Question PC only using 8gb out of 16gb RAM installed

[PCnovice117]

Asking on behalf of a friend of mine. He is currently running into an issue where (I am guessing) only 8GB of ram is being seen by the operating system in contrast to the 16GBs he has installed onto his motherboard. I had advised him to run a mem-test, check if he installed the ram sticks correctly, reset cmos in case it was a setting's issue, individually check the slots and rams used to verify they all work, and double checked the OS to see if it is even capable of running with 16GBs. All the sticks are the same.

For the specs:
CPU: Intel Core i7 950 @ 3.07GHz
Motherboard: MSI Big Bang-XPower (MS-7666)
RAM: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (x2)



Couple of things I would like to note and if possible for someone to provide clarity on would be greatly appreciated.

1. For some reason he is unable to start his computer with only 1 ram stick installed, however when he uses two in the single channel configuration the PC works fine. When using more than 2 sticks he can enable the dual channel configuration. Why is it that the computer is unable to start-up with a single ram stick (4GBs) or in dual channel configuration?
2. This is from my lack of experience, when trying to access his BIOS my friend opens up the CMOS utility page. Is there no other option to obtain the system BIOS for the motherboard? Wanted to see if the BIOS is displaying the amount of ram installed to verify, but was only able to access the CMOS utility page and see that the ram's timings for the associated slots were present.

 

[Calvin7]

Memory manufacturers won't guarantee the use of multiple kits of memory to be compatible together. Typical results of using multiple kits of memory are what you have now.

 

[PCnovice117]

Memory manufacturers won't guarantee the use of multiple kits of memory to be compatible together. Typical results of using multiple kits of memory are what you have now.

How would one go about reducing or negating the risk of multiple kits of memory not being compatible with one another. Is there something on the motherboards side where I can view specs or standards that have been tested with the board to work? Or is it something I will need to further investigate with the ram manufacturer? I understand that manufacturers cant guarantee different products of ram not being able to work with one another since the testing process for verifying would be enormous, but I was under the assumption that as long as the ram sticks are of: same manufacturer, model, voltage, clock speed. etc. there should be a good chance that there is a high probability of it being compatible.