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How does memory speed relate to FSB?

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April 19, 2009 6:57:26 PM

how does memory work with FSB? to my understanding you need them to match each other for speed or it causes a bottleneck. is that true?

for example: if i have a 1066FSB CPU, and my RAM is 2 1GB sticks of PC5300 in dual channel, at 667mhz. does that cause a bottleneck?

if it does, would that mean i would have to use 2GB in dual channel of 1066mhz RAM to not cause any bottleneck issues? as the RAM speed would be identical to the FSB.

thanks.
April 19, 2009 7:09:22 PM

Basically what you do is take take the speed and then divide by the number of channels so DDR2 @ 800mhz would be 400mhz and you would need at least that much of a FSB. For DDR3 divide by 3 etc. Also its not about the amount of ram but rather the transfer rate. If your ram is rated faster than your processor can handle it will just revert to a slower state so that shouldn't be a problem.
April 19, 2009 10:04:00 PM

so then if i had 2 1GB sticks of DDR2 1066mhz RAM and a 1066FSB CPU, id take the RAM and divide it by 2 = 533. then dual channel doubles the bandwidth taking it up to 1066 on each stick, which would equal the FSB and give the best performance?

or am i missing something?
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April 20, 2009 2:23:56 AM

In short:

Ram runs off FSB, but off its base clock which is 266mhz, not 1066. 1066 is a data transfer rate, not actual speed. Its called Quad Pumped FSB, because it can transfer data 4 times per clock, so it has a bandwidth "as if" it was running at 1066mhz with one data transfer per clock.

Its the same with ddr memory. I dont know where that dividing by number of channels came from(lol), thats baloney. DDR = double data rate, same story as with FSB, but here data is transmitted only twice per clock.

Dont know where im really going with that :) 

Bottom line is:
Your current ram is not causeing a bottleneck.

I tried my system with ram speeds ranging from ddr2 533 to ddr2 980 and none showed any gain or loss in performance, apart from the fact that 980 produced more heat which might have been helpful in winter ;]
April 20, 2009 6:23:07 AM

jsrudd said:
For DDR3 divide by 3 etc.

DDR3 is 4 times FSB.
April 20, 2009 2:25:20 PM

MadMagik is the one closest to the truth.

On an Intel Core2 system :

FSB is quad pumped so 4 data transfers per clock
DDR2 = Double Data Rate 2nd generation so double pumped 2 data transfers per clock
DDR3 is 3rd generation DDR and it uses the same technique as ddr2 so it is also double pumped

So if you have a CPU that has a FSB of 1066 you have 1066 / 4 (quad pumped) = 266mhz and your ram is 266 * 2 (double pumped) = 533mhz

Same thing for a 1333mhz FSB
FSB 1333 / 4 = 333
RAM 333 * 2 = 667

There are the numbers for a 1:1 ratio where the memory runs in sync with the FSB. You can run the ram faster or slower than the FSB but there are no significant real world performance increase.
April 21, 2009 6:23:49 PM

smartel7070 said:
DDR3 is 3rd generation DDR and it uses the same technique as ddr2 so it is also double pumped


No, DDR3 effective speed is FSB x 4.
April 21, 2009 7:08:59 PM

theAnimal said:
No, DDR3 effective speed is FSB x 4.


This comes from the wiki page for DDR3:

"In electronic engineering, DDR3 SDRAM or double-data-rate three synchronous dynamic random access memory is a random access memory interface technology used for high bandwidth storage of the working data of a computer or other digital electronic devices.

DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM. The primary benefit of DDR3 is the ability to transfer twice the data rate of DDR2 (I/O at 8× the data rate of the memory cells it contains), thus enabling higher bus rates and higher peak rates than earlier memory technologies."

Now if it were fsb x 4 wouldn't it be called QDR or Quad Data Rate ?
April 21, 2009 10:53:15 PM

smartel7070 said:
This comes from the wiki page for DDR3:

"In electronic engineering, DDR3 SDRAM or double-data-rate three synchronous dynamic random access memory is a random access memory interface technology used for high bandwidth storage of the working data of a computer or other digital electronic devices.

DDR3 SDRAM is an improvement over its predecessor, DDR2 SDRAM. The primary benefit of DDR3 is the ability to transfer twice the data rate of DDR2 (I/O at 8× the data rate of the memory cells it contains), thus enabling higher bus rates and higher peak rates than earlier memory technologies."

Now if it were fsb x 4 wouldn't it be called QDR or Quad Data Rate ?


Yes, it's still double data rate; however, it's also double the data, which effectively makes it 4 x FSB.
April 21, 2009 11:22:55 PM

No, DDR3 is still x2. Like DDR2 and DDR1 is x2.

Internally, the module is made differently, so you can increase the external bus speed without increasing the internal that much. But this is all internal to the module and is irrelevant when talking about its relation to the fsb.
April 22, 2009 1:32:17 PM

Thing is, the speeds given on DDR2 and DDR3 are the DDR1 equivalents. So, take DDR1 400, DDR2 400 and DDR3 400. DDR1 is 400MHz, DDR2 is 800MHz EFFECTIVE and DDR3 is 1600MHz effective. Also, you have to divide the CPU FSB by 4. If it is 1066, it actually is 266MHz. Your 667 RAM is @ 333MHz. Try overclocking your CPU. Getting 1333 from 1066 (That is 333MHz) is not that difficult. Dunno what CPU you have.
April 22, 2009 2:44:14 PM

rags_20, that explanation didn't make much sense.
May 10, 2009 9:05:22 PM

DDR3 need divide for 4!!!!!

its right this... DDR2 800/2= 400 DDR3 1600/4 = 400


DDR 3 is NOT double pumped if is... the 4870 have a crap memory too lol
May 10, 2009 10:58:43 PM

No, you divide DDR3 by 2 always when talking about the bus related to the fsb
!