Young Mandy wrote:
> Current Set Up
>
> ASUS P5B - Deluxe - JUST STANDARD SETTINGS IN BIOS (no overclocking etc)
> Intel E6600 Core 2 Duo 2.4ghz CPU
> Antec Sonata II Ultra Quiet Midi
> 2 x 1Gb Geil Value DDR2 2.0GB PC6400 Dual Channel memory kit (2 x 1GB)
> 800MHz (5-5-5-15) +Aluminium heat spreader, ATI Crossfire Ceritified
> Nvidia 7600GS 256mb DDR Graphics
> 250GB Seagate SATA II 7200 (already had a 160GB one)
> Windows Vista Business 32 Bit
>
> I have the opportunity of buying additional memory (2 x 1GB) exactly the
> same as my existing (Geil Value DDR2 2.0GB PC6400 Dual Channel memory
> kit (2 x 1GB) 800MHz (5-5-5-15) +Aluminium heat spreader, ATI Crossfire
> Ceritified) at a really really good price......practically a give away.
>
> I often hear about motherboards not recognising the 4th slot (ie it wil
> only recognise 3GB instead of 4GB).
> Will this be the case on this board?
> If it is the case is it still worth going ahead and getting it, although
> it will only show 3GB
>
> Any advice will be greatly appreciated.
>
> Mandy
That is a tough question to answer. I think for the most part, people have
just been buying the memory, because it is so cheap, and not worrying about
details.
You could check the vip.asus.com forum for your board, and see if people
report any particular problems with 4x1GB. There may have been a problem
installing Vista when 4GB is present, but you've already installed your
OS, so you might not see that problem.
In terms of what memory may be available to you, I see two potential limits.
1) For a 32 bit OS, where PAE is being used to make DEP work, there is
only room for 4GB of addressing. Some address space is needed for
system busses. Your video card has 256MB on it. The BIOS sets up the
memory map. The BIOS will allocate address space in chunks of 256MB or so.
At least 256MB for PCI, and 256MB for PCI Express. That means that once
address space is allocated for system busses and objects on them, there
is 3.5GB or less, left to allow addressing memory. The remainder of memory
cannot be accessed - it is unreachable. (To give an extreme example, on
one Athlon64 system, which had two video cards for SLI operation, and the
cards had 512MB of graphics memory on each card, only 2.5GB of system memory
was accessible, of the 4GB plugged in. You have one video card, so your
penalty will be less. You might see 3.2GB of your 4GB, for example. It
depends on how the BIOS chooses to assign the address map.)
2) I previously found an article on the Microsoft website, where the number
3.12GB was mentioned for a 32 bit OS. Now, this article doesn't state that
this limit is related to address map - it may be, but they didn't say as
much. As far as I know, if a person switched to a PCI video card, instead
of a PCI Express one, they may be able to get more than 3.2GB free reported,
which may be at odds with this page.
http://support.microsoft.com/kb/888137/en-us
Now, if you installed 2x1GB + 2x512MB, chances are you wouldn't lose any
of the memory, due to addressing limits in a 32 bit OS. On previous
motherboards, ones with DDR, you might have lost a tiny amount of memory
subsystem performance, by using that memory configuration. But when
I read a couple DDR2 era datasheets, to answer your question, I see
a subtle distinction between the way the DDR boards worked, and the way
the DDR2 ones work. It looks like an entire cache line comes from one
channel, and in order for dual channel to work, the cache lines would
have to alternate (fetch an odd line, fetch an even line, causes both
memory channels to respond, and gives the "dual channel boost").
There wasn't mention of optimizations for open pages, implying that
while 2x1GB + 2x512MB would be slightly worse for DDR, it seems
to make no difference to DDR2. At least, that is how I'm interpreting
what I'm reading. (My eyes glaze over when I read parts of the DDR
description, and I've never been able to understand the wiring pattern
of bits used for row and column addressing in the Intel chipset
memory controller descriptions. So I cannot claim to understand all
of it.)
*******
A DDR2 memory guide from Intel - note there is no table prioritizing
performance versus configuration. I wish Intel would fix page 12!
(Figure 4 is messed up.)
http://download.intel.com/design/chi...s/30234403.pdf
A DDR memory guide from Intel - see performance table on last page
http://download.intel.com/design/chi...s/25303601.pdf
*******
OK, so what are the alternatives, based on the little I understand today ?
1) 4x1GB - may see 3.2-3.5GB free memory reported in 32 bit Windows. May cause
problems during OS install - removing a stick or two, during a reinstall,
may fix that, as may a Service Pack.
2) 3x1GB - this would be what you'd attempt to use, if (1) failed. The
consequence of doing this, is the lower 2GB of memory operate in
dual channel mode, and the upper 1GB operates in single channel mode.
That is my understanding of "Intel Flex Memory" mode.
3) 2x1GB + 2x512MB - Due to the use of DDR2 and the different design
scheme used, there appears to be no memory controller penalty for
this mode. You would get very close to 3GB free memory reported.
With the low cost of memory, you might save $25 or whatever.
I think the small cost saving, is why some people don't bother with
these refinements.
HTH,
Paul
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