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PCI and AGP share bandwidth?

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Jun 20, 2001
My friend and I have been arguing over the fact that the AGP slot and the PCI slot directly below it share bandwidth. I say thats not true. He says he heard it was. We both think that the AGP and first PCI slot share an IRQ, correct? Who is write and who is wrong? Also, does the PCI bus interface with the northbridge or the southbridge chipset? Every diagram I see has it intersecting the line running from northbridge to southbridge, makeing me believe that some functions are controlled by the north and some by the south for the PCI bus. Thanks for any insight.


3D God
Nov 20, 2001
Fort Wayne, Indiana
The agp slot was designed as a stand alone slot... with its own bandwidth and mastering. Dedicated for graphics. The IRQ's can be shared on the slots but can also be manually given seperate IRQ's... disabling ACPI will usually give them seperate IRQ's. And I'm not too sure on the chipset... I think with most chipsets the southbridge controls the PCI slots and agp slot... not too sure though so don't quote me on that one.


Mar 4, 2002
Cincinnati, OH
The IRQ assignments for PCI 1 are different depending on the mobo. However, in the past PCI 1 most often shared an IRQ with AGP....that's why good builders always skipped PCI 1 and populated the slots consecutively down starting at PCI 2. Consult your manual, all GOOD manuals (Asus) have a table that shows which slots share.


Just Another Retired Moderator
Dec 19, 2000
Lawrenceville, GA
Northbridge controls CPU L2, memory bandwidth PCI and AGP in interactions with the processor.

SouthBridge handles USB, serial and parallel ports, IDE, audio and ISA I/O.

In short, NorthBridge handles more advanced functions while the SouthBridge handles the more basic forms of I/O.