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So I have an older Intel S5000XVN motherboard, here are the specs

CPU Arch : 2 CPU - 2 Cores - 2 Threads
CPU PSN : Intel Xeon CPU 5160 @ 3.00GHz
CPU EXT : MMX, SSE (1, 2, 3, 3S), EM64T, VT-x
CPUID : 6.F.6 / Extended : 6.F
CPU Cache : L1 : 2 x 32 / 2 x 32 KB - L2 : 4096 KB
Core : Woodcrest (65 nm) / Stepping : B2
Freq : 2985.21 MHz (331.69 * 9)

MB Brand : Intel
MB Model : S5000XVN
NB : Intel 5000X rev 31
SB : Intel 6321ESB rev 09

RAM : 16384 MB FB-DDR2 
RAM Speed : 331.7 MHz (1:1) @ N/A
Slot 1 : 2048MB (5300)
Slot 1 Manufacturer : Kingston
Slot 2 : 2048MB (5300)
Slot 2 Manufacturer : Qimonda
Slot 3 : 2048MB (5300)
Slot 3 Manufacturer : Kingston
Slot 4 : 2048MB (5300)
Slot 4 Manufacturer : Qimonda
Slot 5 : 2048MB (5300)
Slot 5 Manufacturer : Kingston
Slot 6 : 2048MB (5300)
Slot 6 Manufacturer : Qimonda

As you can clearly see, CPU-Z says I have 16GB of PC2-5300 RAM installed.

For some reason in both BIOS and in Windows the maximum usable RAM is 12GB instead of 16GB. I have a dedicated video card connnected, so it can't be stealing RAM to use for the GPU (the S5000XVN does not have any onboard video).

I am running Windows Server 2008 R2 x64 as the primary OS - so there should not be a memory limitation imposed on me from the OS.

Has anyone experienced this before? Any ideas on how I can actually use all 16GB of RAM?

I plan on using this machine to use for a Hyper-V server and have x2 Quad Core Xeon CPUs on the way as I write this post.

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could you throw a linux livecd on and run memtest as well as try booting off it to absolutely make sure its not a software thing? – Journeyman Geek Sep 13 '12 at 0:48
Thats not a half bad idea actually.. I'll try that and report back. The issue is even in the BIOS it says 16GB total - 12GB Available... I cant seem to figure out where that 4GB of ram is going to tho.. – Richie086 Sep 13 '12 at 1:06
Well, Its not the OS's limitations or the motherboard. And i figure if you're going to boot a livecd to check the hardware, might as well doublecheck. – Journeyman Geek Sep 13 '12 at 1:12
In the CPU-Z report, it only lists RAM in 6 slots. At 2GB each, that's only 12GB. Am I missing something here? – techturtle Sep 13 '12 at 2:27
up vote 3 down vote accepted

You may have memory sparing activated, which means that 2 of the 8 memory modules are reserved in case one memory module fails. See the manual for details on this and you should disable this option in the BIOS if you do not want it.

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Yep, it was memory sparing that was causing my issue. thanks! – Richie086 Sep 17 '12 at 0:37

Your motherboard supports up to 32 GB of RAM, so thats not the issue.

Have you tried each stick of RAM individually? One of them could be bad.

Once you establish one as good, try it in all the slots to see if a slot is bad.

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