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Question about multi-channel memory architecture


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#1 HeeHaw5130

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Posted 24 September 2014 - 02:39 PM

I have a question regarding multi-channel memory.

 

If the width of a front-side bus determines how many modules equal one full bank, then one 64-bit stick in a 64-bit bus should be a bank in itself on its own channel, right?

 

Now, for two channels, each with two memory slots each, does this mean that every individual slot has its own single 64-bit bus, meaning that one channel is 128-bits? Or do the two modules share just one 64-bit channel?



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#2 jonuk76

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Posted 24 September 2014 - 09:04 PM

One 64bit DIMM = one bank of memory, which is why you can install them singly.  If it was a 128bit bus, you would be forced to install them in pairs (and a dual channel setup would require 4 DIMMs).  The number of banks doesn't have to correspond with the number of "channels".  It's perfectly possible (and common) for memory controllers to support more than one DIMM per channel.  Some basic chipsets may only offer single channel memory.

 

Thinking back to much older computers, 30 pin 8bit SIMMs (from the 386/486 era) had to be installed in 4's in these applications (32 bit processors), and 72 pin 32bit SIMMs could be installed singly or in pairs depending on the application (IIRC they had to be installed in pairs on Pentium's as it had a 64bit external bus).


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#3 HeeHaw5130

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Posted 24 September 2014 - 10:33 PM

One 64bit DIMM = one bank of memory, which is why you can install them singly.  If it was a 128bit bus, you would be forced to install them in pairs (and a dual channel setup would require 4 DIMMs).  The number of banks doesn't have to correspond with the number of "channels".  It's perfectly possible (and common) for memory controllers to support more than one DIMM per channel.  Some basic chipsets may only offer single channel memory.

 

Thinking back to much older computers, 30 pin 8bit SIMMs (from the 386/486 era) had to be installed in 4's in these applications (32 bit processors), and 72 pin 32bit SIMMs could be installed singly or in pairs depending on the application (IIRC they had to be installed in pairs on Pentium's as it had a 64bit external bus).

 

So (just to clarify), each slot has their own dedicated 32/64/etc. - bit bus through the memory controller and to the CPU, regardless of the channel structure?



#4 jonuk76

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Posted 25 September 2014 - 08:53 AM

No that's not my understanding of it.  A dedicated bus for each DIMM would mean that a typical PC with 4 x DIMM slots would have quad channel memory.  There are PC's with quad channel memory but they are very high end.

 

A dual channel memory controller (for example, the integrated controller on a Core i5 processor) is considered to have 2 x 64bit memory buses.  A triple channel memory controller (for example the MCH on a Core i7-960 processor) has 3 x 64bit buses.  Each bus or channel on these example processors supports up to 2 x 64 bit DIMMS (4 or 6 in total respectively).

 

From Wikipedia

 


Dual-channel memory

Dual Channel memory controllers are memory controllers where the DRAM devices are separated on to two different buses to allow two memory controllers to access them in parallel. This doubles the theoretical amount of bandwidth of the bus. In theory, more channels can be built (a channel for every DRAM cell would be the ideal solution), but due to wire count, line capacitance, and the need for parallel access lines to have identical lengths, more channels are very difficult to add.

 

BTW what is the purpose of the question? General interest or is there a question about a specific system coming?


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#5 HeeHaw5130

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Posted 25 September 2014 - 11:12 AM

I'm studying for the A+ exam; there are some things that textbooks can't always clearly explain.

 

So then it's 2x DIMM slots = 1x 64-bit bus?



#6 jonuk76

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Posted 25 September 2014 - 12:22 PM

OK I believe what I'm saying is right, but I would suggest checking with your A+ instructor (if you are doing an instructor led course) for exactly what information they are looking for in the syllabus, rather than relying on this.  They will be looking for specific items of knowledge and concepts, rather than detailed knowledge about real world examples.

 

Anyway, yes the 2 DIMMs both share the same bus.  In some cases there are potential performance penalties for using more than one DIMM per channel.  As a real example, a motherboard/CPU may support DDR3-1600 when one DIMM per channel is used, but be limited to DDR3-1333 when all four slots are populated.  Here's a few more real examples on a page I found, dealing with server memory.


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#7 HeeHaw5130

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Posted 25 September 2014 - 12:59 PM

Unfortunately, there are no colleges or tech schools that offer any A+ courses, so self-study is my only option.



#8 jonuk76

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Posted 25 September 2014 - 01:28 PM

Well hope I helped in some way then.  As it's quite a wide syllabus I suspect they aren't going to quiz you in huge depth about your knowledge of memory buses, but as long as you have a grip of the basics :)

 

A+ is probably do-able completely self study, but I found Network+ pretty hard going and that was with an instructor..


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#9 HeeHaw5130

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Posted 28 September 2014 - 10:01 AM

Very helpful. Thanks!






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