Installed Memory not showing up in a late 2009 Dual Core Mac Pro

Question

I have a late 2009 Dual Core Mac Pro.
I restarted the my computer today, an a utility popped up saying something about I need to re-arrange the memory in my mac because it was not optimized.
I thought this was weird because all the slots are filled with 2 gigs chips for a total of 16 gigs.
When I checked about this Mac it said I only had 12 gigs of memory in my system.
I went to open up the case, and there are to red light glowing for slots 7 and 8 on the motherboard, and any memory chips I put in those slots do not show up.
Can anyone tell me what is going on and how to fix it.
There was a power outage the other day, but I keep my machine on a surge protector.
I generally try to take really good care of my computers so seeing something wrong out of the blue like this is a little freaky for me.
Any help would be great.
Thanks
Stan

Answer 1

Each "side" is operated independently, so slots 1, 2, 3 and 4 are one set, and 5, 6, 7, and 8 are an independent set. The third and fourth slots on each side share a memory channel, so a failure of one module in these two slots could possibly disable the other one.
The Mac Pro uses the Error Correction Code hardware built into its Xeon processor to provide single-bit (in a word) Error Detection and Correction in Hardware. It also can detect double-bit errors, but these cause a kernel panic, machine-check, error overflow or uncorrected error. This is by design, to avoid poisoning you data.
The Error Correction Code hardware is used in a special way at Startup. Any failure (correctable or not) that occurs in the first few seconds while running the Power-On Self Test causes your Mac to declare the slot(s) of those modules "empty" and simply not use them in the running system. Vigilance is required, because the next time you run this test, those Bad modules may pass.
So the movie version is that you have some DIMM(s) that have failed, and you should remove them from your Mac and consider whether to replace them with good modules, or to add some larger modules.
I expect your recent power failure, protected by a UPS, is merely a coincidence, not a cause-and-effect.