Memory density clarification!!!

Question

I started another thread on this, but the update to that thread is big enough to have its own thread. However, for the history the link to that thread is https://forum-en.msi.com/index.php?topic=127102. In a nutshell in that thread I wanted to know if I could mix memory modules of different densities on my X58 Platinum SLI. I had previously mixed densities on my Asus P4 build, so why not with my MSI I7 build? Unlike Microsoft I knew Intel would not go backwards with capability. However, the moderators on this site, my X58 Platinum SLI user’s manual, self described gurus on this site, and even MSI tech support with 2 different support tickets all told me I could not mix memory modules of different densities! But I carried on thinking that they were all wrong. As you can see by the supporting picture evidence they are all WRONG and my thinking is right. You CAN mix memory modules of different densities. Not only can you mix them, but it works well enough to overclock. I currently have 2GB memory modules in slots A0-C0 and 1GB memory modules in slots A1-C1. As you can see the BIOS as well as Vista both report the correct 9GB. If you do the math there is only one way to get 9GB and that’s to mix memory module densities.
 Moderators: don’t be so quick to tote the MSI line if you have not seen it with your own eyes!
End users: if you suspect something is wrong in your manual it probably is. Most mobo manufacturers copy instructions from one mobo model to another and frequently put out false information. As we can see MSI is no exception since they published the following lines:
  - DDR3 memory modules are not interchangeable with DDR2 and the DDR3
    standard is not backwards compatible. You should always install DDR3
    memory modules in the DDR3 DIMM slots.
  - In Three/ Dual-Channel mode, make sure that you install memory modules of
    the same type and density in different channel DIMM slots.
Anyway . . … I wanted you all to know that you don’t have to throw away memory modules if they are not the same density. If everything about the memory modules are the same except for the density the SPD capabilities of your mobo will sort the rest out!
There were some pictures here, but sorry I had to stop hosting them at 3:42p, 21 May. Just a bandwidth limitation, that's all. Yeah, I know about photobuket and the like.

Answer 1

I am not really sure if you are seeing the full picture here.  The fact that you had no problems when mixing different memory modules on two different platforms does not prove anything.  Statistically, it is not significant enough to come up with a general statement like "the manual is wrong" or "yes, you can mix, no problem".  That is simply not
very responsible as part of general advice.
Nobody here will tell you that unidentical memory modules will ultimately cause problems on every system.  It can work just fine (just like in your cases), however, most of us have had to do with many systems in the last couple of years and we have come across users with many different system configurations here and in other forums.  There are numerous cases in which mixing memory unidentical memory modules led to unstable or unbootable systems.  There are also numerous cases that were issue free.  Even if the success rate is/was (let us say) 66%, are you really serious in trying to tell us to ignore a 33% failure rate just because you did not run into any problems on two systems?
I have mixed my Kingston HyperX KHX8500D2K2/2G sticks with a kit of G.Skill-Kit F2-6400PHU2-2GBHZ memory modules on some of the systems I had here in the last couple of month.  And yes, there were cases in which that worked just fine.  However, it caused horrible stability problems on some of the configurations I tested and these issues could be simply resolved by using identical modules. A friend of mine recently bought a second pair of OCZ DDR2-800 modules (same model he already had) and ran into sincere bluescreens and system crashes.  He talked to OCZ and after they made him compare the production dates of the modules, it came out that the newer kit used different memory chips.  OCZ exchanged the old kit for another kit of the same revision as the newer one.  Why:  Because they are very well aware of the fact that stability problems are not unusual  in such cases.
I am honestly wondering what you are actually trying to prove with this thread.  If your system is stable with your memory mix, that is wonderful for you.  No doubt about that.  But, again, just because you had success regarding your specific configuration, this does not mean that this will be true for all system configurations.  This kind of Inductive reasoning is rather misleading. 
You are claiming the manual is wrong.  Well, it isn't.  If MSI removes the remark "In Three/Dual-Channel mode, make sure that you install memory modules of the same type and density in different channel DIMM slots." from the manual, what are you going to tell users that actually have problems that disappear once identical memory modules are installed?  They will say:  Why isn't there a note in the manual that advises system builders to use identical modules? 
Dual and Triple Channel operation is all about synchronous read & write access to the system memory to enhance system performance.  How synchronic can read/write access be on systems with an asynchronic memory configuration? 
One more thing:  I have read through your other thread again and as far as I see it, you are not referring to the density of the memory modules, but to the capacity/size of the modules (1 GigaByte vs. 2 GigaByte).  Memory Density and Memory Capacity/Size are not the same things:
The term "memory density" refers to the kind and organization of the memory chips that are used on a memory module.  Corsair's Datasheets do not specify the memory density.  But let's have a look at a Kingston Datasheet for Comparison:
http://www.valueram.com/datasheets/KVR1333D3N9K3_3G.pdf [3 x 1 Gigabyte]
This quote from the datasheet refers to the Memory Density:
Quote
eight 128M x 8-bit
"128M x 8-bit" means a memory density of 1024 M-bit (or 1 GBit (note: not MByte/GByte)).
Now look at this kit:
http://www.valueram.com/datasheets/KVR1333D3N9K3_6G.pdf [3 x 2 Gigabyte]
The memory capacity/size of the module has doubled, yet the density is the same:
Quote
sixteen 128M x 8-bit
Again: 128M x 8-bit is 1024M-bit (or 1G-bit).
The memory size/capacity depends on the number of 128M x 8-bit Chips that are used on the module: 16 x 1024 Mbit = 16384 Mbit.  If you devide that number by 8 (bit -> byte) you get your 2GByte. Although the memory capacity differs (1GB vs 2GB), the memory density is identical (1 Gbit in both cases).
One more example for comparison:
Instead of 128M x 8-bit, the density can also be 64M x 8bit for a give 1024MByte Module. 
Example:
http://www.valueram.com/datasheets/KVR1066D3N7_1G.pdf [1 GByte]:
Quote
based on sixteen 64M x 8-bit DDR3-1066 FBGA components.
Density is:  64M x 8-bit = 512 MBit x 16
If 512MBit modules are combined with 1024MBit modules (different density), the chance that the system will not boot at all or be horribly unstable is very high. 
Quote
Yes, the manual is wrong.
No, it is not.   It states the adequate compatibility conditions for synchronous/interleaved multi-channel operation.
Quote
there are no specific DDR3 slots.
There are six slots on your board that are specifically for DDR3.
Quote
DDR3 memory modules are not interchangeable with DDR2 and the DDR3
    standard is not backwards compatible. You should always install DDR3
    memory modules in the DDR3 DIMM slots.
This is actually a very important piece of advice.  For experienced users it may fall into the same category as "Attention! Hot Beverage" on coffee-to-go cups and the "don't use the microwave oven to dry (living) pets" in some microwave user manuals.  In the last two years I have met at least five users that wondered why the board won't boot after forcing DDR memory modules into DDR2 slots or DDR2 memory sticks into DDR3 slots.