Through out last month Ive seen few posts about disappearing ram people with 6gb would see 4gb and with 12gb see 10, 8. 6gb etc.
There is a lot of good points made in this tread and through out the forums. I just tried to put it all together.
I would look at situation with missing ram from two sides Lets call it situation A) and situation B)

Situation A) :
 New everything. You just got all the Parts and putting it all together. Prior to installation its good to check all you slots and make sure there is no dust or debris. Use Dust canister or other compressed air to clean it all. It might be also as good idea to clean all you Ram contacts with Alcohol to remove any factory residue. Same with CPU. Make sure there is no bent pins in the socket. Keep in mind that even slightest misalignment could cause a hoax.

 
^ good socket.

 
^bent pin (Top of the picture)
 
Put your ram in appropriate slots (if 6gb)  Everything at stock, only ram set to manufacturer specs. Set ram voltage to 1.65. DO NOT USE XMP profiles SET YOU RAM TIMINGS MANUALLY.
Note: For people running 12gb+ upping Ram voltage to 1.66-.168 in bios sometimes necessary in order to achieve stability.
Check out those theads there is some very good info there.
http://www.overclock.net/intel-memory/703820-ram-not-showing-up-post.html
http://i4memory.com/f54/asus-x58-6-12gb-memory-overclocking-detecting-less-memory-than-installed-19200/
If you experiencing "Dropouts" (at Post not all ram preasent) Take all but one stick and Memtest it 8 passes min. http://www.memtest.org/#downiso
If all comes up clean. Put it all back again. test. If it still drops out. Check in windows which slot not present and swap sticks around. If a different slot missing: Its you ram. If its the same. Than next step would be trying to loosen your CPU cooler a bit, and or resit CPU. Test it again. If you still missing ram. You board might have a bad ram slot or a defective cpu. Ideally it would be nice to check it in the other machine, If its not an option send your board back describing the steps you've done, I don't think you'll have a problem returning it.
Note: EVGA board are always a bit picky about the ram. So far the best brands that got really good ratings from the forum members are:
Mushkin - seams that people got really good experience with this brand and the price is very competitive.
Corsair - good quality sticks thous some had issues with the earlier versions.
Gskill - Another good brand that seams to work pretty good with EVGA boards.
Even Im myself running OCZ ram I would not recommend it. Since a lot of members here had really bad experience with this brand.
         One thing though worth mentioning Is Max memory option in System Configuration
Control Panel/System&Security/Administrative Tools/System Configuration/Boot/Advance Boot Option/ - now some people say uncheck Max memory some say check  the box. In my situation Checking it would cause 1, 2, 2,5 3, gb disappear, the amount of Memory Low Gap in bios that i set.  If its 2.5 (that what I have now) it would result in 2,5 gb being reserved for PCI devices. I dont know why and dont have any expanation to it But that what happened in my situation. Try playing with it too.
Good read on the subject:
http://i4memory.com/f80/memory-low-gap-settings-16090/
[link=tm.aspx?m=125673]http://forums.evga.com/tm.aspx?m=125673[/link]   post #3
Another thing to mention is Dram Ratio setting in bios. Set it to a specific value according to your ram effective frequency. 1066,1333. If running stock. 1600, 1866, 2000, if overclocking.

Situation B) :
Everything new passed at stock with flying colors. And you are establishing your Ram max clock. You work your way up increasing BClk 10 or 5 point at a time There are a lot of great threads through out the forum about this.

That will give you couple of Ideas.

