DDR2 SDRAM: Does MHz matter?

[Tristan Miller]

Greetings.

I've got a Samsung X20 laptop which I am considering giving a memory
upgrade. The user guide states only the following with regard to the type
of memory required:

Memory: 256/512/1024 MB DDR2 SDRAM, 2 DDR socket

I did some Internet window-shopping and note that this memory is available
at different clock speeds -- 512 MB @ 333 MHz or 512 MB @ 266 MHz, for
example.

Does it matter what the clock speed of the memory chips are? Can I mix and
match, or do both the installed chips have to be running at the same clock
speed?

Regards,
Tristan

 

[Barry Watzman]

Yes, it matters, although a memory module made for a speed faster than
the computer is running at will normally work ok. A memory module made
for a slower speed either will not work at all, or it will work but this
will be accomplished by slowing down the entire memory system to match
the slower memory.

Mix and match? Yes it will work, but: If you mix memory modules on a
machine that is capable of dual channel operation, it will revert to a
single channel mode, cutting the memory subsystem throughput in half.
[See my thread a few days ago about "dual channel memory"]. For proper
dual channel operation, although the modules need not be truly
identical, both their architecture (including their internal
architecture, which you normally have no way of determining] and ALL of
the timing specs must be identical. Also, if you mix different speeds,
not only will you disable dual-channel operation, but the entire memory
system will operate at the speed of the slowest of the two memory
modules (and if that is lower than the "normal" speed of that
CPU/chipset, there is a chance that it won't operate at all, but more
commonly it just slows the system down to match the memory).

 

[Tristan Miller]

Greetings.

Thanks for your helpful explanation. I do have some further questions,
though.

Does it matter what the clock speed of the memory chips are?

For proper
dual channel operation, although the modules need not be truly
identical, both their architecture (including their internal
architecture, which you normally have no way of determining] and ALL of
the timing specs must be identical.

Do the memory capacities also have to be identical? That is, can I use one
512 MB chip and one 1024 MB chip?

And how would I go about determining the specs for my existing RAM chips,
if there's no "normal" way of doing it? Would it be indicated on the
chips themselves? Or must I write to Samsung, the manufacturer of my
laptop?

Regards,
Tristan

 

[Barry Watzman]

You can mix capacities, but if you do so you definitely preclude
dual-channel memory system operation. In precluding such operation, you
may be cutting the memory system speed in half. [However, not all
computers can run dual channel interleaved memory anyway, without regard
to whether the modules are matched or not.]

There is no easy way for an end-user to determine all of the relevant
specs for his memory modules. If your system doesn't support dual
channel operation anyway, then you can just ignore most of the specs and
get any module of any size of the right type (SDRAM, DDR, DDR2) and fast
enough to allow full speed operation of the computer (again, if you mix
speeds, at best the memory system will run at the speed of the slowest
module).

If your system does support dual channel operation and your objective is
enable such operation, the best way for an end-user to do this is by
obtaining two identical modules (usually by purchasing them together at
the same time from the same dealer).

The next best option (again, see my earlier post and response on dual
channel memory) is to TRY to obtain a non-identical module which is the
same in all of the ways that matter. In this regard, on some modules
the adhesive part number label has a lot of detailed information (I was
lucky, in my case it did), but in other cases the label is essentially
useless.

If you cannot determine the necessary specs from the module label, it
may be possible to determine them from the actual part numbers on the
chips theselves by finding the chip manufacturer's data sheet for those
particular parts (I'm talking here about the chips on the module, not
the module itself). The data sheets are usually available online,
however, this is ***WAY*** beyond the ability of 95%+ of all computer
owners. In that case, the best thing to do may, again, be to replace
both modules and just sell the removed single module on E-Bay.

Greetings.

Thanks for your helpful explanation. I do have some further questions,
though.

Does it matter what the clock speed of the memory chips are?

For proper
dual channel operation, although the modules need not be truly
identical, both their architecture (including their internal
architecture, which you normally have no way of determining] and ALL of
the timing specs must be identical.

Do the memory capacities also have to be identical? That is, can I use one
512 MB chip and one 1024 MB chip?

And how would I go about determining the specs for my existing RAM chips,
if there's no "normal" way of doing it? Would it be indicated on the
chips themselves? Or must I write to Samsung, the manufacturer of my
laptop?

Regards,
Tristan

 

[Tristan Miller]

Greetings.

You can mix capacities, but if you do so you definitely preclude
dual-channel memory system operation. In precluding such operation, you
may be cutting the memory system speed in half. [However, not all
computers can run dual channel interleaved memory anyway, without regard
to whether the modules are matched or not.]

So how do I know whether my system (a Samsung X20) can run dual channel
interleaved memory? I don't see any specific mention of it one way or
another in the spec sheet.

