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CPU, FSB??

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February 8, 2008 7:46:30 AM

I am a little confuse about the effect of the FSB on the processing power. Assuming that I have the same proc at the frequency but different FSB ( and multiplier), would I see any diferrence in the computing power? Assume that the memory also runs at the same frequency.

Pasoleatis

More about : cpu fsb

February 8, 2008 8:07:39 AM

Basically the FSB is the base speed of the CPU and RAM. This FSB is then multiplied for the CPU and RAM.

E.g. a 2.4GHz CPU could have a FSB of 266Mhz and a Multiplier of x9 .. while the RAM would use the same FSB of 266MHz with a multiplier of x2.5 to give 667MHz. These examples are based on the defaults for the E6600 and any pc2-5400 ram.

Therefore the FSB x Multiplier = CPU speed.
FSB x Mem Multiplier = Mem Speed

So increasing the FSB would in effect o/c your cpu. But you then need to start thinking about voltages. Depending on your mobo the FSB, Multiplier, Voltages.. can all be adjusted.

Hope this isn't more confusing..
February 8, 2008 9:09:32 AM

Thank you for the answer. I understand app how is thething with base frequency and multipliers. I was wondering if there is any difference in running a proc at same frecquency, but with different FSB (of course it would have also differnt multiplier).
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February 8, 2008 9:55:00 AM

It shouldn't matter too much.. your mobo will have min/max FSB and your CPU will have a max multiplier. E.g. I use a FSB of 400MHz with Multiplier of x8 giving 3.2GHz. I assume you want to do this, so it matches up with the speed of your memory i.e. 1:1 ratio.

All you can do is experiment and see if it boots.. if my pc fails to boot after o/c it will reset itself to defaults allowing me to change things... I assume yours probably has the same process built-in.
February 8, 2008 9:58:37 AM

I want to buy a custom laptop which is built on Clevo D901C. I know that the chipset supports only 1066 MHz FSB, but some company oferrers the Qx6800 on it at 3Ghz. I was wondering if the proc is working at its full capacity or should I wait for other versions of the laptop which has a chipset with 1333MHz.
February 8, 2008 10:09:14 AM

Just one thing to point out.

If we are talking basically about Intel C2D chips, certain chips have a rated FSB.

When you have a rated FSB, that is the specified speed set for that chip. For example:

E4xxx/E2xxx is a 800 FSB chip
E6xxx/E6x20 are 1066 FSB Chip
E6x50 are 1333 FSB chip
E8xxx are 1333 FSB chip

All that determines is the actual speed the chip will run at. To give another understanding of it, my E4400 is rated for FSB@800mhz. Since I OC'd it, it is now rated for a FSB@1200mhz:

10(multi) x 300(FSB Speed)=3000mhz or 3ghz speed or 300(FSB Speed) x4 (quad pumped) = 1200mhz FSB Rating

So its not that a chip is working at full capacity, but how far you can push it stably.

Edit:

So what determines the actual speed is the FSB rating and multiplier.
February 8, 2008 10:13:00 AM

According to ebuyer the QX6800 uses a 1066MHz BUS.. @ 2.93GHz This would indicate a 266MHz FSB (Quad) using a x11 multiplier at default. Therefore the QX6800 advertised is basically set to default capacity.

further info: http://techreport.com/articles.x/12210
February 8, 2008 10:32:41 AM

Yes you are write. I amde a mistake it was the qx6850. so from your posts I understand that the FSB does not affect the performance. A qx6850 at 3Ghz (9*333Mhz) would have same performance as a qx6850 at 3Ghz (11*266Mhz).
a b à CPUs
February 8, 2008 10:41:22 AM

Raising the FSB is the only way to overclock most processors, as the multiplier is locked. Years ago, even if you had a processor that you could unlock the multiplier on, raising the buss speed resulted in better performance than just raising the multiplier, as buss speeds were realitively low. The buss is what your processor communicates over to the rest of the components in your PC. So raising the buss speed results in a larger overall performance gain than raising the multiplier to achieve the same actual speed.
Today's processors run pretty high buss speeds already. Raising the FSB speed does not result in a significant increase in speed over just raising the multiplier(if you could somehow unlock the multiplier) But, as I said, most CPU's have the multiplier locked.
For instance,
100Mhz buss X a multiplier of 10 gives you a 1000Mhz processor or 1Ghz.
If you could unlock the multiplier, or go with a higher FSB, here are 2 results.
100Mhz FSB x12 gives you 1.2Ghz
120MhzFSB x10 gives you 1.2Ghz
The second senerio would result in higher performance.

I stress again that this is a fact of years gone by, the buss speeds that modern CPU's run at are high enough now that raising the buss speed vs raising the multiplier does not have much difference in overall performance. (although it still does have some impact towards better system performance) The buss used to be the main limiting factor of PC performance, hence the move by manufacture's to raise the buss speeds and lower the multipliers to achieve the same final speed on modern processors.
a b à CPUs
February 8, 2008 10:45:57 AM

pasoleatis said:
Yes you are write. I amde a mistake it was the qx6850. so from your posts I understand that the FSB does not affect the performance. A qx6850 at 3Ghz (9*333Mhz) would have same performance as a qx6850 at 3Ghz (11*266Mhz).


No, this is not correct. The processor running at 9x333 will show slightly better overall system performance.
But, it's not the processor it's self that is faster, it is that it can communicate with the rest of your system faster.
February 8, 2008 10:56:41 AM

I see. So this FSB is in a way similar to the bands of a highway. So the FSB is important also when the proc comunicates to the memory, hdd, video card or other components?
a b à CPUs
February 8, 2008 10:57:44 AM

Yes, exactly, a very good way to look at it.
February 8, 2008 12:09:44 PM

Actually the quad pumped part would be more similar to a highway in lanes.

The FSB speed, is simply setting the highway speed limit. :D 

Edit: or somewhat a visual:

Lane one-------->speed limit 266
Lane two-------->speed limit 266
Lane three-------->speed limit 266
Lane four-------->speed limit 266

above total-------------------------1066
multi--------------------------------10x266

Actual speed-----------------------2660 mhz or 2.66ghz
!