http://forums.evga.com/tm.aspx?m=5247

http://forums.evga.com/tm.aspx?m=5266

http://forums.evga.com/tm.aspx?m=7528 

So say you are at you ram rated speed say 1600mhz (when finding out ram max speed I try to use 2:10 divider meaning raising BCLK you raising your ram in a nice ratio 133 * 10 (RAM divider) 1330 mhz 160 * 10 = 1600 mhz. Why do I prefer to do it like that? Well my reasoning was at 1600 mhz my lowest CPU speed (always Twice RAM) would be 3.2ghz Most i7 are able to reach that overclock without too much tempering with Bios. Mine did all on auto besides RAM voltage 1.65 and VTT +100 (in that case) So it eliminates a lot of tweaks leaving you with only ram settings and VTT voltage (Internal MC voltage)
Just a note: Keep in mind All slots populated and high ram frequencies put a lot of strain an all the parts, So If you got a weak memory controller you might not be able to run 12gb at 1800, 1866 or 2000mhz ever with particular chip.
            Say, you just skipped some steps and here you are loading at 3,2 ghz cpu and 1600 mhz ram and only 10gb present on post. Get in bios and without any changes start dropping your BCLK till missing stick appears again. This is important. Keep in mind that not all ram will be working at top speed they are rated for. It all depends on the Ram you are using and how strong you Internal memory controller is. If you Ram shows up then you are at  max ram clock on stock cpu settings. Meaning that that's your ram max speed and you wont be able to squeeze any more. If you want more you have to get other ram or CPU depends on which is the weaker link.
Here is few thing to try in order to get slight ly higher BCLK.
FirstlyI would change Is QPI PLL you good all the way to 1.4 they say. I wouldn't go over 1.375
Mine is at 1866mhz is 1,325. Your safe starting point would be 1.225v
If your ram is all there  Get in windows Run Prime 95 and Linx Pack 20 pass minimum  to be sure everything runs great.
Get back to bios and start raising BCLK till it disappears.
Do QPI pll tweak. Test with Linx.
Another thing to try is TRFC setting you might go as high as 110 on it if necessary. People who run 12gb might have to set it to 100 right away.
Another two timings that worth trying are: look at the picture

Shamino
      \\ tRFC: This is also very dependent on how high you're running the Memory frequency and the voltage you feed Memory. 88 seems to be a high enough spot when you're trying to max out the Memory Frequency, and when you do sub 70 timing on this one, be prepared to feed enough VDIMM. 59 is a great performance value, but you may not get the same overclocks or may need a big amount of voltage when you're running 2000+ Frequencies.



http://g.imagehost.org/0180/2_6.jpg

    quote: ORIGINAL: Shamino quote: ORIGINAL: AuDioFreaK39

     -  Very nice! Much appreciated Shamino, thanks for your efforts. What are tsrRdTRd and tsrWrTWr timings responsible for doing?
       -  no idea , 6, 6 works good for me so i just set that///

Here what i did in my case (going for 1866mhz , dropping from 6 to 8gb at 1833mhz)

 So I set QPI to 1.3 and changed above mentioned two timings and 183 BCLk pass with everything all ram still available. Nothing bad happened.
     I upped QPI to 186 :
1st post 12gb (f10 and exit)
2nd post 12gb (f10 and exit)
3rd post 10gb (f10 and exit)

Something else needed be changed. I tried QPI pll 1.325

1st post 12gb (f10 and exit)
2nd post 12gb (f10 and exit)
3rd post 12gb (f10 and exit)

 Success. Got in windows passed all tests again, all ram was there. More yet I can post with 187 QPI now all 12gb there... anything above 187 leads to disappearing ram again.
(Im still playing with it, but i think Ive reached the limit with 12gb speed) Remember Its 12gb at 186 BCLK 2:10 ram ratio. Pretty good.
Other seting is TRL timing in bios try setting it to +2(only available in the newer BIOSs. Refer to this thread to track changes in the bios versions. Thanks to Davabled for keeping it all together.
  [link=http://forums.evga.comarchive/tm.asp?m=100618335]http://www.evga.com/forum..hive/tm.asp?m=100618335[/link]
http://forums.evga.com/tm.aspx?m=6305
 
Thanks to rjohnson11 who made another troubleshooting thread. Making the issue more relevant. so check it out.
 
http://forums.evga.com/tm.aspx?m=479626
 
And the last. If you got you board and updated you bios right away rolling back sometimes might fix the issue as well. Im still running s22d bios since its worked the best for me so far. But don't hold me to it cause I dont update bios often: if it works don't break it. :)
 
I would like to thank all forum members who shared there thoughts and experience with this particular issue. Special thanks to  Linuxrouter for giving me some good insights on the problem and for support. HalloweenWeed for keeping it real and helping so many people. :)


I hope it was of help.

I have the same issue when testing overclocks.  12 GB turns into 8 GB.  I have been upping the VTT, but the manufacturer says 3.75 is the limit.  I haven't been messing with QPI PLL VCore much, so I'll give that a shot next.  Gets a bit tiring getting the RAM to detect and the watch it fall off when you get a "successful" overclock.

As it happens, doing the i7 certified memtest86+, I still get errors with all default values(ram timings entered).  And this is new ram tested by mushkin via an RMA I just did.  Seems like the first bank is the one to freak out.   