Regards,
Tristan

 

[The little lost angel]

the timing specs must be identical. Also, if you mix different speeds,
not only will you disable dual-channel operation, but the entire memory
system will operate at the speed of the slowest of the two memory

While the speed will slow down to match, I don't think it will affect
dual channel availability just because the DIMM are rated for
different speeds.

 

[George Macdonald]

Greetings.

I've got a Samsung X20 laptop which I am considering giving a memory
upgrade. The user guide states only the following with regard to the type
of memory required:


I did some Internet window-shopping and note that this memory is available
at different clock speeds -- 512 MB @ 333 MHz or 512 MB @ 266 MHz, for
example.

Are you sure about those speeds and that you're looking at the correct
memory type, i.e. DDR2 and not DDR? DDR2 is generally quoted at the double
clocked rate and starts at DDR2-400 and goes up. Samsung's web site is a
little hazy on the subject but you should be looking at DDR2-400(PC2-3200)
or DDR2-533(PC2-4200), depending on the version of the system you have -
apparently there are two X20s in the DDR2 class??

Does it matter what the clock speed of the memory chips are? Can I mix and
match, or do both the installed chips have to be running at the same clock
speed?

Yes the speed does matter - you don't want to get a lower speed than you
currently have. In general, a higher spec part should work but I'm not
sure how finicky DDR2 memory might be in that respect. Either way,
PC2-3200 or PC2-4200, unless you buy the authorized dealer Samsung part,
you want to be sure *before* buying that it's guaranteed to work, and in
that respect you can't beat Crucial... and their price is usually much
lower than dealer parts.

Check out www.crucial.com - they have a U.K. and EU site - just enter your
product data and see what they recommend. Since you'll be taking off the
memory slot cover to do the upgrade, pop it off first and see if the
current part that's in there is labelled with speed info for matching to
etc.

Since Samsungs specs on the system seem to cover at least two different
models, and are umm, kinda generic, you might try running a system ID
utility like http://www.cpuid.com/cpuz.php or Sandra to see what they make
of speeds and memory single/dual channel configuration.

 

[Tristan Miller]

Greetings.

Are you sure about those speeds and that you're looking at the correct
memory type, i.e. DDR2 and not DDR?

No -- I didn't realize there was a difference.

Check out www.crucial.com - they have a U.K. and EU site - just enter
your product data and see what they recommend.

Thanks; they had an entry for my system:
<http://www.crucial.com/store/listpa...ooks&mfr=Samsung&tabid=AM&model=X20&submit=Go>

I'll check out those system ID programs you mentioned.

Regards,
Tristan

 

[George Macdonald]

Greetings.



No -- I didn't realize there was a difference.

Be *very* sure about this. I was assuming your X20 is quite a recent
system - older versions used DDR memory. I wish mfrs would quit using the
same model number, or at least adopt a suffix, for systems which use
different technologies.

For the dual channel thing, again assuming a newish system, the i915PM/GM
chipset supports dual channel mode but the mbrd would have to be
implemented to support it too. Samsung is err, silent on the subject on
their web site.<shrug>

 

[Barry Watzman]

Interleaved dual-channel memory is a function of the CPU chipset,
although such operation could be blocked by the BIOS if the laptop mfgr.
was playing stupid laptop bios tricks. The X20 is a Centrino system
with an Intel 915 chipset. It should be capable of dual-channel
interleaved memory operation.

Also, it's a 533MHz FSB, so you need DDR2 PC2-4200 (or faster) memory
(Samsung actually says PC2-4300, but I've not seen such memory commonly
available). Plain DDR (or SDRAM) won't work (at all), and slower memory
will slow down the system. For interleaved dual channel operation, the
two modules must have identical specifications, although, strictly
speaking, they don't have to be absolutely identical (I have a very
similar Toshiba laptop, and successfully run the system with interleaved
dual channel memory operation using one Hynix and one Elpida module, but
the two modules have identical specs and identical internal and external
architecture).

Greetings.

You can mix capacities, but if you do so you definitely preclude
dual-channel memory system operation. In precluding such operation, you
may be cutting the memory system speed in half. [However, not all
computers can run dual channel interleaved memory anyway, without regard
to whether the modules are matched or not.]

So how do I know whether my system (a Samsung X20) can run dual channel
interleaved memory? I don't see any specific mention of it one way or
another in the spec sheet.

Regards,
Tristan

 

[Barry Watzman]

I think it will. Normally the bios reads the specs of the memory
modules from the SPD (serial presence detect) chip on each memory module
and compares them. And, normally, if they don't match exactly, dual
channel interleaved mode is disabled, even if it might work at the speed
of the slower chip. This is all up to the bios, so it might not always
be the case, but that's how it's usually done. The settings on laptops
(which normally have no ability to explicitly set or override anything,
unlike many desktop motherboards) are very conservative.

