Triple channel memory in building a computer

Intel's i7-900 series microprocessors feature a triple channel memory controller. Each channel is electrically independent from all other memory channels.

When memory is installed on an x86 memory channel it is configured in what's known as a "rank". A rank is a set of DRAM integrated circuits which when their IO pins are concatenated together form a 64 bit data bus. Typical configurations are 4 chips each with 16 IO pins (4x16 = 64), and 8 chips each with 8 IO pins (8x8 = 64). The former is common on laptops via the SODIMM formfactor, and the latter is common on desktops via the DIMM formfactor. Ranks are arranged onto DIMMs, typically one per side. If a DIMM has integrated circuits on both sides, it is typically called a "Dual Rank DIMM".

Most motherboards are manufactured with either one or two DIMM slots per channel. In typical consumer usage this allows for up to four ranks per memory channel in the following configurations:

• 
  no ranks
  one single rank DIMM
  two single rank DIMMs
  one dual rank DIMM
  one single rank DIMM and one dual rank DIMM
  two dual rank DIMMs

There is very little difference electrically between a pair of single-rank DIMMs and a single dual-rank DIMM. The only real difference is where the chip-select signal gets sent. The former are rarely found in enthusiast systems and almost all XMP modules are certain to be of a dual-rank design.

Regardless of how they are arranged, each additional rank both increases the electrical load (in terms of signal power) placed on the memory controller, and reduces the integrity of the signals themselves.

When transfer rates are increased (via an increased IO bus frequency) the margin between a stable signal value and an unstable signal value gets increasingly smaller.

The effects of both the increased electrical load and the transfer rate are cumulative with each other. Higher data rates and higher installed capacity are more likely to be unstable than either a higher data rate or a higher capacity individually. To combat this, Intel specifies that XMP certified modules be stable in an environment with no more than two ranks (typically one dual rank DIMM) per memory channel. This allows manufacturers to market the modules more reliably.

This does not mean that XMP cannot be used in an environment with more than one DIMM per channel, it just means that XMP alone may not be enough and even in the best of circumstances proper operation cannot be guaranteed.

For example, I run 32 GiB of DDR3-2133 in my PC, arranged as 8x4GiB DIMMs. Each DIMM is dual-rank with 2GiB per rank for a total of 4 ranks per channel. This is outside Intel's XMP testing specification and indeed the XMP profile alone is not enough. I need to boost the memory controller voltage slightly in order to keep it stable at that speed.

I hope that this answered your question.

Basically this is an older mobo that dates back to it's original release in 2009, these support the 1600 and up DRAM as with a CPU OC, it's native to 1066 DRAM and can run 1333, your best bet will be 1600 sticks in a 3x4GB config to hit 12GB, which will be about the max you can carry, might be able to run 24GB but down at 1333 or 1066....