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I have been running 2x2GB of Corsair Dominator 1600 dual channel sticks on a ASUS P8P67 motherboard.

Was intending on buying the same again, so I could run all four sticks on the mobo. instead I accidentally ordered the 2x4GB Corsair Dominator 1600 kit.

I swapped the current 2x2GB with the two new 2x4GB (black slots on mobo). This appears fine in BIOS and in System (running Windows 8).

I then put the 2x2GB sticks back into the blue slots on the mobo.
BIOS still only registers 8GB, and so does System.

However, CPU-Z is reporting the expected 12GB. enter image description here

Whats the deal here? Is there an approach to troubleshoot, or is it that even though the kits are same speed/brand, they wont work in conjunction.

The answer I am looking for is either a workaround for my machine to support (and use) the current configuration of 2x2GB + 2x4GB, or for the answer to be that it is not possible.

Let me know if you need me to include more info via the comments.

Added research

The P8P67 has a limit of 32 GB (i.e. 4x8 GB DIMMs) as per specs on Asus website. – Pincopallino

The manual on 2-6 says "You can install 1GB 2GB and 4GB.." no mention of 8GB in that line. But in smaller writing on that page it says "According to Intel Spec, The max. 32GB memory capacity can be supported with DIMMS of 8GB (or above)". and on Page x it "repeats" that line about Intel stating 32GB is supported.

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Have you tried it with the 2*4GB modules only? How much does memtest report? –  ott-- Feb 11 '13 at 10:49
    
Can u take ur banter to chat please. I feel the title is sufficient, but if anyone wants to edit, feel free. –  Simon Feb 11 '13 at 12:02
    
I well spell it out for both of you. The OP had 4GB of RAM. He intended to buy another 4GB of RAM for a total of 8GB. Instead he got an 8GB kit giving him a total of ... wait for it ... 8GB. Hence this question, and at least two people that don't understand irony. –  ta.speot.is Feb 11 '13 at 12:10
    
I would start by upgrading the BIOS to the latest version and clearing the CMOS settings. (Take note of the current settings, particularly any RAID/AHCI settings so you can put them back.) –  David Schwartz Feb 11 '13 at 13:23

3 Answers 3

The most probable explanation is that your CPU is not properly seated and it's causing the BIOS to detect the memory in the blue slots but not use it. Modern CPUs have the memory controller on the CPU die, and many of the CPU pins are the memory channel address and data lines.

Test by putting just two modules in the blue slots. If the system doesn't work, this is the most likely explanation. While you're at it, check for any physical damage to the memory slots or any debris in them.

If you confirm that the blue sockets are not working, carefully remove the heatsink and fan, remove the CPU and carefully inspect the pins in the socket. Look for any foreign material on the CPU or pins or for anything bent. If you find anything, fix it. Either way, carefully reseat the CPU, remount the heatsink/fan with new heatsink compound, and re-test.

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Why would the CPU not being seated properly have this effect? –  ChrisF Feb 11 '13 at 13:35
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good idea to add how often you've seen this happen! that's a factor in considering how probable it is! I would guess 90%+ of people haven't ever seen that, including me. It is quite unusual for a CPU To not be seated properly as it's typically so easy for the kind of person that'd install a CPU, to seat it properly! –  barlop Feb 11 '13 at 13:39
    
Particularly these socket 1155's are difficult, maybe impossible, to seat improperly. The pins slide in horizontally after you pull the lever to seat the cpu, if you did it wrong it there would be noticeable carnage under the chip... bent pins, difficulty pulling the seating lever, and I doubt it would work at all after bending the pins 90 degrees in the other direction. –  Kyle Feb 11 '13 at 13:44
    
@ChrisF: Because the memory controller is on the CPU. All it would take is one bent pin on any of the address or data pins for that channel. –  David Schwartz Feb 11 '13 at 14:04
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@DavidSchwartz - while you may know that you can't assume that everybody knows that. Including that information in your answer would make it a better answer. –  ChrisF Feb 11 '13 at 14:05

Have you referred to the motherboard manual on how to mount the DIMMs? I have the same motherboard, and on the manual (page 2-6) I read

You may install varying memory sizes in Channel A and Channel B. The system maps the total size of the lower-sized channel for the dual-channel configuration. Any excess memory from the higher-sized channel is then mapped for single-channel operation.