 

[dungeonfleet]

Get the one with the most clock speed, so that the ram would work
faster, you dont have to have both the rams at the same clock speed

 

[Barry Watzman]

Yes you do, on most systems, for dual channel operation.

 

[George Macdonald]

Get the one with the most clock speed, so that the ram would work
faster, you dont have to have both the rams at the same clock speed

All the memory will run at the speed of the slowest SO-DIMM in the system
so, while strictly speaking some degree of mismatching is tolerated by the
chipset, I believe that most who have experience with this kind of setup
would advise getting as close a match as possible in terms of DRAM
technology, chip select sizes and nominal DRAM clock speeds.

Note that even if a chipset tolerates certain asymmetries in channel
configuration, we've all had experiences where the BIOS was not as smart as
the chipset would allow.

Bottom line: if you're buying, why buy something which *might* work?

 

[Gnu_Raiz]

Greetings.

Thanks for your helpful explanation. I do have some further questions,
though.

Does it matter what the clock speed of the memory chips are?

For proper
dual channel operation, although the modules need not be truly
identical, both their architecture (including their internal
architecture, which you normally have no way of determining] and ALL of
the timing specs must be identical.

Do the memory capacities also have to be identical? That is, can I use one
512 MB chip and one 1024 MB chip?

And how would I go about determining the specs for my existing RAM chips,
if there's no "normal" way of doing it? Would it be indicated on the
chips themselves? Or must I write to Samsung, the manufacturer of my
laptop?

Regards,
Tristan

Their is some good advice in this thread, normally you want to match the
size and the speed of the modules. But if you already have the memory then
it might work as long as it's the same brand and is set to the same speed.

For example, I just built a desktop system based on a 939 socket. I
already had one stick of 512 another of 256 so I bought another stick of
each from the same manufacture and speed as the ones I already had.

I was expecting to run in single channel mode but to my surprise the bios
boot screen got it working in dual channel mode. That is to say, I had the
256 sticks on channel one, and the 512 sticks on channel two. To verify
stability I ran memtest86+ for over 24 hours and did not have any errors.

Now this might be a little off topic, but really could the average laptop
user tell the difference between dual channel and single channel mode? The
only exception might be if your number crunching, or running a scientific
application that might benefit from the dual channel mode.

But face it in the real world a laptop user would probably benefit more
from a faster harddrive then from dual channel memory. Then you have to
consider the bigger benefit of more memory, vs faster memory. I think the
the more memory side wins out, if you consider price. You also have to
consider the FSB, if your running a Intel chip then you have already
reduced the benefit of dual channel memory.

Gnu_Raiz

*** Free account sponsored by SecureIX.com ***
*** Encrypt your Internet usage with a free VPN account from http://www.SecureIX.com ***

 

[Nate Edel]

You can mix capacities, but if you do so you definitely preclude
dual-channel memory system operation. In precluding such operation, you
may be cutting the memory system speed in half. [However, not all
computers can run dual channel interleaved memory anyway, without regard
to whether the modules are matched or not.]

And in the case of some laptop systems, while they can run dual channel,
there's no advantage to doing so - this would be the case for recent
533mhz-FSB Pentium M laptops if they have a separate video chip (dual
channel is only an advantage for those if using the intel video chip which
uses main memory for video ram).

 

[Quaoar]

You can mix capacities, but if you do so you definitely preclude
dual-channel memory system operation. In precluding such operation, you
may be cutting the memory system speed in half. [However, not all
computers can run dual channel interleaved memory anyway, without regard
to whether the modules are matched or not.]

And in the case of some laptop systems, while they can run dual channel,
there's no advantage to doing so - this would be the case for recent
533mhz-FSB Pentium M laptops if they have a separate video chip (dual
channel is only an advantage for those if using the intel video chip which
uses main memory for video ram).

Whoa! Do you have a reference for this dual channel RAM?

Q

 

[George Macdonald]

You can mix capacities, but if you do so you definitely preclude
dual-channel memory system operation. In precluding such operation, you
may be cutting the memory system speed in half. [However, not all
computers can run dual channel interleaved memory anyway, without regard
to whether the modules are matched or not.]

And in the case of some laptop systems, while they can run dual channel,
there's no advantage to doing so - this would be the case for recent
533mhz-FSB Pentium M laptops if they have a separate video chip (dual
channel is only an advantage for those if using the intel video chip which
uses main memory for video ram).

How so? The i915GMS doesn't support dual channel at all and all the other
i915s do, including the PM. So why would the i915PM which doesn't include
Intel integrated graphics not benefit from dual channel memory?