Entire RAM not usable (repost)

Xttony

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I have 16gB or 8gB ram installed at a time but can only use 5.91 in either case on my main OS. I recently installed the same OS on another drive for backup, if virus screws up my current installation. Anyway this new installation uses almost the entire ram. Why does one OS use the entire ram 16gB or 8gB installed and the other only 5.91gB?

I also tried different ram cards and everytime my main OS was stuck on 5.91 if I gave beyond 6gB but the backup OS used full.

Both OS are installed from the same DVD.
My main OS is on a SSD with a lot of installed files so reinstallation will me a issue. Activated
Backup OS on a 6year old 160GB hard drive. Not activated

Additional question
On my backup OS there is still 39mB reserved and I think less than 10mB should be reserved for computer components and I/O. The rest should be consumed for graphics ram, but I donot have internal graphics on my motherboard.

CPU AMD FX8350
Ram corsair vengeance 4x4gB or corsair XMS 4x2gB
GPU AMD 290X
OS Windows 7 Ultimate 64bit

I think my problem is software based and not hardware based also I tried everything on the hardware level
 

Xttony

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Yes I did that and also I tried to uncheck the maximun memory and put different values in the box. But after rebooting it just won't go over 5.91. Also I noticed that after putting a value in the ram to be used box, I save it by clicking on ok but if I go to advanced again the maximum value check box is ticked and there is some arbitrary value in the box below.