What is difference between FSB Speed of CPU and FSB Speed of Motherboard

Hi Everyone:

         This question is more academic in nature as opposed to any application motive.  Lately, I have been working with motherboards more so than usual which has naturally sparked my curiousity on some things.  One thing in particular which I find somewhat confusing relates to FSB or Front Side Bus Speed.  For instance, what is the difference between FSB of the CPU and the FSB of the Motherboard?  With my current setup, the FSB of my processor is 333, so, I had to use a jumper to set my motherboard FSB speed to 166 because the FSB speed of the processor is always 2X more than the motherboard for Athlon chips as explained by an expert from a previous post.  In any case, I am just curious about why there is this difference?  Does it have anything to do with the processor being inherently faster than the motherboard?  

            Any shared insights on this post will be appreciated.

            Thank you

           George
GMartinAsked:
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J-A-LConnect With a Mentor Commented:
It will be different for AMD as compared to Intel as well.
The AMD chips are known to be dual-pumped.. the Intel chips are known to be quad-pumped.

The Actual FSB speed is 100, 133, 166, or 200MHz, what-ever the chip can handle... but, because it can send data on a rising and falling edge of the signal, it sends twice the amount of data... so, they call it a FSB speed of 400MHz.  The Intel is the same story but 800MHz.... even though the actual bus speed is 200 MHz.

Jeff
at yourtechonline.com
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GMartinAuthor Commented:
Hi There:

        Thanks for the followup.  Could you elaborate upon the difference between dual-pumped and quad-pumped?

         Thank you

         George
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originalbiffmalibuConnect With a Mentor Commented:
Basically it comes down to this:

Athlon-based:  100MHz CPU bus = 200MHz Ram bus, 133=266, 166=333, 200=400
Intel-based:  100=400, 133=533, 200=800 and now, 266=1066  unless you're talking RAMBUS where CPU=100, FSB=400 and RAMBUS=800
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Glen A.Connect With a Mentor IT Project ManagerCommented:
GMartin:

Quad pumped means 4 bus transfers per clock cycle, 2 on the rising edge & 2 on the falling edge.   This is how intel can have an 800MHz cpu running on a motherboards bus speed of 200MHz.

Double-pumped means 2 bus transfers per clock cycle, 1 on the rising edge & 1 on the falling edge.

Although this SOUNDS like Intels chips would then be twice as fast as AMD's, it's not the case.  Although intel has a faster cpu bus speed, AMD processes a much larger amount of instructions per clock cycle, and is more efficient at other things like branch prediction, etc.  This is why their Athlon64's are a superior processor to the intel P4's, even at a much lower overall clock speed.
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J-A-LConnect With a Mentor Commented:
Oh yes... Dual-pumped means that per 1 FSB clock cycle... it sends twice the amount of data... once per rising edge, once per falling edge giving it a multiplier of 2x.

On the Intel FSB bus... it is quad-pumped... meaning per 1 Bus clock cycle, it sends 2 x per rising edge, and 2 x per falling edge... giving it a multiplier of 4x

Jeff
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GMartinAuthor Commented:
Hi Everyone:

         After going over the numbers presented along with the technical information, I have drawn one conclusion which I desire to entertain with everyone.  Bascially, it appears the nead for the "multiplier" is to help match the board to the chip.  In other, the FSB speeds of the chips to their corresponding FSB motherboard values to obtain speed symmetry or balance between the chip and board.  Is that a fair conclusion to draw?

         Thanks

         George
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originalbiffmalibuCommented:
this is correct
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GMartinAuthor Commented:
Hi Everyone:

         Thanks so much for breaking down the comparisons between FSB of CPU with FSB of motherboard.  I found the numbers used within the various responses helpful in gaining a better understanding, especially when these numbers where used to compare the AMD based chips with the Intel based chips.  While I already had a little familiarity with some of the information presented, I must confess there were several shared insights which I never thought of regarding chips.  In addition, I really appreciated each expert for taking the time to break down some of the more technical sides of this post (e.g. dual pump versus quad pump of data).

          Thanks again everyone for the help and have a great weekend.

          George

         
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