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Memory: Single-Rank, Dual-Rank, what?

F

freezerburrn

macrumors member
Original poster
Aug 20, 2006
79
0
La Habra, CA
I was wondering if someone could please clear up some confusion for me. I've been looking into getting a 4x 1 GB FB-Dimm package from vendors such as Ramjet.com, Otherworldcomputing.com, Datamemorysystems.com...etc., and some of these sites list their memory as Dual Rank.

Now, I've read articles and have sent questions to some of these vendors asking if their FB-Dimms are Dual Rank or not and some of the information they have sent me regarding Dual Rank memory seems to be conflicting. and even some review sites such as Anandtech.com that say Dual-Rank memory is preferable (Anandtech.com article linked here: http://www.anandtech.com/mac/showdoc.aspx?i=2832)

Here is a couple of replies I got from John Mayer from Ramjet.com after asking whether or not their memory they sell for the Mac Pro is dual rank or not.:

"Brian,
Al of the different FB-DIMMs that we sell for the Mac Pro are single
rank. There are dual rank modules out on the market for a lower
price, but they are not recommended for use in the Mac Pro."

and this one after I copy/pasted him to a passage from the Anandtech article:

"Brian,
I don't know where that text came from but it is incorrect (perhaps
they are confusion dual rank with dual pair). The Mac Pro has 8
slots, and is capable of addressing 8 ranks. Therefore, if you
install 4x dual-rank modules, you will be using up all 8 ranks, and
you will not be able to install any further RAM (i.e. the remaining 4
slots must be left open). This is why Apple specifically recommends
against using dual rank modules.

John"


So which is it? Is the term Dual Rank being used erroneously by some of the vendors and review sites out there or is Dual Rank what I want contrary to what John from Ramjet.com told me?
 
This is all I could find on it. It looks like the ranking has something to do with memory chips being on both sides of not. I am sure if that isn't correct someone else would chime in.
 
Apple's documentation clearly shows dual rank modules being used in the 1, 2 and 4 Gb size.

"A rank is a side of an FB-DIMM; 2 ranks indicate a 2-sided FB-DIMM.

1 GB FBDIMM, Device size 512 Mb, Configurqation 16 M x 8 x 4, Devices per rank 9, Rank size 512 MB, Number of ranks 2
"

I have never heard of single rank 2 Gb FB-DIMMs, if they exist they would be hella expensive. I really don't know where the vendor is coming from UNLESS they are talking about the 512 Mb FB-DIMMs only.

The MacPro is definitely able to address 16 ranks of memory, otherwise it would be limited to 4 Gb of RAM (given that almost all modules use 512 MB devices and 512 Mb per rank).
 
Hm, my 4x 1Gb kit just arrived from Ramjet.com and it looks like it is all single rank since the chips appear to be on one side only.

Now, what will see better performance? 4x 1Gb Single Rank or 4x 1Gb Dual Rank?
 
Looks like Steve from Ramjet cleared up all the confusion for me in the following email:

"Sir:

My name is Steve Moberley, and I am a tech here at Ramjet. Unfortunately, I am also the source of the bad information you received about our 1GB FB-DIMMs for your Mac Pro. I was engaged in other activities when John asked me about the ranking issue, and I gave the correct response, only it was for the wrong machine!



I have read the articles you forwarded to John Mayer, and yes, definitely the dual rank is preferable in the Mac Pro.



I sincerely apologize for causing this problem. At Ramjet we pride ourselves on being a definitive source for information about all Macs, and I really dropped the ball on this one. I’ll definitely be more careful in the future.



Our 1GB, 2GB, and 4GB modules are dual rank (a single rank module would be significantly more expensive), and always have been. You can populate your machine with any paired combination of our 512mb, 1GB, 2GB, or 4GB modules without any problems. The 512mb modules are single rank modules.



I can forward you a copy of our manufacturer’s complete technical specifications for the 1GB modules you received, if you would like. It’s a Word document and is six or seven pages long. I am not aware of anywhere within Mac OS where it will state whether the installed modules are single or dual rank.



Apologies again, and thanks for your business!



Steve Moberley

www.ramjet.com

1-800-831-4569"



Thank you all for your feedback!
 
I've got to say Ramjet has been most helpful with everything. 1 out of 8 modules I received from them was bad and they overnighted me a new pair!
 
There is some REAL CONFUSION here about memory 'rank' !!
It is NOT the same thing as 'sided'ness!

Rank is a JEDEC standards group term. A memory rank is an independently
addressable 64-bit area of the memory module, 72 bits wide on EEC modules
If the 'rank' (address) is for the whole DIMM, whether or not double sided = "single rank"
If there are different addresses for each side of the DIMM = "double rank"
If there are 2 address per side, sides are divided up = "quad rank"
Only registered DIMMs (servers) have full range of options, unbuffered dimms are always dual ranked.

BE CAREFUL the amount of memory you can hook into a board is based on RANK, not on GB of RAM or number of slots!

For EXAMPLE when let's say PC2-3200 memory is involved, you are limited to 8 ranks (again, this is regardless of # of slots/DIMMs !!)
using up 8 ranks: 16 GB ram = 8 DIMMs, SINGLE RANK, 2GB each.
using up 8 ranks: 4 DIMMs, DUALrank, 1 GB each

Usually the memory manuf. says what the rank is for each memory product, and computer manuf. says what the motherboard can handle.
Example: http://www.kingston.com/ukroot/serverzone/pdf_files/Mem_ranks_ENG.pdf

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