A memory rank is a set of DRAMs connected to the same chip select, and which are therefore accessed simultaneously. In practice they also share all of the other command and control signals, and only the data pins for each DRAM are separate (but the data pins are shared across ranks). There is little difference between a dual rank UDIMM and two single rank UDIMMs in the same memory channel, other than that the DRAMs reside on different PCBs. The electrical connections between the memory controller and the DRAMs are almost identical (with the possible exception of which chip selects go to which ranks). Increasing the number of ranks per DIMM increases memory density in the channel, but does not enhance performance; too many ranks in the channel can cause excess loading and decrease the speed of the channel. DRAM load can be lessened by using registered memory. Memory Rank
举例说明，单面的内存一般都是8个8bit的芯片，所以有一个物理BANK，但是双面的内存有16片8bit的芯片，所以有两个物理BANK（16*8/64）并不是所以的单面的内存都是一个BANK，例如单面8片16bit芯片的内存就有两个BANK，也不是所有的双面内存都是双bank,例如16片4bit的芯片的内存只有1个bank ,
传统内存系统为了保证CPU的正常工作，必须一次传输完CPU在一个传输周期内所需要的数据。而CPU在一个传输周期能接受的数据容量就是CPU数据总线的位宽，单位是bit（位）。当时控制内存与CPU之间数据交换的北桥芯片也因此将内存总线的数据位宽等同于CPU数据总线的位宽，而这个位宽就称之为物理(BankPhysical Bank，下文简称P-Bank）的位宽。 在一些文档中，也把P-Bank称为Rank（列）。
single rank单列 dual rank双列 1.首先，需要知道CPU数据总线的位宽,现在一般是64bit, 这个位宽就称之为物理Bank.. 3.所以无论是1RX4 ,2Rx4或者 1RX8,2Rx8，代表的含义可以理解为memory的内存颗粒的个数.而不是单面或者是双面内存.
What is meant by memory bank, rank ang channels???
Inside a memory chip, there are "four rectangle" of memory.
Each one of those is called a "bank".
The dimensions of the rectangles inside the memory chip,
are defined by "rows" and "columns".
Memory chips use those three dimensions for internal addressing.
Memory busses on current generation systems are 64 bits wide.
If you have a 512MB DIMM, it might have 8 chips on each side
of the DIMM. 8 chips, each having 8 data bits, forms a 64
bit wide array, and those 8 chips are called a "rank". A
double sided 512MB DIMM has two "ranks". (Note that some
people use the term "rank" or they use the term "bank"
for this. But using the term bank for both the side of
a DIMM, and for the internal operation of a single memory
chip, can lead to confusion. "Rank" is a more unique
term for this purpose, at the DIMM level.)
There are other chip formations possible. A 1GB DIMM, constructed
from 128Mx4 memory chips, also has a total of 16 chips. But
since each chip is only 4 bits wide on the data bus, it takes
16 chips to build a 64 bit wide "rank". Thus, a cheap 1GB module
constructed with x4 chips, has a single "rank", but is double sided.
Thus, "ranks" are not the same thing as "sides" of memory.
But many people abuse the terminology, and pretend that
they are equal concepts.
"Channels" are independent memory interfaces on a Northbridge.
A memory channel consists of a 64 bit data bus, some address
bits and control bits.
A Northbridge can have two channels. When they work together
for a common cause, that is termed "dual channel". Effectively
the two channels work together, and look like a 128 bit wide
data bus, instead of the 64 bit data width of a single channel.
Typically, the user is expected to use matching DIMMs, in
equivalent locations, on each channel, to get the benefit
of dual channel operation.
It is possible for a chipset to have more than two memory
channels. This chipset, for example, has four memory channels.
Difference between memory bank and memory rank?
Memory ranking on the other hand, happens or is done in the memory module level. Rank of a memory decides priority. General system comes with ranking 1R. If you look at servers, then you might find larger rankings. To identify the rank, just look on the sticker of the memory module and you will find an entry like 1R or 2R. That is the rank.
Read more: http://wiki.answers.com/Q/Difference_between_memory_bank_and_memory_rank#ixzz1w8A55CGL
内存的BANK其实分为两部分，逻辑BANK和物理BANK。 先来讲讲逻辑BANK。芯片的内部，内存的数据是以位（bit）为单位写入一张大的矩阵中，每个单元格我们称为CELL，只要指定一个行（Row），再指定一个列（Column），就可以准确地定位到某个CELL，这就是内存芯片寻址的基本原理。这样的一个阵列我们就叫它内存的逻辑BANK（Logical BANK）。 再来说说物理BANK。通常主板上的每个内存插槽分为两段，这个大家从VIA主板BIOS设置中的BANK 0/1 DRAM Timing选项很容易推理得到，实际上也就是两个BANK，不过这里的BANK概念与我们前面分析芯片内部结构时提到的BANK可不一样。 有些内存条单面就是一个物理BANK，但有些双面才是一个物理BANK，所以不能一概而论。256MB内存条就是一个典型的例子，虽然是双面并多达16枚芯片，但仍然是单个物理BANK的。要准确知道内存条实际物理BANK数量，我们只要将单个芯片的逻辑BANK数量和位宽以及内存条上芯片个数搞清楚。各个芯片位宽之和为64就是单物理BANK，如果是128就是双物理BANK。
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