I'm testing now with the 1.325 QPI PLL Vcore and +125(3.75v) VTT settings at a 4.2 Ghz overclock.  Perhaps I'll get fewer errors now than I did at the defaults.

Hey, very informative thread.  I'm not having a problem with 12 gb dissappearing - I'm having trouble getting it to appear in the first place.
   EnochRoot, I'd love to see your results.  I am running the same Mushkin Ram and have not been able to get 12gb running stable with stock settings let alone an overclock.  I've spent the last two nights noodling settings but with no success.  Sometimes 8gb more often 10gb.  (Same problem other people have where the system properties shows 12gb but task manager and monitors show only 8 or 10gb)  I did get 12gb to register with the system with a high CPU VTT but the system was very unstable.  I have been running the system for a while with 6gb and a modest 3.4ghz OC with no problems.

I've been trying to get the 12gb to work stable with these settings.  I've tried various CPU VTT and Dimm voltages combinations but no success so far.

Memory Frequency: 1600MHz / 2:12
tCL Setting: 6
tRCD Setting: 7
tRP Setting: 6
tRAS Setting: 18
Command Rate: 1T
DIMM Voltage: 1.65V

   If you don't mind me seeking some advice as I'm running out of ideas myself.  Have you or anyone else had problems with this board and 12gb of mushkin 998691 memory at stock settings?  If you have run 12gb of this ram could you let me know what settings did work for you? 

  I should probably try testing each of the new modules one by one for errors. 

Hello Evrybody,

I have the same problem ,but only on the first boot of the day .
I have 6 Gb of GSKILL Trident Pc 16000(9/9/9/24,1t@1.65Volts) and when i boot for the first time  ( when my mobo is cold i think)the bios and the OS only see 4 Gb of ram usable . In windows 7 64 ,the system show the 6GB but only 4 Gb are usable .In the bios i saw only 4Gb of Ram installed .So I stop my Computer completely (Not a reboot!) and i restart it and now the bios show me 6Gb of ram installed and windows too !!!

Heri is my bios template

Mother Board ( EVGA X58 Classified E-760 )
Bios ( S61D )
CPU ( Ci7920DD0 ) 
CPU Cooler ( Noctua  )
Memory ( Gskill Trident Pc 16000@9/9/9/24,1T@1.65V )
PSU ( Coolermaster Real Power 1000 W)
GPU ( GTX295 )
Drivers ( 191.xx )
Operating System ( Windows 7 64 Ultimate )


Frequency Control
CPU Clock Ratio ( 21X )
CPU Host Frequency (Mhz) (200)
MCH Strap ( auto )
CPU Uncore Frequency (Mhz) ( 4000Mhz { 20X } )
CPU Clock Skew ( 0 ps )
Spread Spectrum ( Disabled )
PCIE Frequency (Mhz) ( 100 )


Memory Feature        
Memory Speed ( Standart )
Memory Control Setting ( Enabled )
Memory Frequency ( 2000Mhz / 2:10 )
Channel Interleave Setting ( 6 Way )
Rank Interleave Setting ( 4 Way )
Memory Low Gap ( Auto )
tCL Setting ( 9 )
tRCD Setting ( 9 )
tRP Setting ( 9 )
tRAS Setting ( 24 )
tRFC Setting ( 74 )
Command Rate ( 1t )


Voltage Control
EVGA VDroop Control ( Without VDroop )
CPU VCore ( 1.350 )
CPU VTT Voltage ( +350 )
CPU PLL VCore ( Auto )
**IOH PLL VCore ( Auto ) 
DIMM Voltage ( 1.65 )
DIMM DQ Vref ( +0mV )
QPI PLL VCore ( Auto )
IOH VCore ( Auto )
IOH/ICH I/O Voltage ( Auto )
CPU Impedance (Auto )
*QPI Signal Compensation ( Auto )
ICH VCore ( Auto )



CPU Feature
Intel SpeedStep ( Disabled )
Turbo Mode Function ( Enabled )
CxE Function ( Disabled )
Execute Disable Bit ( Disabled )
Virtualization Technology ( Disabled )
Intel HT Technology ( Disabled )
Active Processor Cores ( All )
QPI Control Settings ( Enabled )
QPI Link Fast Mode ( Enabled )
QPI Frequency Selection ( 4800 GT/s )
OC Recorvery ( Enabled )

PnP/PCI     PCI Express
Maximum Payload Size ( 128 )

Ohh!  I'm closer!  Just got 2:12/1600Mhz running.  Seems pretty stable so far.  Running some more tests.  But under initial full load no problems.  I still can't get it Manu timings stable at 1600 but at least another step closer.  All I done for this attempt was to change the mem divider to 1600/2:12 and left everything else at the settings that worked with the 2:10 divider.
So lower timings (see below) and It never occurred to me to try a lower voltage.  1.6v  works good with these timings. 

