Single- and Double-Sided RAM?

[largon]

I have noticed that there is a very common misunderstanding regarding memory sidedness in this forum. I've done a little research on the matter and decided to write down the facts.

First things first:
Being Single- or Double-Sided has nothing to do with the physical placing of ICs on the circuit board (PCB).

So what does make a module single- or double sided?

Let's examine for example Winbond W942508CH-5:
The IC is configured 32x8.
"32" means IC's capacity is 32 megawords. Edit: to High5 for correction
"8" is the amount arrays inside a chip (ie. the bit count per word). Only one bit can be read/written at a time to a single array, so eight bits at a time can be written to a x8 chip. Soon you'll notice this very number is the key to our guest for dimm sidedness.


Definition for "single/double sided module":

Single sided module can read/write 64 bits at a time.
Double sided module can read/write 128 bits at a time.


If we know the IC configuration, and the amount of ICs on a DIMM, we can simply calculate how many "sides" a module has:

Let's say the module has 8 Winbond W942508CH-5 chips -> to figure if this module is SS or DS we only need to multiply the amount of ICs with the amount of the arrays ie. bits per chip ->

8 (ICs) times 8 (bits) = 64 -> Single sided module !
...and in case we have a 16 chip module:
16 (ICs) times 8 (bits) = 128 -> Double sided module !


Good links about the subject:
http://www.realworldtech.com/page.cf...WT110401204523
http://arstechnica.com/paedia/r/ram_...e.part1-1.html

[high5]

you got the most things right except for this:

The IC has configuration of 32Mx8. Where "32M" means the IC's capacity is 32 Megabytes (32*1024*1024=33554432 bytes) cells, "8" is the amount arrays inside a chip.

32M doesn't mean 32 megabytes but rather 32 mega 'words'. x8 means that one word containes 8 bites. in this case it 32M coincides with actual capacitiy of the ic.

take 64Mx4 for example: 64 mega 'words', each word is 4 bites. the effective capacity of this ic is equal to 32Mx8 one, that is 32megabytes.

of course (as you stated yourself), x8 ic is 'wider' and it takes two times less of those ic's to build one rank (64bits 'wide').

btw, here you have unbuffered ram utilizing x4 ic's on sale.