Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <c59410e0.0406170346.4b9fb4@posting.google.com>,
sarangdw@hotmail.com (sajandra) wrote:
> >
> > It will need new memory.
> >
> > In fact, everything currently listed here uses DDR:
> >
http://www.asus.com/products/mb/overview_amd.htm
>
> Since there is an abundant variety of DDR memories
> pc3200,2700,2100,1600
> I'm kind of lost ... How do I decide between these different types ?
> I want it to be as fast as possible keeping in mind that I would like
> to upgrade the other components on my mobo
> in the future (upgrade to Athlon3200 and GForce6800 ?? ).
>
> THX, Sajan
There are two desirable properties for memory. One property is
latency (delay until the first data is available) and the other
is bandwidth (the ability to fill a cache line as quickly as
possible). The PC3200 transfers data at the highest rate, and
in terms of the resale value of the ram, is a better investment
than the other rates. Since PC3200 is backward compatible with
the other rates, you can use it in place of PC2700 etc.
The latency is partially determined by the column address strobe
(CAS) number. Smaller is better, but smaller costs a lot of
money. It turns out, at PC3200 speeds (DDR400), that CAS doesn't
have quite the impact that it has at the lower speeds. To see
this for yourself, see this Tomshardware article.
http://www6.tomshardware.com/motherboard/20040119/index.html
You can get by quite comfortably with some CAS3 PC3200.
As for the math, there is the clock rate (200MHz), and
two chunks of data are transferred per clock cycle. That
is where the DDR400 rating comes from (double data rate).
The memory is 64 bits wide or 8 bytes. Multiply the 8 bytes
by the transfer rate of 400Mega transfers per second gives
a 3200MB/sec theoretical rate. Now, memory controllers are
seldom 100% efficient, so you will never see the entire
3200MB/sec rate. The reason for this inefficiency, is there
are generally some gaps or wasted cycles, due to the design
of the protocol. DDR2 will reduce this when it becomes
available, very soon.
An AthlonXP processor has a 64 bit data bus, and transfers
data twice per clock cycle, as does a single DIMM. If your
processor is given a 200MHz clock rate, and it transfers
64 bits of data twice per clock cycle, this is termed FSB400.
This matches a memory clocked at 200MHz and its DDR400 data
transfer rate. Dual channel on an AthlonXP is overkill, as
a single DIMM is a good match for the AthlonXP bus design.
There are other processors in the Athlon family, and some
of those (ones with 939 or 940 pin sockets) are dual channel
capable. What that means, is internally the processor has
twice the effective transfer rate of its predecessor. Also,
since Athlon64/AthlonFX/Opteron processors have the memory
interface on the processor, and not on the Northbridge, the
latency is reduced even further compared to other existing
processors. So, in terms of your upgrade strategy, if you
buy unbuffered DDR400 DIMMs, buy two matched ones, so that
if in the future you buy a Socket 939 motherboard, the
matched DIMMs can be used in your new board. Avoid the
temptation to buy a motley collection of unmatched RAMs,
as they may not be usable in the very latest boards.
I noticed that just today, while reading processor datasheets
from the AMD.com site. AMD has stopped using the "dual channel"
moniker in their data sheet, instead referring to 128 bit
mode. What this means, is, unlike the Intel 875/865 ability
to operate in virtual single channel mode, the 939 socket
processors have only one memory controller per pair of
DIMM slots, which means the memories must be matched in
terms of their row_addr/column_addr/chip_width, to work.
So, the best you can do in terms of future proofing, is to
buy two of the same product, at the same time. For example,
2 x 512MB CAS3, should work fine now and in the (near) future.
(This recommendation applies if you want to use all the slots
on the motherboard. There is still a single channel mode, but
it is restricted physically to using only one of the memory
busses, and either one or two slots only.)
At some point, DDR2 will become the favored memory, so don't
expect your DDR memory to be useful for upgrading forever. DDR
will still be around for at least a year, maybe a bit more,
in mainstream products.
HTH,
Paul
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