Don't Fill Memory Slots?

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PC Mag tip: ". . . if your PC has four memory slots, for best
performance you should avoid filling all of them. Usually filling of
just two slots is best."
It doesn't meet the test of reason. In order for this statement to be
true, the system would have to deduct more than the value of the
fourth piece. This says two 512s will yield 1 G, but four 256s won't.
That they might actually yield less than 768MB.

Ever hear of this?


Re: Don't Fill Memory Slots?

gecko wrote:
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I wonder if they are referring to putting in 4 x 1gig chips.
XP 32bit only addresses max 4 gigs and the hardware consumes some of the
4 gig space thus you only get like 3.2 to 3.5 gig of usable memory.
Thus putting in 3 chips is almost as good as putting in 4 chips.

I'm over simplifying this, but you get the basic idea.   There have been
lots of writeups about this in hardware NG's.

Re: Don't Fill Memory Slots?

gecko wrote:
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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.

John McGaw
[Knoxville, TN, USA]

Re: Don't Fill Memory Slots?

John McGaw wrote:
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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.


Re: Don't Fill Memory Slots?

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Yes, I've heard of this.  It all depends on the capacity of the memory
modules you insert.  The more capacity, the more current get consumed.
There may be a maximum current draw across all the slots that you must
not exceed.  4 256MB sticks in 4 slots will draw less current than 4 1MB
sticks in those 4 slots.  The mobo may even say that it supports 2GB
memory modules but you cannot put more than 2 in because those 2 already
draw the max current and 4 2GB sticks in 4 slots would be beyond the
milliamps that the mobo can supply to all 4 slots.

There is also the issue that you might use memory modules that exceed
the addressing bus width for the memory slots.  You will have to stay
under the max capacity per memory module that the mobo specifies (while
also staying under the max current draw).  However, exceeding the max
addressing width applies to all slots.  The only reason why I mention
this is because users often replace with higher capacity modules which
can then exceed the address bus width.  The result is that they won't
get the full capacity of the memory modules.

It might also refer to dual channel mode which requires memory modules
in pairs.  If you add 2 memory sticks (and in the correct slots) then
dual-channel mode is enabled.  If you add 1 memory stick, you obviously
can't do dual-anything.  If you add 3 memory sticks, you may or may not
get dual-channel mode.  For some mobos, the first 2 sticks (in the
paired slots) will be dual-channeled while the 3rd stick is not.  In
some mobos, you lose dual-channel model altogether.

So is this "tip" somewhere online so the rest of us can read what you
read to know what it really said?

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