CPU Front Side Bus clock frequency

  23790954 12:37 08 May 2003

Being a retired elderly gentleman, who spends his time building computers for grand children, I am having difficulty understanding how to ascertain what speed the CPU should be set at in the bios. The user guide for my MSI KT 3 ultra motherboard states the CPU FSB clock item allows you to select the CPU FSB frequency in Mhz, and settings option ranges from 100 to 220. If installing a CPU other than 100Mhz you should change the setting to corresponding FSB frequency. The CPU I have iknstalled is a MDA Athlon 1300, so what should I set the frequency to.
The CPU Core Speed Derivation Procedure in the user guide given as:
If CPU = 100MHz
Core Bus ratio = 7
CPU core speed = Host clock x Core/Bus
= 100MHz x 7
= 700 Mhz

Any assistance and explanation of this would be much appreciated.

  Paranoid Android 13:12 08 May 2003

You need to determine whether the CPU is a 200 Mhz FSB model, or 266 MHz FSB. My guess would be 200 MHz because (to the best of my knowledge) the 266 FSB was put into the 1.33 and 1.4 GHz Athlons not the 1.3's.

The CPU communicates with the motherboard and memory at twice the host clock speed. Therefore if your CPU supports 133 MHz host clock speed, it is referrded to as 266 Front Side Bus (FSB).

This is separate from the speed that the CPU operates at internally.

It stands to reason that a 1300 MHz CPU which communicates with the motherboard at 266 MHz is faster than a 1300 MHz CPU that communicates at 200 Mhz.

I think your setting will need to be Host clock = 100, multiplier = 13.


  LastChip 13:12 08 May 2003

= Front Side Bus. This is the basic frequency at which your processor "talks" to other parts of the system.

So, for a 1300 Athlon, you would set the FSB speed to 100 and multiply it 13 times. eg 100x13=1300. (if it is a 200 chip - see further on).

However, you have to determine

a; what your motherboard will support in terms of FSB.

Some will only support 100, while others support 133, and yet others support both. When you read about a 200 or a 266, all that is happening, is the frequency is being double clocked.

and b; what your processor was designed to run at.

Most new AMD processors are designed at either 200 or 266. You can determine what the processor can run at, by decoding the OPN (ordering part numbers). More about this can be found at the AMD web site. click here

  LastChip 17:51 08 May 2003

23790954 e-mailed me and wrote; "I am still unsure of one vital piece of information. My bios page reads as follows:

Spread Spectrum: (which I have left alone)
it reads: +0.25% onwards etc
CPU FSB clock: which I have put to 133
CPU ration: which is on auto;
CPU Vcore: auto;
DDR voltage:(v): auto;
AGP voltage:(v): auto;

Where and how do I multiply my processor times the amount required. e.g. 13 x 100"

OK, sorry for the delay, but I had to track down your motherboard manual on the net, download it, and see what you were confronted with!

You have to keep in mind, you cannot use a mouse when navigating in the BIOS, so if you take a look at the bottom of the BIOS screen, it explains how to change things.

In order to go from item to item, use the up and down arrows on your keyboard. In order to change a value, use the + and - keys.

Remember, the BIOS is designed as failsafe. In other words, you will not change anything, unless you select "Save" at the end of the operation. Just exiting from the BIOS, will leave all your existing settings in tact, and you exit via the "Escape" button.

So, to resolve your problem. First navigate to the Frequency/Voltage Control page, which you have already found.

Use the arrow keys to highlight "CPU FSB Clock" and use the +/- buttons to change the value to 100. Next move to the "CPU Ratio" line, and change the value to 13. For now, that's it! Now use Escape to return to the previous menu, and at some point, the program will ask you if you wish to save the settings. Select "Y" for yes, and press enter. The system should now re-boot with your new settings, and the correct speed should appear on the set-up screen.

If you find the system is OK and stable, the manual advises you to return to the BIOS set-up, and disable the "Spread Spectrum". This will improve overall system response.

Hope this helps and makes sense!

  Squiggler 20:01 12 Sep 2004

I have a AS Rock motherboard that says that it is a 400Mhz FSB. Is there any way of telling what speed it is actually running at from within Windows etc. I would like to check the speed of the FSB as I am almost certain that this speed is depending on the processor and RAM etc. It this is true does any one know how to confirm the speed of the FSB.


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