov 30 '14 at 4:48
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    Enter "4GB RAM" in the search box at the top of the page and you will get 1,224 previous questions on this general topic. The odds are high that your answer is waiting for you. We try to avoid reinventing the wheel on common questions. If you don't find an answer, update your question to identify what is different in your situation, which will jump-start diagnosing the problem. But you understand that "available" memory is what is left after loading the OS and associated infrastructure? – fixer1234 Nov 30 '14 at 5:02
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    Sometimes shared video ram takes from hardware ram – Sun Nov 30 '14 at 5:45
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    possible duplicate of Why does Windows only show about 3.5 GB of my more than 4 GB of RAM? – bwDraco Aug 26 '15 at 22:04
  • en.wikipedia.org/wiki/PCI_hole – bwDraco Aug 26 '15 at 22:05
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The usable memory may be less than the installed memory on Windows computers

On a computer that is running Windows 7, the usable memory (RAM) may be less than the installed memory.

For example, a 32-bit version of Windows 7 may report that there is only 3.5 GB of usable system memory on a computer that has 4 GB of memory installed.

Or, a 64-bit version of Windows 7 may report that there is only 7.1 GB of usable system memory on a computer that has 8 GB of memory installed.

...

The reduction in available system memory depends on the configuration of the following:

  • The devices that are installed in the computer and the memory that is reserved by those devices
  • The ability of the motherboard to handle memory
  • The System BIOS version and settings
  • The version of Windows 7 that is installed (For example, Windows 7 Starter Edition only supports 2 GB of installed memory.)
  • Other system settings

The above applies to Windows 8 and 8.1 as well.

Source The usable memory may be less than the installed memory on Windows 7-based computers:

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