Dual Channel RAM Question

[darkghost101]

Not too familiar on how dual channel RAM works,
lets say if i use Asus A7N8X-E Deluxe mobo,
and Kingston HyperX 512MB PC3200 Dual Channel DDR Kit (2 x 256MB), the mobo has 3 RAM slots, what happens if i add another stick of memory?

 

[Prof133]

Good explanation <A HREF="http://nforcershq.com/forum/viewtopic.php?t=3199&sid=cbe67cd25ef05020a1b0c89ff201eb21" target="_new">here</A>:

In order for DualDDR memory to run at it's optimal speed, you need to have an equal amount of RAM in each channel. Slots 1 and 2 are channel 1, and slot 3 is channel 2. Right now I've got 256MB in slot 2, and 256MB in slot 3. This means 256MB in each channel. If I was to upgrade and wanted to keep full Dual DDR, I would need a 512MB stick, and put it in slot 3, and the 256MB sticks in slots 1 & 2. This would give me 512MB in each channel, and an equal balance. This would give me an optimal Dual DDR setup.
If I were to have, say, 512MB in one channel, and 256MB in the other, only 256MB of the channel with the 512MB stick would be running in Dual DDR, since the channels would not be balanced. The rest of the 512MB stick would run in standard single channel mode, giving below optimal performance.

 

[Prof133]

If three memory modules are installed in the board, DualDDR is in effect because there is memory in both channels. It's just a question of how much is accessed in DualDDR (128-bit) access mode and how much is accessed in 64-bit access mode. The optimal configuration is all of it accessed in DualDDR access mode - memory balanced across both channels.

The amount accessed in DualDDR access mode is equal to the smallest amount between the two channels times two. The rest is accessed in 64-bit access mode. If moving from 512MB (2x256MB) to 1GB, add a 512MB stick and place it in the memory slot that appears to stand alone and the two 256MB sticks in the two remaining slots. In which case you'll have 512MB on both channels and the entire amount in DualDDR access mode.

If you had 2x256MB and then added another 256MB stick, you'd end up with 512MB of it in DualDDR access mode and 256MB of it in 64-bit access mode - not an optimal configuration.

 

[Slasher42]

Prof-

DOes that hold true for all Motherboards?

I just got an Abit IS7 board, and due to cost at this time I only got 2x256mb, if I want to upgrade say 2 months from now I could simply by a 512mb dimm and place it in there on one channel and the 2x256 in the other channel and all would run in dual channel access mode?

 

[Prof133]

For enabling dual-channel for IS7, which uses the i865pe chipset, you'd have to use two or four memory modules; and rules apply. To make the move to 1GB you'd want to add two more 256MB memory modules.