Question about Dual DDR setups

[Kythorn]

I'm looking to put together a dual DDR system. I only actually want to put in a gig of ram now, but preserve my ability to go higher later. I don't know how the 'pairing' actually works with 4 DIMMS. Do all four have to exactly match? Or just bank 1 + 2 and then 3 + 4?

Not knowing this has left me concerned that I won't be able to find another 2 DIMMS later that exactly match the configuration of what I get now. If I have to I could buy 2 gigs now - I'd just rather not unless necessary.

Does anyone know for sure how this works?

 

[Lord Evermore]

Each "channel" needs to have the same amount of memory for the best performance, and all modules will run at the speed and timings of the slowest module. Not sure of the exact design of a P4 dual-channel chipset works, you most likely need to have the exact same modules (such as the cell configuration and number of chips) for each channel in order to work, while with an nforce2, you can have varying modules.

What type of motherboard are you using?

 

[Kythorn]

I don't have a specific model picked out yet, but I'd like to pick up something based on the Canterwood chipset.

 

[Lord Evermore]

A similar thread here: http://forums.anandtech.com/messageview.cfm?catid=27&threadid=1026044

Intel chipsets only have one memory controller, so each channel needs matched modules with the same density and size and double or single-sided. I'm still not sure whether you can have to different types in the two slots on one channel, and the same types in the other slots.

Like, slots 1 and 3 are channel 1, slots 2 and 4 are channel 2. Put two modules of 256MB in 1 and 3, you get dual channel 512MB. If you then put 512MB modules of a higher density in slots 2 and 4, will all the slots fall to single-channel operation? Or will the controller recognize that both channels have matched memory, even though the slots aren't? I don't know yet.