Please explain how this works.

[HWF]

How does a Mother Board keep the agp/pci buses at 66/33 when you overclock the fsb?

 

[AdvancedRobotics]

It doesn't. The fsb of the agp and pci slots are determined by the fsb of your processor multiplied by the divider, which is motherboard specified.

AGP divider is typically 1/2 (one half). PCI dividers are diverse and go up to (I belive) 1/7 (one seventh).

So If you have a 133MHz fsb, then the AGP bus will be 66MHz and the PCI bus will be 33MHz (With a 1/4 divider) as you said. As the fsb of your processor increases then you'll need to increase the dividers to keep the fsb in safe parameters.

 

[jackschmittusa]

I am only guessing, buy I would imagine that it has its own clock generator that is set in the bios to either the standard freq or triggered by the fsb to rise proportionatly with an increase in fsb speed. When you lock it in the bios, it is not triggered by the fsb speed. Hopefully, someone will know for sure.
-edit-
AdvancedRobotics posted while I was writing my post. Some mobos allow you to lock the pci/agp freq.

 

[acidpad]

As advanced robotics said, the divider is controlled by the motherboard. basically there is just a variable divider (as far as i understand it) and the divider is changed based on the increase in FSB to maintain the proper agp/pci bus lock.

 

[HWF]

My question comes from reviews saying Intel Mother boards having the ability to lock the agp/pci buses. This is supposed to help with overclocking. I've been wondering how this is done.

 

[Shalmanese]

In the olden days, you had fixed dividers. Now, you can actually lock the bus to 33 Mhz. Basically, its all done with buffers. Buffers let you have completely independant bus speeds. However, latency suffers a bit.

 

[Peter]

Depends on two factors:

* What does the clock synthesizer chip provide?
* What does the chipset allow?

With chipsets that don't support anything but a certain selection of speed ratios (like e.g. Intel 440BX), you cannot use clock synthesizers in asynchronous ("locked") mode.

If the chipset does do that, you can. It's not a new invention either ... ye olde SiS 5571 and VIA Apollo VPX did that already, and that was waaaaaaay back in the early Pentium days, when Cyrix introduced 75 MHz CPU front side bus.

 

[ProviaFan]

Originally posted by: HWF
My question comes from reviews saying Intel Mother boards having the ability to lock the agp/pci buses. This is supposed to help with overclocking. I've been wondering how this is done.

How that is done has already been explained. Many AMD motherboards these days come with PCI and AGP locks as well, FWIW. How this helps with overclocking is that it allows you to increase the FSB (and thus the memory and processor speed) without increasing the speed of the PCI and AGP busses (because many times PCI peripherals, graphics cards, and hard disks - which are run from an IDE controller on the PCI bus - are very sensitive to clockspeed).