VIA memory - again.

[Charles Steinkuehler]

Chris wrote:
> O.k., so call me cheap.  But call me cautious as well.
> 
> Buying a new  motherboard - a VIA P4M266.  The specs just say "PC2100 
> DDR memory".
> 
> However, MANY websites that sell PC2100 memory seem to have two 
> groupings (looking at PC2100 256 Meg); 
> - average price $50 and higher
> - inexpensive price $27 - 30 with caveat "for VIA chipset only"
> 
> Anyone have any insight as to what makes VIA chipsets so special, and if 
> I can use the less expensive memory?  For example (and I have no 
> affiliation with this site, just the first on the list at Pricewatch 
> today); http://buyaib.com/25pc21brinst.html
> 
> All I have been able to find is a somewhat inferred reference to memory 
> interleaving that VIA implemented.  Seems to me if VIA needs special 
> memory, it would be more expensive, not less?

I don't know about the VIA chipset details, but if it *DOES* interleave 
memory, it's quite believeable the VIA chipset could use memory with 
less stringent timings (ie: cheaper).

Modern memory systems (along with the CPU and front-side bus) run at 
very high frequencies, where "black-box" effects caused by trace 
impedence, chip parasitics, signal propagation times, etc. all shrink 
the already razor thin timing margins.  If you're interleaving memory, 
that means you have two completly seperate banks of memory that arn't 
connected at all.  On a normal non-interleaved system, everything is 
tied together on one big bus.

So, for instance, if you have 4 DIMM sockets in an interleaved system, 
each DIMM control signal is physically connected to the chipset's 
north-bridge and two DIMM sockets.  In a non-interleaved system, each 
DIMM control signal is tied to the north-bridge and *FOUR* DIMM sockets. 
  With some control signals (like the address lines, RAS, CAS, etc) 
being tied to *EVERY* chip on a DIMM, you could potentially have 72 
chips connected to a single control line (if you used 4 dual-sided ECC 
DIMMS with 18 parts per DIMM).

At the speeds modern systems operate at, the couple of pico-farads of 
capacatance per DRAM chip can become very significant if there are 72 
parts hooked to the same signal.

So...I'd test it extensively before you install a mission-critical 
service or something, but you can probably get by just fine with the 
lower cost DIMMS.

-- 
Charles Steinkuehler
charles at steinkuehler.net