registered vs. unbuffered compatibility (P5BWD2)

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This _must_ be a FAQ, yet I couldn't find the answer anywhere (or
rather, I found completely conflicting answers):

Will Registered ECC RAM work on a motherboard that calls for
"Unbuffered non-ECC / ECC RAM"?

(First, I'm not even sure how "Unbuffered non-ECC / ECC RAM" is to be
parsed: it could be "Unbuffered non-ECC" / "ECC" or "Unbuffered"

Specifically, the RAM type would be DDR2-533 Registered ECC, and the
motherboard is a P5WD2.

Note that I am _not_ asking about mixing unbuffered and registered
RAM: I am aware that this will not work.  I also understand that
registered RAM might loose a clock cycle for every memory access: this
is not a concern.  I am merely asking whether the memory will work at
all (and as ECC).

Many FAQs explain the difference between registered and unbuffered
RAM, and between ECC and non-ECC, and imply that certain configs
require registered RAM (e.g., dual processor motherboards, any kind of
Opteron, etc.), but the point about certain motherboards requiring
_unbuffered_ RAM is not made.  The motherboard's manual also makes no
particular warning (whereas it mentions a whole lot of things that
will _not_ work, such as trying to use DDR instead of DDR2).  I am
also told that ECC RAM will work on non-ECC motherboards.

The reason for my concern is that it seems extremely difficult, if not
downright impossible, to get Unbuffered ECC RAM - let alone DDR2
Unbuffered ECC - here; in fact, one broker could not even be convinced
that it exists.  It's most unfortunate having a motherboard that
supports a kind of memory that is impossible to find!  On the other
hand, Registered ECC is relatively easy to find.  (Also, it seems to
me that it would be massively stupid not to allow for the mere extra
clock cycle delay it takes to make Registered RAM usable on all
motherboards, but, well, stupidity is hardly ever a reason for things
being otherwise.)

     David A. Madore
    (, )

Re: registered vs. unbuffered compatibility (P5BWD2)

David Madore wrote:

"  Will Registered ECC RAM work on a motherboard that calls for
"Unbuffered non-ECC / ECC RAM"?  "

No, it won't.  You can get non-registered (unbuffered) ECC RAM, and it
is that which it refers to.  Registed ECC RAM will not work if that is
how it's stated on the specs.

Re: registered vs. unbuffered compatibility (P5BWD2)

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If you need information on memory the best place to get it is here at the
OCZ forum:
You will get answers from one of the tech support staff from OCZ.
Hope his helps,

Re: registered vs. unbuffered compatibility (P5BWD2)

Buffered/registers vs unbuffered/unregistered and ECC vs nonECC are two
different issues.  What I am about to post is very general and thus not
accurate, but it should do.

ECC just adds error correction capability to the module.  In most cases
you can use ECC or non-ECC memory, you just WON'T get the benefit of ECC
unless you use all ECC memory in a ECC capable system with ECC enabled.
ECC memory just adds additional bits to the width of the 'word' of
memory being transferred - instead of a 'word' being 8/16/32 bits
10/18/36 bits are transferred - the calculation and checking of the
'parity' is done by the memory controller.  Most nonECC systems just
don't know they are there.

I have seen some vendors specifically state that ECC capable memory is
not supported on their board.  I haven't tested it, so I don't know if
there was a 'real' hardware compatibility issue or the vendor didn't
what the hassles of someone putting ECC memory in and magically
expecting it to work.

What is the downside of using non-ECC memory?  You don't know if what
your getting from memory is what you put there.  Is it a big thing?
Depends.  For most of us: no, the likely hood of a bit being corrupted
is very small.  The enabling of ECC does slow the system down a little -
there is overhead in calculating the checksum when storing or retrieving
each word of memory.  ECC memory is also more expensive as there are
more chips on the module to support the ECC encoding.

Registered and buffered memory are the same thing.  The memory module
has a buffer register between the memory chips and the memory
controller.  Why?  Basically it is a signal booster.  The memory
contoller can only communicate with a limited number of chips - each
chip it communicates with requires a little current and the controller
has a limited amount of current it can generate.  The use of the buffers
reduce the number of chips it has to talk to directly.  So, with
unbuffered memory the controller might be able to talk to 3 DIMMs worth
of memory chips, with the use of buffers it might be able to talk to 24
DIMMs - the limit here is the number of chips it can communicate
directly with not the amount of memory it can address.  

Your system specs should say whether you need registered memory or not.
If it doesn't state, then it probably  wants unbuffered memory.

If you system only supports 1/2/3/4 slots, it probably uses unregistered
memory.  If your system is a server supporting 5+ slots, it probably
uses registered.  Some systems are capable of supporting either, but not
both at the same time, certain Opterons for example.  

If you are having problems finding the right memory for you system, try
one of the online retailers. Here are two that have a presence in France
or the EU. / /

Re: registered vs. unbuffered compatibility (P5BWD2)

On Fri, 5 Aug 2005 13:04:19 +0000 (UTC), (David
Madore) wrote:

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Almost certainly not.

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As a general rule of thumb a motherboard either requires registered
memory or it requires unregistered memory.  These days there are very
few boards that will work with either (older boards are a bit
different in this regard, but for new boards it usually one or the

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To put DDR memory in that board would require a hammer... don't do it!

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Generally speaking yes, though that's definitely not a sure-thing.
Usually if you put ECC memory in a board that doesn't support ECC it
will just ignore the extra chip and work exactly as if it was non-ECC
memory.  However you are taking a bit of a chance doing so.

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It most definitely does exit.  There's absolutely no requirement for
ECC memory to be registered/buffered.  In fact, there's absolutely no
requirement for registered memory to be ECC, though non-ECC registered
memory is EXTREMELY rare.

Here's a link to some DDR2 667 ECC/unbuffered memory:

Seems like this is what you were looking for?  Crucial also carries
1GB parts as well as some DDR2 553 memory that should fit the bill.

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Your motherboard does also support bog-standard non-registered,
non-ECC memory, so if ECC is not a requirement for your applications
you could go for that.  If not, Crucial can definitely provide you
with memory that will work for you, though perhaps not with the
maximum speed DDR2 800.  Kingston also has non-registered ECC memory,
as should most of the other bigger memory companies.

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There's more to it than just an extra clock cycle.  It's rather tricky
(read: extra cost) to make a board that support BOTH registered and
non-registered memory these days.  As such you're pretty much left
with one or the other.

Tony Hill
hilla <underscore> 20 <at> yahoo <dot> ca

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