Should I try the stock timings 6,7,6,18 with a tRFC of 88 instead of auto (56) and see how it fairs with the 1.6v?  It seems the more voltage I put through the less stable it gets, Yet muskin recommends upping the VTT a few steps and Dimm voltage to 1.65/1.7 for 12gb

Memory Speed ( Standard )
Memory Control Setting ( Enabled )
Memory Frequency ( 1600Mhz / 2:12 )
Channel Interleave Setting ( 6 Way )
Rank Interleave Setting ( 4 Way )
Memory Low Gap ( Auto )
tCL Setting ( 8 )
tRCD Setting ( 8 )
tRP Setting ( 8 )
tRAS Setting ( 19 )
tRFC Setting ( 88 )
Command Rate ( 2t )

Voltage Control
EVGA VDroop Control ( With VDroop )
CPU VCore ( 1.3 )
CPU VTT Voltage ( +175 )
CPU PLL VCore ( Auto )
**IOH PLL VCore ( Auto )
DIMM Voltage ( 1.6 )
DIMM DQ Vref ( +0mV )
QPI PLL VCore ( Auto )
IOH VCore ( Auto )
IOH/ICH I/O Voltage ( Auto )
CPU Impedance (Auto )
*QPI Signal Compensation ( Auto )
ICH VCore ( Auto )

fanf074

Hello Evrybody,

I have the same problem ,but only on the first boot of the day .
I have 6 Gb of GSKILL Trident Pc 16000(9/9/9/24,1t@1.65Volts) and when i boot for the first time  ( when my mobo is cold i think)the bios and the OS only see 4 Gb of ram usable . In windows 7 64 ,the system show the 6GB but only 4 Gb are usable .In the bios i saw only 4Gb of Ram installed .So I stop my Computer completely (Not a reboot!) and i restart it and now the bios show me 6Gb of ram installed and windows too !!!

Heri is my bios template

Mother Board ( EVGA X58 Classified E-760 )
Bios ( S61D )
CPU ( Ci7920DD0 ) 
CPU Cooler ( Noctua  )
Memory ( Gskill Trident Pc 16000@9/9/9/24,1T@1.65V )
PSU ( Coolermaster Real Power 1000 W)
GPU ( GTX295 )
Drivers ( 191.xx )
Operating System ( Windows 7 64 Ultimate )


Frequency Control
CPU Clock Ratio ( 21X )
CPU Host Frequency (Mhz) (200)
MCH Strap ( auto )
CPU Uncore Frequency (Mhz) ( 4000Mhz { 20X } )
CPU Clock Skew ( 0 ps )
Spread Spectrum ( Disabled )
PCIE Frequency (Mhz) ( 100 )


Memory Feature        
Memory Speed ( Standart )
Memory Control Setting ( Enabled )
Memory Frequency ( 2000Mhz / 2:10 )
Channel Interleave Setting ( 6 Way )
Rank Interleave Setting ( 4 Way )
Memory Low Gap ( Auto )
tCL Setting ( 9 )
tRCD Setting ( 9 )
tRP Setting ( 9 )
tRAS Setting ( 24 )
tRFC Setting ( 74 )
Command Rate ( 1t )


Voltage Control
EVGA VDroop Control ( Without VDroop )
CPU VCore ( 1.350 )
CPU VTT Voltage ( +350 )
CPU PLL VCore ( Auto )
**IOH PLL VCore ( Auto ) 
DIMM Voltage ( 1.65 )
DIMM DQ Vref ( +0mV )
QPI PLL VCore ( Auto )
IOH VCore ( Auto )
IOH/ICH I/O Voltage ( Auto )
CPU Impedance (Auto )
*QPI Signal Compensation ( Auto )
ICH VCore ( Auto )



CPU Feature
Intel SpeedStep ( Disabled )
Turbo Mode Function ( Enabled )
CxE Function ( Disabled )
Execute Disable Bit ( Disabled )
Virtualization Technology ( Disabled )
Intel HT Technology ( Disabled )
Active Processor Cores ( All )
QPI Control Settings ( Enabled )
QPI Link Fast Mode ( Enabled )
QPI Frequency Selection ( 4800 GT/s )
OC Recorvery ( Enabled )

PnP/PCI     PCI Express
Maximum Payload Size ( 128 )

Your template lookes good.
Try those changes and tsrRdTRd and tsrWrTWr timings set from 4 to 6 See how itll goes.

