Shared Memory

[Lennart Sorensen]

On Fri, Jan 28, 2005 at 09:36:13AM -0500, John Wildberger wrote:
> I am in the market for a new computer and would like to sort out some of the 
> advertising hype. 
> What does "shared dual channel DDR2 SDRAM" imply?
> -shared-: 
> Is this a HW or SW feature? Does it share memory with a graphic card or does 
> it share it between running processes?
> -dual channel-:
> Is this a feature of the memory or how the memory is used by the mobo to 
> achieve an effective 800MB  data channel using a 400MB rated memory?
> -DDR2-:
> What is to be gained by using DDR2 versus DDR ?
> I see more and more SATA drives being part of packaged computers. What are the 
> pro's and con's for them? Are they more difficult to get installed with 
> Linux?

Usually shared memory means something other than the CPU/chipset is
using the meomry for it's own use.  Often this would be the case for
somethign that has onboard video, such as the crap made by intel in some
of their chipsets.  This is a bad thing (but cheap to manufacture).

Dual channel means the chipset runs the memory in two banks to give you
twice the memory bandwidth.  This is a good thing.

DDR2 can theoretically be faster than DDR, but so far nothing seems to
ahve really taken advantage of it yet.  I suspect it still costs extra
too.

SATA is great, since the cable can be longer, and blocks less air flow
tahn the old IDE ribbon cables.  It also allows a higher transfer rate
although no current drive is anywhere near that limit.  SATA currently
comes in 1.5 or 3.0Gbps (150 or 300MB/s).  IDE is normally 100 or
133MB/s max (although shared between up to two drives per channel).
SATA doesn't share.  Of course with the bus speed of PCI usualyl being
132MB/s this only matters when the chipset connects the controller to
something faster than plain PCI (which many new systems fortunately do).
Linux support for SATA works great on many chipsets, but not all.
Sil3112 works fine, intel ICH5 works, nvidia's nforce4 works, and via's
sata works.  Sil3114 is a pain to get drivers for as far as I can tell,
and I am not sure about any of the promise chips either.  Make sure the
bios is set to native mode or you will have some issues (for example the
intel won't run your regular cdrom/dvd in dma mode if set to shared or
compatible mode, but will in native mode).  Of course XP users need a
floppy drive to load SATA drivers if in native mode, or a custom made
install CD that includes the drivers (often called slipstreaming).

Lennart Sorensen
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