Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <b55lv0l5fhffu10nqqfhnmttcn2hj3m4kg@4ax.com>, DC
<crowe4215@yahoo.com> wrote:
> I have a p4p800e dlx md. I have one piece cosair xms 512 pc3200 and
> one piece cosair xms 512 pc3000. Both pieces have the same timing and
> are running at 333 mhz. Can I use dual channel with these?
> Thanks
> DC
>
Well, they don't have exactly the same timing, in the sense that one
is rated at PC3000 and the other at PC3200. But the distinction in
this case is probably nothing to worry about. For simple timing
differences, the slowest stick sets the timing.
Matching is a function of the memory controller in question.
An Nforce2 dual channel motherboard, needs virtually no matching
at all. Each slot in the motherboard functions independently,
and in true dual channel fashion, the two channels interleave
(take turns providing 64 bits of data), to increase the
data rate.
The next level is the Intel chipset method. It is called
"Uber Dimm", and the memory controller sends the exact same
control signals to two sticks positioned for dual channel
operation. This means the row/columns/banks of the two sticks
have to match, as does their total memory size. If the
rows/columns/banks match, but there are trivial differences
in clock rate (PCxxxx), Tcas, Trcd etc., the BIOS can select
the lowest common denominator setting that both DIMMs can
accept, and the DIMMs will be run with those settings. (And
in your case, as long as your PC3000 and PC3200 stick have the
same rows/columns/banks, they should be workable in dual
channel mode, barring any BIOS bugs.)
The third level, is the AMD Athlon64 method. It is like
the Intel method, only as far as I can tell, virtual single
channel is missing. That means, if you have widely disparate
sticks of RAM, then you run them in single channel mode, and
that means using the sockets of only one of the two channels.
In other words, on a four slot Athlon64 motherboard, you
cannot place a 128, a 256, a 512, and a 1024MB stick, and have
a single channel 1920MB configuration (like you could on an
Intel 875/865 board). The best you could do is a 1536MB
configuration, placing the two largest of those DIMMs in the
two DIMM slots identified in the manual, as the ones to use
in single channel mode.
I think the Athlon64 method would likely prevent a matched
pair of DIMMs, placed in a pair of slots that arranges them
as a 128 bit wide "Uber DIMM", from working with a third
stick placed in the remaining single channel slot. Once you
decide to run dual channel, it means you either use two matching
sticks, or use two pairs of sticks matched with one another.
A matched pair of 512MB and a matched pair of 1024MB would give
a 3GB configuration on the Athlon64 dual channel. (And interleave
might be disabled with that 3GB configuration.)
For both the Intel and the Athlon64 methods, there are
opportunities to interleave between sets of DIMMs, to take
advantage of the ability to keep memory pages open. If you
use four 512MB double sided DIMMS, where rows/columns/banks
match on all four sticks, then some lesser significant address
bits can be used to control chip select on the DIMMs, and the
resulting interleave pattern maximizes the number of pages
being kept open at any one time. A page that is kept open,
does not need to be reopened, so the overhead on next
access is reduced.
Some refererences:
For Intel dual channel:
865P memory guide
ftp://download.intel.com/design/chipsets/applnots/25303601.pdf
875P memory guide
ftp://download.intel.com/design/chipsets/applnots/25273001.pdf
For Athlon64:
BIOS writer's guide:
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/26094.PDF
Socket 754 datasheet (single channel):
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/31410.pdf
Socket 939 datasheet (dual channel unbuffered):
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/31411.pdf
Socket 940 datasheet (dual channel registered):
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/31412.pdf
For Nforce2, there is no datasheet available for download, and
user experience is the only guide as to what works. No idea
how dual channel on the KT880 works.
HTH,
Paul
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