Voltage Control
EVGA VDroop Control ( Without VDroop )
CPU VCore ( 1.350 )
CPU VTT Voltage ( +350 )
CPU PLL VCore ( 1.65 ) **************
**IOH PLL VCore ( 1.65 ) **************
DIMM Voltage ( 1.65 )
DIMM DQ Vref ( +0mV )
QPI PLL VCore ( 1.3 ) ********* Try 1.325 as well
IOH VCore ( Auto )
IOH/ICH I/O Voltage ( Auto )
CPU Impedance (Auto )
*QPI Signal Compensation ( Less ) ****************
ICH VCore ( Auto )



CPU Feature
Intel SpeedStep ( Disabled )
Turbo Mode Function ( Enabled )
CxE Function ( Disabled )
Execute Disable Bit ( Disabled )
Virtualization Technology ( Disabled )
Intel HT Technology ( Disabled )
Active Processor Cores ( All )
QPI Control Settings ( Enabled )
QPI Link Fast Mode ( Enabled )
QPI Frequency Selection ( 4800 GT/s )
OC Recorvery ( Enabled )

PnP/PCI     PCI Express
Maximum Payload Size ( 128 )

ty_ger07

Try increasing QPI PLL Vcore some.  QPI PLL Vcore is linked to the onchip memory controller hub; increasing it a bit can help stabilize the memory controller.  You can also try increasing IOH Vcore a bit.  I would try 1.2v for both IOH Vcore and QPI PLL Vcore.  IOH Vcore is voltage to the onchip northbridge which connects the CPU to the memory and rest of the system.

I'm not running an i7 rig but I'm still pretty sure the memory controller is on the CPU die itself and has a direct link to the DRAM called QPI. My understanding of this is that there is no involvement of the northbridge when CPU and memory communicate. Therefore IOH (northbridge) voltages wouldn't effect RAM stability, as they do with my core 2 duo. Am I wrong?

Ok i will change this timings
 tsrRdTRd and tsrWrTWr timings set from 4 to 6
I put also my qpi voltage to 1.325v and increase the voltage of IOH and QPI Pll to 1.2 volts .I also set the QPI signal compensation to "LESS"

I would tell you tomorow if this settings resolve this problem .

Thanks for helping me !!!(I'm a little french guy and my english is poor ,so sorry)

Ok i'll try to put QPI PLL to 1.350v and i will tell you tomorow if it's change something to me .
But after the settings i 've mentionned yesterday the problem is always here .Always 4 GB of Ram detected by the bios !

(Ok i will change this timings
 tsrRdTRd and tsrWrTWr timings set from 4 to 6
I put also my qpi voltage to 1.325v and increase the voltage of IOH and QPI Pll to 1.2 volts .I also set the QPI signal compensation to "LESS")

I am completely disapointed ;I answer me if i would disable the turbo mode ; maybe it will help me ....

Today when i boot my Computer,always 4Gb of totaly ram installed instead of 6 Gb!

So i decide to shift cpu , reinstalling my Noctua properly and  wait one hour before  start my computer (because when the motherboard is hot ,my 6Gb dont gave me any problems).

One hour later i start my computer and my 6 gb of ram were here !!!So i prefer to wait tomorow to tell you if my problem have disapeared  .

Thanks all for your interest for my bug .

 

posted this in another thread...but it appears to be the memory socket on the motherboard. i do have the dissappearing ram from time to time. and it only seems to happen on the 760/761 boards for me. i found that on both boards..i would have to wiggle the ram around in it's socket (number 5) then very carefully put the fan back on. then it was fine. i would rma yet again..but this board can boot to windows at 230+ bclk or 4.8 ghz...so i have been reluctant to send it in....

i figured it was more than just the 760/761, but you know how that goes...lol

fanf74

Hello ,

So after removing the Heatsink, remove the Cpu and reinstalling these parts ,i have always a problem with my ram detected by the BIOS (4gb instead of 6 Gb) .For me this problem is a problem of Shadow Memory i think .Because when the bios saw 4 Gb of ram totaly installed my OS (WIN 764 bits) saw the 6 Gb but tell me that only 3.99 Gb are usable !!!

