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