Effects of Overclocking on AGP, PCI and ISA buses

[Garet Jax]

I have done a lot of research on overclocking and I am preparing to start overclocking my home machine.

One of the areas I have not been able to find much discussion on is how OCing affects the AGP, PCI and ISA slots.

I will write what I believe to be true, but I would appreciate the more knowledgable folks to correct me when I am wrong and answer my questions at the end.

Thanks a lot in advance.

ISA
I have no idea how OCing affects ISA slots and devices. Any knowledge or advice is appreciated.

PCI
Since the PCI bus normally runs at 33MHz, we need to have access to FSB dividers to ensure that we can set the correct speed regardless of the FSB. The best speed range to set the PCI bus to is between 33MHz and 40MHz.

Overclocking the PCI bus yields very little performance gain due to the types of cards that use PCI. As a result, it is more important to find a divider that allows the PCI cards to run in a stable fashion than it is to maximize the PCI bus speed.

AGP
Since the AGP bus normally runs at 66MHz, we need to have access to FSB dividers to ensure that we can set the correct speed regardless of the FSB. The best speed range to set the AGP bus to is between 66MHz and 83MHz.

Unlike the PCI bus, speeding up the AGP bus can yield significant performance improvements since we are speeding up the computer's ability to process and manage video output. As a result, it does make sense to maximize the AGP bus' speed.

Questions
(1) What is the base speed of the ISA bus?
(2) Is there a divider for ISA bus on OC'able motherboards that have ISA slots?
(3) Where can I find information on which types of AGP cards are more overclockable than others?

 

[drewski]

There should be a PCI to ISA bridge, and with it a multiplier from PCI speed to ISA. This will exist in boards that do not have ISA slots since I'm pretty sure that the PS2 Keyboard & Mouse, as well as some of your other ports are handled through ISA.

It is my understanding that this is a 1/4 multiplier (i.e. 33MHz PCI speed => ~8MHz ISA base speed).

I thought speeding up the PCI bus would speed up some things like File transfers or downloads through NIC/Modem. Could be wrong though.

 

[AndyHui]

In most cases overclocking no longer affects the ISA bus on modern motherboards, since its frequency is no longer linked to the FSB. The nominal frequency of the ISA bus is 8MHz.

Speeding up the PCI bus does bring big benefits, such as faster transfers from IDE drives, and generally any other data that is sent across the PCI bus, as there is more bandwidth.

In general, most of the latest AGP cards are able to handle AGP speeds of up to 100MHz, such as the nVidia GeForce family, very late TNT2s, and Voodoo 4/5s. ATI Radeon cards should handle these very high speeds quite well too.

 

[shathal]

Side-effects of OC-ing PCI/AGP.

AGP - no benefit in OC-ing here (not that you can specifically OC the AGP) - the bottleneck with GFX is not the bandwidth of the AGP-bus, it's the GFX-card internal memory-bandwidth. The only effect here is potential instability.

PCI - some PCI devices/cards don't like over/under-clocked PCI-busses and tend to misbehave in rather unpleasant (and fatal/crashy) ways. Things that often tend to demonstrate such behaviour are certain NICs and various other devices which I can't be asked to list here.

As such, I don't see the benefit of OC-ing the PCI-bus, it's brought me more grief than was worth it. If I had a choice, I'd disconnect the link between CPU FSB and PCI - but ah well ... .

Just wrote this to make you aware that OC-ing here too has its unpleasant side-effects .

Hope it helps .

And AFAIK the ISA clock is 1/4 that of the PCI one, so that's another vote in that direction .

BTW - why would you *BOTHER* OC-ing ISA? Just wondering - seems like waste of effort/time to be honest .

 

[puppet]

"The PCI bus is much faster than the ISA bus. So, for ISA cards to work properly with I/O cycles from the PCI bus, the I/O bus recovery mechanism adds additional bus clock cycles between each consecutive PCI-originated I/O cycles to the ISA bus.

By default, the bus recovery mechanism adds a minimum of 3.5 clock cycles between each consecutive 8-bit I/O cycle to the ISA bus. The options above enable you to add even more clock cycles between each consecutive 8-bit I/O cycle to the ISA bus. Choosing NA sets the number of delay cycles at the minimum 3.5 clock cycles.

So, set the 8-bitand/or 16-bit I/O Recovery Time at NA if possible for optimal ISA bus performance. Increase the I/O Recovery Time only if you are having problems with your 8-bit or 16-bit ISA cards. Note that this function has no meaning if you are not using any ISA cards."

stolen from....




Bios linkage