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explanation of how to get compatible memory for a motherboard

Question title says it all.  What the hell is 16X16 ?  I thought it meant 16 chips each with 16MB, for a total of 256MB on the memory stick.  However, I got the following message from a vendor:

[quote]
We have reviewed the transaction and found out no error from our system and the shipment.

The memory sticks you purchased have 8 chips on each stick as described:
http://www.cesell.com/product.php?productid=20798&cat=0&page=1

Btw, 16x16 is the parameter of DRAM, not the number of chips and the size of chips.

Thanks,
Tech Support
CESell.com
[/quote]
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mraeryceos
Asked:
mraeryceos
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1 Solution
 
Irwin SantosComputer Integration SpecialistCommented:
Here is a good memory resource for you...and they sell the RAM too.

http://www.crucial.com/
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garycaseCommented:
"... What the h... is 16X16 " ==>  The memory your bought was "16X16 8CH", which means it was internally organized as a 16K x 16K array using 8 chips.   Externally all SDRAM is 64 bits wide (unless it's ECC, in which case it's 72 bits).

By the way, the source you used provides one bit of information that's hard to find from many memory vendors (including Crucial & Kingston) -- the # of chips on the stick;  which lets you know if it's single-sided or double-sided memory.   (the link to the module you bought does clearly show "8 chips")  Crucial and Kingston do NOT provide this information, which is unfortunate, as there are many motherboards which require a specific density of memory.  
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mraeryceosAuthor Commented:
garycase:

By density, you mean MB or Mbits (the amount of memory) per chip?
Is the number of CH (channels?) the same thing as the number of chips on the stick?
On the Intel site:
http://www.intel.com/design/chipsets/mature/index.htm
The row that says Mbit support means the number of Mbits per chip?  For example, if that number is 512, then 512/8=64MB per chip, and the maximum memory a slot can take would come from a double sided stick with 8 chips per side (64MB x 16 chips = 1GB).  Correct?

I don't understand what you mean by "internally organized".  What exactly is an array?
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willcompCommented:
Unless you have a great deal of experience, the best method to ensure compatibility is to use a memory configurator from one of the major memory manufacturers.  I prefer Crucial, but Kingston is also good.

SDRAM comes in single, dual, and quad densities (referring to capacity of each chip where single density is 64 Mbits).  Most motherboards that accept SDRAM do not support quad density RAM which is what you purchased.

If you provide your motherboard model or its chipset, we can help determine appropriate memory.  The number of chips for a given capacity is essential information that is not always provided by vendors.  The designations used by memory manufacturers usually provide capacity and bit width, but not density.
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willcompCommented:
Correction - dual should have been double, as in double density.
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garycaseCommented:
All memory is organized in an array of rows and columns -- when you access it there is what's known as a "Column Address Strobe" and a "Row Address Strobe" to identify the exact location you want to access.

Individual memory chips have a particular amount of memory on them -- say 64MB as in your example -- and the number of chips on a stick depends on the size of the individual chips and the total capacity of stick => so yes, if you had 64mb chips then a 1GB stick would have 16 chips on it (so it would be double sided).   The details of accessing the memory are slightly different for a single-sided array and a dual-sided array, and some motherboards require specific organization to work correctly in specific modes, as they may not support proper addressing of dual rank (double-sided) modules, or may only support dual rank modules in specific configurations.

Yes, memory density means the amount of memory per chip.   The number of channels is NOT the same as the number of chips.   A dual-channel memory controller can access two memory sticks at the same time ("dual" channels) IF the specifications of the memory sticks are identical (that's why you should buy matched pairs of memory for a dual-channel motherboard), essentially doubling the memory bandwidth.

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garycaseCommented:
... I agree with willcomp that the best approach is to use a memory selector => crucial, kingston, and 4allmemory (www.4allmemory.com) are all good.
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mraeryceosAuthor Commented:
garycase, If I understand correctly, dual-channel memory access has nothing to do with dual rank memory sticks (modules)?  Or related?
I understand the array thing now, and I understand that dualrank/doublesided sticks have 2 separate arrays on each side, and the motherboard must be compatible with the way the 2 arrays are organized.
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garycaseCommented:
Yes, you understand correctly ==> there's no relationship between dual rank memory sticks and dual channel memory access.
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mraeryceosAuthor Commented:
garycase, can a 128MB stick be 16x16?  I saw this in another receipt I had.  If the array is 16K x 16K, doesn't that imply 256,000,000 locations in the array?  Is each location in the array a bit or a byte?
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garycaseCommented:
The 16x16 is referring to the chip layout -- size vs bytes.

So each chip is 16mb x 16 bits => or 32MB / chip.   So a 256MB module will have 8 chips;  a 128MB module using the same chips would have 4 chips.

When I first read your comments I thought this was a memory layout designator;  but looking at the page for the vendor it's clearly a size x bits indicator.

Note that the chips are combined in groups of 4 on the module(s), since each actual access is 64 bits wide.
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mraeryceosAuthor Commented:
I can't find anything having to do with sdram on the internet that mentions "size vs bits" OR "size x bits".  I did find at least one page that mentions "mb x bits":

http://developer.apple.com/documentation/Hardware/Developer_Notes/Macintosh_CPUs-G4/iBookG4/4_Expansion/chapter_5_section_3.html

Although I've found some wikipedia articles that were really good (I'm still reading), I haven't come across memory organization within each chip.
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