Have some ideas ?

With a price of 400 ,i wish this problem will resolve rapidly : maybe a RMA ?

Another thing I would try. Test One stick of ram in Different memory slots. To make sure its not one of them malfunctioning.
Is the ram sits pretty snug in them?
It just my guess but from what you say. (I only missing of the cold boot but appears when Computer is warm) so it sounds like something somewhere is poorly contacted and when its gets warm and expends its works. Go ahead try move you ram around. If it wont work. (Do you have an option to test different set of ram (friend) if not, Id say RMA it.

I love reading threads like this.. Good info all around, keep up the strong work guys...

Sorry for the absence. I was on vocation for a while.

Beakerbox

Hey,  Sorry for the delay in getting back to this.  (been slammed at work)  I finally got the ram working harmoniously at Manu Spec.   My problem was I didn't feed it enough QPI,PLL voltage.  Once I put it up to 1.375 the full 12gb is running great @ 6,7,6,18,1t  DramVoltage 1.67, QPI/Pll Vcore 1.375.

Thanks very much for the advice and very informative thread.  I'm an enthusiast but not a hardcore OCer,  I mostly build rigs for fun and to meet my work requirements at a reasonable price.  My last board was an Asus with p45 chipset, so a lot has changed with QPI and I neglected to tinker with too much because I knew so little about it.  This tread really consolidated some key bits of information to help me understand my new system.   Your effort is much appreciated Ghost!

Your are welcome and thank you for a good word. :) Enjoy your rig

hulett9957

I love reading threads like this.. Good info all around, keep up the strong work guys...

Thank you for a feedback. Im glad you found it informative. Im very hesitant about posting a lot Most of the time its just trivial. Ill try to add more info as I get it.
Cheers

shamino

I've explained quite a couple of times before, every time at bootup every part is 'trained' as intel calls it, the memory is trained and sized, the qpi is trained.
if a chanel or dimm doesn't pass the training for any reason at all, it is not sized.
I'm all for pushing clocks as high as possible, but do bear in mind that:
 
We are overclocking the MCH on the CPU regardless of what type of rams we have got.
I have 2200MHz RAM for example, but no matter that, the CPU is rated at 1066MHz MCH and 1333 for some.
Anything over it is overclocking, even if CPU stays at same speed. This is exactly what Intel will tell you if you ask them, "Why does my CPU not run 2000MHz RAM for example"
 

Thank you for the Info Boss. Much appreciated.  So If I understood right when Manufacturer states 1866mhz ram it means 1333mhz Ram Factory overclocked and  rated/tested for 1866?

>>>

   One more thing That I found out just couple of days ago. For some reason  I happened to change one setting In control panel.
system tools/ System configuration/ Boot tab/ check Maximum memory. 12gb
Boots fine all that jazz all 12 gb present on a POST However In control panel /system shows 12gb installed 9.499 useable. (sounds familiar Right :) )
Same in resources managment - it showed all 12gb present but 2.5 gb reserved for Hardware reserved. ( I have memory low gap set to 2.5 (two GTX 285 1gb + gts 250 512mb)
Went to bios, set MLG to auto Reboot, All 12gb present at POST,
In system and Resources Management 12gb installed 9.99 usable, and 2gb reserved for Hardware.
To be honest I got confused for a while. I started thinking Is my memory acting up now. Am i loosing my mind. Ran Memtest again No red, on 8 passes.
Finaly I unchecked that stupid box in Boot Tab. And all Ram is there. System 12gb installed and RM shows 11109 mb available (OS always reserves 900-1.2 gb for itself)
I have no Idea what this MLow Gap feature is for. (reserved for PCI devices) If you got 1videocard with 1gb of ram, with this feature on auto it will reserve 1gb of your ram for it, Two GPUs - 2gb In system feature in OS you will see 4.99 usable and 3.99 usable accordingly.
       So again just to make it clear: go to Control Panel/Administrative Tools/System Configuration/Boot tab/Advanced options/ uncheck maximum memory box.   Apply, reboot. You should see 12gb or 6gb installed in CP/System.
As for MLG in bios I set It manually to the combined amount of GPU ram. So Tri SLI should be set to 3gb Sli to 2gb, etc.
I hope it was of help