The Big Q!

[Tiberius13]

Ok... when people overclock their fsb, they always have some concerns about devices they are running in the AGP and PCI slots on their motherboard since they are affected by changes in the fsb speed.

The Big Q is:

What affect does the change in fsb have on these AGP and PCI devices?

I have heard it said that the pci bus runs faster until you hit 150, then it falls back down (the divider changes?). Does this same rule apply to AGP? If you hit certain key fsb levels (such as 150, 167, etc.) will it remove the strain from your AGP and PCI devices?

All responses and input is appreciated

 

[xxsk8er101xx]

Big Q, always the Big Q! Well what about the little Q huh!? No one ever takes into consideration for the little Q you fat bastard!

j/k

<A HREF="http://www.anandtech.com/mysystemrig.html?id=9933" target="_new"> My Rig </A>

 

[Smilin]

Not unless YOU change the multiplier...it doesn't happen on it's own. You can set the FSB higher and higher until your shiz blows up.

 

[FatBurger]

Newer Abit P4 boards will lock the PCI and AGP so they don't move. Other boards offer dividers that you can change at certain levels so your PCI and AGP remains in spec.

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Don't step in the sarcasm!

 

[Tiberius13]

So basically on a normal/average motherboard... if you raise the fsb bus, all of the PCI and AGP cards go for the ride?

My current understanding is that the PCI bus runs at 33mhz (1/4th of the fsb)? So if you push your fsb to 150, the pci bus is pushed to 37.5, and if you push the fsb to 166, the pci bus reaches 41.5?

I could be completely wrong on that assumption as I only picked up that 'knowledge' from someone elses post in a forum...

My only worry in overclocking my system is my ATI AIW card. There is a lot of 'stuff' packed onto that card that might not agree with the faster bus...

 

[FatBurger]

That's exactly right, which is why there are different dividers that will drop the PCI and AGP back down.

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Don't step in the sarcasm!

 

[Raystonn]

The actual default divider depends on the default external clock on your system. The Pentium 4 uses an external clock of 100MHz. The PCI would then be on a 1/3 divider, with the AGP at twice that. Most modern Pentium 4 motherboards have an 'auto' option (which is the default setting) for PCI/AGP that will actually change the PCI/AGP dividers for you to attempt to keep it as close to specs as possible, without underclocking them.

The Abit TH7II and a few others also include the ability to actually 'fix' the PCI/AGP at spec, so they never increase.

-Raystonn


= The views stated herein are my personal views, and not necessarily the views of my employer. =

 

[Tiberius13]

My guess is that my Asus A7M266 doesn't change the divider on it's own... or support manually changing it....

 

[FatBurger]

No, it doesn't, and yes it does.

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Don't step in the sarcasm!

 

[Raystonn]

Based on <A HREF="http://firingsquad.gamers.com/hardware/a7m266/page4.asp" target="_new">this</A> information, I would say you are likely correct. I do not see any mention of PCI or AGP dividers.

-Raystonn


= The views stated herein are my personal views, and not necessarily the views of my employer. =

 

[Tiberius13]

No, it doesn't, and yes it does.

If the 'yes it does' refers to manually changing the divider, please let me know how this would be done. Thanks!

 

[jclw]

One other thing to consider:

When the PCI/AGP busses are locked and you raise the FSB they become out of sync, and you end up with a higher latency, which in some cases will cause a performance decrease. Ideally every bus can be multiplied by an integer to give the FSB.

- JW

 

[FatBurger]

Looking at the link Raystonn provided, it looks like you can't change multipliers. I could've sworn you could.

<font color=orange>Quarter</font color=orange> <font color=blue>Pounder</font color=blue> <font color=orange>Inside</font color=orange>
Don't step in the sarcasm!

 

[Tiberius13]

Aren't we talking about a different divider/multiplier than the cpu multiplier?? The A7M266 does not support changing the cpu multiplier, but I didn't realize that was a factor in determining the effect raising the fsb would have on agp and pci devices?

I can see that this is a wonderfully confusing topic.. LOL...

I know a few motherboards let you set a divider such that you can overclock to settings like 166/133 (166 would be fsb, and 133 would be ? agp/pci bus?) For P4's based on what was posted here it seems that some motherboards make changes like that on their own?

 

[Unolocogringo]

Different boards are set up differently. Some boards allow you to change the PCI,AGP dividers as you see fit.

Other boards automatically change at spicific fsb settings.

And some of the newer boards allow all three mentioned above.

The 133/100 or 166/133 settings that you are refering to are the cpu/memory settings.The 133/100 setting will run the cpu at 133fsb and the memory at 100fsb. Likewise some boards also allow yuo to run 100/133 which will give better memory performance on a 100fsb cpu without overclocking the cpu by running the memory at 133

I aint signing nothing!!!

 

[Crashman]

Most boards only drop when changing from 66 to 100 or 100 to 133. Only the newest boards have the proper dividers to drop at 150 or 166.

What's the frequency, Kenneth?

 

[svol]

Just to make everything clear:

100 MHz FSB: 1/3 PCI, 2/3 AGP
133 MHz FSB: 1/4 PCI, 1/2 AGP
166 MHz FSB: 1/5 PCI, 2/5 AGP (not sure about the AGP divider)
200 MHz FSB: 1/6 PCI, 1/3 AGP


It's not a bug, it's a feature
THGC: before: :frown: 😱 , after: 😎