I can't tell if they mean that only one DIMM on the higher-sized channel is mapped, which probably is what you are experiencing. You can try switching the channels, but I guess it won't fix the problem.

As for CPU-Z readings, they differ from the system because CPU-Z only reads the SPD information from the RAM modules, while the system shows the memory that is actually mapped by the memory controller.

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Thanks. So are my 2x2GB not being used at all? –  Simon Feb 11 '13 at 11:59
    
@Simon - Yes...Because your motherboard doesn't support using 4GB DIMMS in one channel and 2GB DIMMS in another. –  Ramhound Feb 11 '13 at 12:01
    
And is there a workaround anywhere? BIOS perhaps? –  Simon Feb 11 '13 at 12:01
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@Simon. I'm not sure they are not used at all. It might be that the two 4 GB DIMMS are mapped as they were 2 GB in order to have a dual channel. As far as I know, the only thing you could try is to move around the DIMMs and check what's the total amount of memory that is mapped. I don't think that a BIOS update would solve the problem (which isn't actually a problem) based on the changelog on ASUS website, but It could be useful for other improvements. –  Pincopallino Feb 11 '13 at 12:15
    
@Ramhound: He doesn't have 4GB DIMMs on one channel and 2GB DIMMS in the other. That would be a poor setup and it's not the one he's using. He has 6GB on each channel, which is the correct setup. –  David Schwartz Feb 11 '13 at 13:29

From the manual:

You may install varying memory sizes in Channel A and Channel B. The system maps the total size of the lower-sized channel for the dual-channel configuration. Any excess memory from the higher-sized channel is then mapped for single-channel operation.

As I understand it, that means you need to install

  • 1 x 4GB in DIMM_A1 (black),
  • 1 x 4GB in DIMM_A2 (blue),
  • 1 x 2GB in DIMM_B1 (black),
  • 1 x 2GB in DIMM_B2 (blue)

Then the lower 8GB will be in Dual-Channel Mode, whereas the upper 4GB will operate in Single-channel mode (as channel B only consists of the 2GB modules). Doing this will mean that the specs (MHz, timings) of the 4GB and the 2GB modules should ideally match.

And to explain CPU-z: it only displays the installed memory size, i.e. it adds up the sizes of all memory modules, but due to the wrong configuration you run at the moment, the motherboard can't use the whole capacity (you have one 2GB and one 4GB module on different channels, hence the motherboard reduces the usable size to the smaller of both).

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You're directing him to the worst possible memory configuration! He can put 6GB on each channel. –  David Schwartz Feb 11 '13 at 13:06
    
Well, as I see it, channel A will hold 8GB and channel B will hold 4GB in this configuration. The system will use the full 12GB, 8GB in dual-channel and 4GB in single channel. Having 6GB on each channel is what he tried and it didn't work! –  Stefan Seidel Feb 11 '13 at 13:19
    
Why do you think your suggested configuration will work when his doesn't? It's definitely inferior, since it ensures that some of the RAM will not run at dual channel. (And you are seriously misunderstanding what the manual is saying if you think it's saying you must unbalance your channels.) –  David Schwartz Feb 11 '13 at 13:22
    
Yeah, manuals are mostly written by non-native English speakers, so it's not always certain what's meant. I understand "You may install varying memory sizes in Channel A and Channel B" as "You may install size xGB DIMMs in channel A and size yGB DIMMs in channel B and it will work". Yes, it is inferior performance-wise, that's for sure. But quite obviously the alternative is to run with 8GB of usable memory even though there are 12GB physically installed. –  Stefan Seidel Feb 11 '13 at 13:26
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It says that you can do so if you must. It will work. You will wind up with some memory mapped in a single-channel configuration. And likely the channel with more memory on it will bottleneck. But it's a supported configuration. You can resort to it if you have no choice. (For example, if you only have a 4GB module and a 2GB module, what else can you do.) –  David Schwartz Feb 11 '13 at 14:48

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