assigning irq’s in bios

You cannot manually assign irq resources to pci cards in windows. I have learned that you can set plug and play os to disabled in the bios and attempt to assign irq recorces to different pci bus slots in the bios. I have found this to be difficultat best, the goal being a seperate irq for each pci device. Sometimes it seems the only solution is to switch cards and slots manually to achieve the desired results. Some slots seem to share irq resources no matter what. I would like to have some basic guidelines given that bios are different on different motherboards.