John said:
I can think of interpretations but without seeing the original "tip" I
won't venture a final reply. How about a link so we can see exactly what
it is they wrote?
One idea that comes to mind is that some chipsets don't handle the extra
loading of more sticks gracefully. In other words, they can use more
aggressive memory timing with two sticks than they can with four sticks.
I think that is what they're getting at. If you had an Athlon64 with four
DDR memory slots, you could run two sticks at DDR400 Command Rate 1T.
If you install four sticks, then DDR400 2T or DDR333 1T would likely
be speed/timing settings for the RAM. The lost in memory bandwidth
is about 20%, by going to 4 sticks on Athlon64, with an application
performance loss of around 6%. (This assumes the sticks are double
sided, and the biggest capacity sticks will be that way.)
On a modern DDR2 motherboard, I feel you have less to worry about.
At one time, Anandtech used to include 2 stick versus 4 stick
test results, and on DDR2, there was hardly any change to the
CAS and other settings. So for DDR2, the performance loss is
not a guarantee. Perhaps if you were operating at extremely high
DDR2 clock settings, there would be enough of a loading effect
to care (so it you were trying to take top spot in the Futuremark
ORB, you might dispense with four sticks and try two).
Naturally, there are exceptions to every observation. There are
some boards, where if you read the reviews, you can see they're
not that good when it comes to the memory bus design. They
seem to be unstable with four sticks. But that doesn't mean
it is the fault of the technology, but rather a badly designed
instance of motherboard or chipset. Which is why you should read
the reviews, before buying that motherboard to take four sticks.
In terms of getting memory capacity stated, you always get the
capacity in bytes. If you install 4 * 256MB, then there will be
1GB of storage locations present in the computer. But if the
BIOS is planning an address map for all the hardware (which
routes certain addresses to certain hardware resources on the
board), then you could find in that case, that some memory
is "unreachable". If a system was limited, for whatever
reason (*), to a 4GB address space, you have 4GB of memory installed,
two 512MB video cards, then *something* in the box is not
addressable. The BIOS chooses to not completely map the
memory in that case, so only the bottom section of the
memory is available for use. The top part of the memory
is still there, but no bus operation of the processor
can read it or write to it. In that case, the memory
reported in Windows, has to be less than 4GB, since
Windows will not be able to access the upper part
of memory. With the low price of memory, for most
people, that loss is a "don't care". For those
worried about getting their "money's worth", they
could always try a 2x1GB + 2x512MB config of DDR2
sticks, for a total of 3GB.
(*) To make the previous paragraph shorter to write,
I didn't go into details of the limitations caused
by a 32 bit OS, versus a 64 bit OS, versus chipsets
that support memory remapping and ones that don't.
And so far, I haven't seen a single web page, that
does a good job of addressing the issues thoroughly.
Paul
