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Base Clock, CPU , FSB and Setting Interrelationships

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Nov 7, 2014
Trying to understand how the Front Side bus speed impacts the CPU.

Right or Wrong (please correct if Wrong, this is what I have so far.

1) For determining CPU frequency (MHz or GHz) validity, both AMD and Intel “appear” to use a base clock speed of 100 MHz.

2) Based on the outcome of this CPU verification, processors are “graded” by their highest and stable clock speed.

3) Based on this final grading a multiplier is determined for the CPU. This multiplier is a multiple of the 100 MHz base used for testing.
(e.g. a Athlon 1100 MHz processor / 100 Mhz base clock) = 11 multipler.

4) The multiplier is then locked into most CPU’s so the CPU cannot be overclocked.

5) The motherboard chipset manufacturers incorporate a base clock into the chipsets which reside on the motherboard. This base clock chip may be separate or part of a controller such as the Northbridge.

6) The Front Side bus speed is determined based on which manufacturers (AMD, Intel, etc) processor will be supported.
The chipset selected which supports this processor contains a base clock.
The FSB speed is then adjusted by taking the base clock times some multiple to peg it to the manufacturers CPU Series.
--- For AMD it appears to be 2 times the base clock (2 x 100 MHz)
--- For Intel it appears to be 4 times the base clock ($ x 100 MHz)
The “true base clock frequency” can be determined by taking the FSB and dividing by the
CPU manufacturers multipler since the true base clock value is not normally published.

7) In order to accomodate a wider range of CPUs, the motherboard manufacturers allow the FSB to be configured with several different speeds.
In my case I can set the FSB at either 200 MHz or or 266 MHz.
Since this is an AMD CPU I calculate a base clock of either:
100 MHz = (200 / 2) or
133 MHz – (266 / 2).

>>>>>>>>> Now this is where I’m confused. <<<<<<<<<<<<<<<<<

If I set the FSB at 133 MHz then my multiplier would need to drop to 8.27 (1100 / 133) in order to
keep the CPU operating at the designed 1100 MHz.

So my guess is if the CPU has a multiplier “hard coded” into it as an 11, increasing the FSB speed would allow me to have a window from 8.27 to 11 in order to up the CPU speed greater than 1100 MHz.

If Wrong what am I missing??

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Gulper Nozzle Co-Owner
Dec 15, 2008
Clockspeed of the CPU = BCLK/FSB/HTT x Multiplier. BCLK(Baseclock) is the modern equivalent of the FSB(Front Side Bus). A CPU is not binned quite like that. They are essentially agnostic out of the wafer. They take cpus and test them to fit in buckets called bins. These bins are divided up by specific clockspeeds and power envelopes. The 'guts' of how it works, the bclk/fsb/htt bus is in the architecture.

The flexibility(overclocking headroom) of the any of the bus vary by chipset.


Nov 7, 2014
EarthDog thanks for responding.

1) HTT is a new acronym for me. What does it stand for?
2) Is your formula (BCLK divided by FSB divided by HTT) ?

The flexibility(overclocking headroom) of the any of the bus vary by chipset.

I'm still trying to understand:
1) whether the different FSB frequencies (200 and 266 in this old board) are put there for use with different CPU's
-- or --
2) since the FSB has these different frequiencies at my disposal ( I can select), whether selecting the faster FSB speed (266 Mhz)
and lowering the multiplier (8.27) will cause the CPU to run at the same 1100 MHz -- and --
whether an adjustment of the Multiplier up or down from 8.27 at 266 MHz will have "any" effect of the CPU's frequency if the CPU is locked
and if unlocked?

3) If changing the FSB and Multiplier will burn up the CPU and/or

4) what other things (e.g. voltages, memory speed, etc) do I need to change -- and -- how do I calculate those values (Formulas)
if I set a new FSB speed (other than the 100 MHz now being used)?

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