DRAM Memory Rank Knowledge

Some knowledge of DRAM, first recorded and then sorted out.

1. What is memory rank?

A memory rank is a set of DRAM chips connected to the same chip select, which be therefore accessed simultaneous Ly. In practice they also share all of the same command and control signals, and only the data pins for each DRAM is Separat E (But the data pins is shared across ranks).

the term "rank" was created and defined by  JEDEC, the memory industry standards Group. On A DDR, DDR2, Or ddr3 memory module, each rank has a a-bit wide data bus (wide on DIMMs SUPPORT ECC). The number of physical drams depends on their individual widths. For example, a rank of x8 (8 bit wide) drams would consist of eight physical chips (nine if ECC are supported), but a rank of X4 (4 bit wide) drams would consist of physical chips (if ECC is supported). Multiple ranks can coexist on a single DIMM, and modern DIMMs can consist of one rank (single rank), both ranks (dual rank) , four ranks (quad rank), or eight ranks (octal rank).

There is little difference between a dual rank Udimm and both single rank Udimms in the same memory channel, other than tha T the drams reside on different PCBs. The electrical connections between the memory controller and the drams is almost identical (with the possible exception O F which chip selects go to which ranks). Increasing the number of ranks per DIMM is mainly intended to increase the memory density per channel. Too many ranks in the channel can cause excessive loading and decrease the speed of the channel. DRAM load on the CA (command/address) bus can is reduced by using registered memory. "1"

2. How can I identify the number of SODIMM's memory rank?

"R" in "Module model" (red circle) is the meaning of "rank", that is, there is only one rank in this module. "x8" is the particle's bit width (bit width).

For this memory corresponding block diagram, it can be seen that all CS chip selection signals are connected together.

Memory block Diagram "2"

3. Some discussions on rank (quoted as "3"):

1. How do I calculate the rank number of the module according to the module number?

Answer: According to the module composition principle can know: if the depth of the module is equal to the depth of the particle, is a rank, if the module depth is equal to twice times the particle depth, is two rank. For example, the module depth of the Samsung module numbered M378B5673DZ1 is 256M. And because this kind of module uses the K4T1G084QD granule. The density of this particle is 1024Mb and the bit width is 8b, so the particle depth is 1024mb÷8b=128m. That is, the module depth is twice times the depth of the particle, so it is two rank.

In addition, the rank number can be calculated from the size of the particle and the actual particle number given by the module number or particle number. For example, when the particle bit width is 8b, the module uses 8 particles, 8x8b=64b, is a rank, if 16 particles, 16x8b=128b, is two rank.

2. What is the rank number of the module related to the number of modules?

A: The number of faces (side) of the module and rank number is two different concepts. And the number of memory is not reflected in the number of polygons. However, the module faces, not one, is two, and the current rank number is the same. Therefore, it is easy to confuse them when they are represented by symbols. However, it can be very clear that: "R" in the memory tag is the expression of rank, not the number of polygons, memory "face" of the English text is side, if the expression two face words, should be "2S" is Ah!

3.2rx8 on the memory label indicates that the memory is 8 particles on both sides?

Answer: No. "R" means rank, which has been explained above. "x8" is the particle width is 8bit meaning. Because 1 rank is 64bit, two rank is 128bit, so the symbol "2rx8" means that the module has 2 rank, the particle's bit width is 8b. Therefore, the module uses particles of 128b÷8b/= 16, not 8. Similarly, when the label on the memory bar is labeled "1rx16", it indicates that the module is 1 rank and the particle bit width is 16bit. Its number of particles is 64b÷16b/= 4 capsules. It's not 16.

Reference documents:

"1" Https://en.wikipedia.org/wiki/Memory_rank

"2" computing_ds_4gb_ddr3l (b-ver) Based_sodimms (rev.1.0). pdf from "https://www.skhynix.com/products.view.do?vseq=963 &cseq=75 "

"3" http://blog.csdn.net/force_eagle/article/details/7961317

DRAM Memory Rank knowledge