Strange RAM behavior - overclock CPU, 1/2 RAM disappears

[Matrices]

(I think this began happening after updating BIOS on MSI GD80 motherboard, after initial flash attempt went bad. This is on Vista 64 with Corsair DDR 1600mhz memory.)

Here's the problem: task manager only recognizes 2GB of RAM, even though BIOS and system properties shows 4GB of RAM. This happens when I overclock the "frequency" of the CPU, i.e., 19x190 mhz or 20x190 mhz, and run the RAM on a divider.

It's kind of convoluted. At first, I thought the problem only happened if I enabled XMP - Corsair RAM only shows up as 1333mhz rated, but when I enable XMP, it shows up as 1600mhz, as it should. So I disabled XMP and the problem went away, or so I thought.

But the problem persists. If I enable XMP and set everything to auto with the RAM, all my RAM appears again, but that automatically makes my CPU "frequency" run at 160mhz. If I increase frequency to 190mhz, then I lose half my RAM in Windows.

No idea what the fuck is going on here.

 

[Matrices]

This is a queer problem. I figured out why, when I enable XMP, the multiplier is forced to 160 - that's the number that produces exactly 1600mhz DDR3 speed when the divider is set to Auto.

So I disabled XMP and chose some numbers and dividers that would also produce 1600mhz memory speed, and all my RAM showed up again. If I chose a different combination of multiplier and frequency and divider - which results in 1520mhz RAM speed - only 2GB shows up.

What is this craziness?