Taurus - interesting, I went to cut the directions for dual ddr out of the asus manual and got an interesting surprise.
The wording is ambiguous. There are 2 seperate sentences. One says "to enhance system performance use the dual channel feature". The other says that the DIMMS "must be installed in socket 1&3 or 2&3 or in 1,2&3". It's the same in the older a7n8x or the deluxe version. They don't explicitly mention that dual ddr only works in pairs.
So then I went to the ABIT nforce2 manual and it was even less
informative. It just said when using 2 3200 dimms use DIMM 2 and 3 slots.
I'm digging further to see if any manufacturer actually tells you that you are restricted when using the dual channel feature.
My sources have been tech reviews and the experience of friends with the asus boards who could not even boot in dual channel mode if a pair of dimms weren't in the right socket.
Note they make 2 sockets one color and the other socket a different color for identification (usually black and blue) and 2 sockets are right up against each other to limit timing delays in the MB interconnect.
Dual DDR uses the same type of trick that RAID striping uses. While one stick (or HD) is being read the other is in it's "wait state". You need 2 physically seperate devices of the same size to make this work. RAID allows you to use 2 different sized drives but the partitions have to be the EXACT same size. There is no equivalent to a partition in windows memory access so you have to have 2 same sized sticks.
Actually thats how DDR works too. They have 2 "rows" of chips on one DIMM and interleave access to get the double data rate. Dual channel DDR is actually quad data rate achieved by using 2 seperate memory controllers each one accessing one DIMM. There are not enough pins on a DIMM to do the dual channel trick on one DIMM. It requires 2 seperate databuses.
Now I'm on a crusade to find a manufacturer who admits this in writing. I'll post the link when (if?) I find it.
------------------------------------------------------------------------------
EDIT: Manuals for Giga, asus, abit were useless on this quest.
From the FIC site:
http://www.fica.com/site/html/services/faq_detail.asp?c_id=240000645
Q4 How do I enable the Dual-channel memory mode for the AU13?
A To enable the Dual-channel mode you must have the a minimum of 2 memory modules. These must be identical modules which means they must have the same IC chips, same specs, same size and same brand. The Dual-channel mode is automatic once you install the one module in each of the Mainboard's 2 memory channels.
For example of Dual-channel mode:
Channel 1 - DIMM 1 and Channel 2 - DIMM 2
Channel 1 - DIMM 1 and Channel 2 - DIMM 3
Channel 1 - DIMM 1 and Channel 2 - DIMM 2 & DIMM 3
For example of Single Channel mode:
Channel 1 - DIMM 1
Channel 2 - DIMM 2
Channel 2 - DIMM 3
Channel 2 - DIMM 2 & DIMM 3
Still does not address what happens with a third stick since it says memory must be the same size and type???
Biostar M7NCG Has best description so far implies 3 sticks can be used and still get dual channel. Does not say if third stick has higher bandwidth tho?
• Memory:
3 x 184-pin DDR memory modules
Maximum 3GB total system memory
Dual Channel DDR 200/266/333/400
(For dual channel 128-bit performance, at least 2 modules must be installed.)