Memory question with A64 motherboards

[wraith3k]

From what I've learned here you can have either 3GB of DDR333 or 2GB of DDR400, unbuffered, on an Athlon 64 (not FX) motherboard. Does this mean if a board has 3 memory slots, would I be able to fill all 3 with DDR400 as long as it doesn't exceed 2GB? (For instance, a 1GB stick and 2x512). Or would I only be able to use 2 of the slots?

(Sorry if this has been asked before, new to these forums)

Thanks

 

JEDEC specifies one unbuffered DIMM of DDR400 per channel, and two for DDR333. You can use more DIMMs per channel if they're registered DIMMs.

 

[Peter]

In practice, 3-DIMM configurations at 200 MHz ("DDR400" in marketingspeak) do not ever work stably. Hence the limitation - if you want to run three DIMMs, you need to step back to 166 MHz ("DDR333"). If your DIMMs have a lot of chips on, you might even have to adhere to the specification

 

Peter, does that limitation apply to buffered or registered DIMMs too? Or are all types affected?

 

[Peter]

No, that in fact is the very point of registered DIMMs - reducing the electrical load imposed by the DIMM onto the common bus. Where an unbuffered DIMM imposes up to sixteen loads onto the address lines and up to six onto each clock, a registered one just attaches the register chips - meaning each DIMM is one electrical load, regardless of how many chips are on it. As a side effect, a registered DIMM may contain twice as many RAM chips as an unbuffered one.

But don't ask me what's the theoretical limit on 200 MHz operation with registered DIMMs. IIRC this isn't even officially specified just yet. On the other hand, there are boards that implement four slots per channel, e.g. MSI's dual Opteron board.