Originally Posted by alexan_e
The list I refer to is http://www.asus.com/ROG/CROSSHAIR_IV_FORMULA/#support_QVL
I can see several 1600MHz models supported for 4 dimm configuration and several 1600MHz which are not
This is also true for 1333MHz, 1066MHz etc so I'm not sure what the problem is with those that "can't work", it doesn't seen to be the frequency.
When motherboards mention the RAM speeds supported, they are not including all the CPUs in their test run. Even though 1600mhz is supported officially on the motherboards, it will only be on select processors (the FX series in this case) whose memory controllers can take handle that frequency.
ie: officially Thubans dont support mem freq above 1333, but because they are good cpus for overclocking they can handle 1600mhz ram speeds easily
this explains it better: http://hardforum.com/showpost.php?p=1037427142&postcount=28
All AMD motherboards are capable of setting memory at 1600Mhz (x8 multiplier, often undocumented), just the AMD CPU IMCs themselves are only rated for 1333Mhz. The reason for this is IMC limitation; according to this official AMD document, you would require an NB speed of 2400Mhz (above default 2000Mhz spec and requiring more than 1.15V) to not bottleneck 1600Mhz. NB speed must be 3 times the speed of the RAM (before DDR). 667 [1333 DDR] x 3 = 2000. 800 [1600 DDR] x 3 = 2400. However, if you overclock the NB to 2400Mhz and run 1600Mhz memory, you will see the performance benefit; otherwise you won't, so AMD did not bother rating the IMC at above DDR3-1333. If the IMC/NB were capable of being within spec to run 1600Mhz, then 1600Mhz would run with no problem across all of the AMD IMC revisions (C2, C3 and E0) and provide adequate performance benefit. And in most cases, the NB can be overclocked/overvolted to meet that minimum 2400Mhz spec.
So if you want to go by the official supported speeds you have to consider what speeds your CPU can handle and not only the motherboard.