What is "dual-channel DDR"?

[Lennart Sorensen]

On Tue, Mar 15, 2005 at 08:49:53PM -0500, William Park wrote:
> What is dual-channel DDR?  Is it also called "DDR2"?
> 
> I'm interested in Abit AV8 (AMD64, S939)
>     <http://www.abit-usa.com/products/mb/products.php?categories=1&model=175>
> which takes dual-channel DDR.  But, I don't know what that means.  My
> experience ends with SDRAM and Pentium 3.

Dual channel means you take two sticks of memory and run them in
parallel so you have twice the bandwidth available.  This works by
having two memory controllers running the memory as two seperate chunks
of memory.  Access to memory is then split evenly across the two chunks
(usually works best if you have each chunk identical size so you can
alternate between the two chunks for every address).

So assuming you have 64bit memory (most is today) you would have bytes
layed out like this:

Channel1 Channel2
01234567 89ABCDEF

So if you go to read 128bytes you get the throughput of both channels at
the same time.  Most benchmarks I have seen indicate a 5 to 10% boost in
speed on AMD systems, and usually a bit more than that on P4 (the P4 is
more bandwidth hungry because of a very large cacheline size, which
makes it read a big chunk every time it accesses memory).

DDR2 is simply a new version of DDR designed to allow higher clock
speeds and lower latencies.  Some boards support both (but NOT at the
same time).

Lennart Sorensen
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