Dual-Rank 8GB Vs 4GB Single-Rank

[Kiyoka]

Hi tom's forum.
As you probably knows when talking about Pc RAM we are looking on the technology called DDR3/4 SDRAM chips on DIMM Slots.

The RAM is splitted for many things like 1 sided, 2 sided, single and dual rank , frequency and capacity.

Iv'e searched every where I could and couldn't find the appropriate anwer for the question below:
What the heck is a dual rank memory?

For that question I want you to check the datasheets for the next 2 DIMMS:

1)KVR13N9S8/4 which is a 4GB DIMM 1Rx8 DDR3-1333MHZ
As you can see in the datasheet, it contains 8 sdram chips that each has a capacity for 512MB which means a 512Mbit - 8pins chip and it has 8 of those which means its a 8*8=64 bits for a total of 4GB that can be transferred on the data bus parallel.


2)KVR16N11/8 which is a 8GB DIMM 2Rx8 DDR3-1600MHZ.
As you can see in its datasheet, it says 1Gx64-bit(8GB) which has to mean that it should be capable to transfer 8GB on the 64bit data bus but as you keep reading it says that the DIMM contains this time 16 chips of sdram and each of them is a 512Mb-8 bit (output pins) so that means 16*8=128 bits.
That means the RAM has 8GB overall but it can't transfer the whole 8GB at once on the 64 bit data bus!!
So this RAM is dual rank and it has 4GB for 64 bits and another 4GB of 64bits but they cannot work simultanously and act as 8GB for 64 bits at once as mentioned on the beggining of the datasheet!!



So right now im so confused about the meanning of the terms single,dual and quad rank modules and I can't understand how is it possible for a more ranked module to perform better as you could find online benchmarks than a single module on about the same frequency lets say an 2400MHZ and the same capacity lets say 8GB DDR4 RAMS????

(And lets not talk about the latency which superior for the single ranked ones).


I hope some1 could realy clear this nonsense for me once and for all!!
Thanks you very much.

 

[camieabz]

Try this - https://forums.crucial.com/t5/tkb/articleprintpage/tkb-id/dram@tkb/article-id/71

Performance differences are probably negligible, and will be scenario-specific.

 

[Kiyoka]

Thanks you, but it still doesn't explain the above compability with the datasheet versus the explenation about it, so I still couldn't understand my question about how could it be faster a dualn ranked versus single rank, if there are any single rank modules with 8gb on them though

 

[musicmasteringservice]

As far as I understand, the main difference between Single Rank and Dual Rank memory is density (the number of chips on the PCB and therefore the RAM size). A likely reason one would pick dual rank over single rank is to be able to put more memory in a system. CPUs and motherboards usually have limitations on how much memory can be recognized and accessed; using dual rank memory modules would be one way to maximize the amount of memory you can put in your system.
The downside of this is that with dual rank memory, only one rank can be accessed at a time, which potentially makes dual rank memory slower. But this is not always the case; it depends on how much memory is in your system, the speed of the memory (the FSB / DRAM ratio), the latency of the memory (along with other factors like CAS, RAS, CR, FAW, etc.), what type of memory is installed (buffered vs. unbuffered; ECC vs non-ECC, etc), how the memory controller accesses the memory, whether the memory is running in dual channel (or quad channel) mode (not to be confused with dual rank or dual sided, which are all different things), the efficiency of the memory controller (usually built into the CPU these days), the design of the motherboard and possibly even the settings of the operating system..
Dual channel mode (not to be confused with dual rank), is when two memory modules are running in tandem on a single data bus, which doubles the bandwidth between the memory controller and the memory channel. Dual channel mode is different from dual rank and is still subject to the limitations of a dual rank module (where only one rank can be accessed at a time). But most people will not notice much of a difference between the numerous factors that affect memory speed on a modern system; RAM is still the fastest part of a modern computer system outside the CPU; paying double for slightly faster memory is usually a waste of money. Buying memory from a reliable manufacturer with a good warranty is usually better than paying a fortune for memory with slightly better specs.
In short, single rank memory can be fully accessed by the controller at any time (i.e. in a single command cycle); dual rank memory requires accessing only one rank at a time (full access requires two command cycles), but usually increases the amount of RAM that can be installed in the system.