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Interrupt Request (IRQ) Conflicts

If your machine hangs at regular intervals, slows down to a crawl for no apparent reason, and generally behaves as if possessed of a mind of its own, the cause could be a hardware interrupt conflict. When two hardware devices are assigned to the same IRQ, one or both of the devices may decide not to function properly or not function at all.

Tracing IRQ Conflicts

Symptoms:

One or both devices on a shared IRQ will not function.
The machine locks up, or will not boot.
DO NOT modify your BIOS IRQ assignments. If you have come to this page after manually assigning IRQs in your BIOS then put the settings back the way they were and let the motherboard assign the IRQs automatically.
If your BIOS has an option for "Plug and Play Operating System", or similar, and if it is set to 'Yes' or 'Enabled' then set it to 'No' or 'Disabled' and leave it at that setting. XP does not need this option enabled.
The overall goal of IRQ troubleshooting is to track down shared IRQs at the hardware level, before XP boots, and to avoid having and add-on cards sharing IRQs with onboard resources. The only proper solution to an IRQ conflict in XP is to find a slot for the offending card that is not shared with other resources. And if you don't have a spare, unshared slot, either toss the card or toss your motherboard and get something better and more modern. That is not a joke.

Windows XP will reassign BIOS hardware IRQs and set up its own list of shared IRQ vectors, which it does a very good job of. If your machine is locking or a card is not functioning, and if the problem is caused by an IRQ conflict then the conflict is more than likely occurring before XP can assign new IRQs to the devices. What this means is that it is nigh on pointless to consider modifying XP's automatic IRQ assignments, especially since XP will not let you modify some of its assignments anyway, and also because if you override XP's automatic assignments then you are likely to cause more problems for yourself if you add a new device at some later point.

You need to determine what slots in your machine share what IRQs. Some BIOS have an option that will list the slots in your machine, along with the IRQ that each slot uses, and if you are lucky, the list will indicate if the slot is shared with onboard resources. However, most modern BIOS have no options at all for managing IRQ assignments. For example, many Intel workstation and server boards use ACPI therefore they do not support modification to IRQ assignments.

Manually assigning IRQs in BIOS is generally only applicable to some very old, non-ACPI compliant systems, and if ACPI support is enabled then XP will ignore manual BIOS assignments anyway. If you really must assign IRQ addresses manually in the BIOS on an ACPI-enabled motherboard then you may need to reinstall XP (or do an in-place upgrade) and force it to use the Standard PC HAL. How to force a Hardware Abstraction Layer during upgrade or installation of Windows XP. Also see Manually Assigning IRQs in Windows XP, lower down the page.

Some BIOS will list the IRQ configuration at boot time. If you're quick with a camera or know how to use the keyboard Pause button to suspend the processor, you can get a look at the list of IRQs assigned by the system.

Conflicts on a shared IRQ for onboard USB and a slot in the motherboard are common, especially if the card is a sound card or internal modem. Check the BIOS IRQ assignments, if possible, and do not put cards in slots that are shared with on-board devices. All BIOS are different so you will have to look for something like "PnP/PCI Configuration", "IRQ Resources", or "PCI IRQ Assignment". Consult the manual for your motherboard.

If you cannot find the motherboard IRQ assignments from your BIOS, consult your manufacturer's website or the manual that came with your system.
Pull out all unnecessary cards:
  • Remove all cards and peripherals that are not needed to make the machine boot
    • CD drives & DVD drives
    • Additional hard disk controllers
    • Secondary hard disks
    • Sound cards
    • TV cards
    • Any other non-essential PCI/USB cards
    • All USB devices, including hubs
    • Add-on serial port cards, etc.
If you have an onboard video card as well as a PCI or AGP card installed, remove the plug-in cards and enable onboard video. If you have multiple video cards and no onboard video, remove any PCI video card and keep the AGP card, if you have an AGP card. You must remove all non-essential cards even if you are 100% convinced that a certain card is not causing the problem.

If you have an internal modem, take it out. If you have an external modem that is plugged into an onboard serial port, disconnect it.

If you have modified the BIOS entry point addresses for any serial or parallel ports then put them back to the factory default settings.

You should be left with a "bare-bones" system with keyboard, mouse, a single video output, a single hard disk and not much else at all.

If the machine operates in this stripped-down mode then it's fairly certain that one or more of the devices you have removed are the at the root of the problem.

Turn the machine off, put one device only back in and start the machine up again. Repeat this process until the symptoms reappear. If the symptoms do not recur then it's possible that you have put a device back into a different slot to what it was in originally. Just moving a card to a different slot can often cure the problem. If the problem persists, go to the next step.
If you still cannot determine what IRQs are shared between onboard devices and slots in your motherboard, take one card, keep moving it from one free slot to another free slot and restarting your machine until the device conflict is resolved or you run out of either slots to move to or cards to move, whichever comes first. Repeat for each card. Swapping the positions of cards might also resolve the issue.
If the card has a switch or jumper for setting the IRQ then it's probably quite old, as most cards nowadays do not support manual IRQ assignment via jumpers. You may have to cut your losses and toss the card.
Manually Assigning IRQs in Windows XP

If you are unable to resolve your IRQ conflict by slot-swapping, and if you are determined to modify XP's IRQ assignments against kadaitcha.cx's recommendations then in Device Manager, expand "Computer" and right click the expanded entry. Select Update Driver, then choose "Install from a list or Specific Location (Advanced)", click Next. Select "Don't Search, I will choose the driver to install.", click Next. If Standard PC is shown in the list, choose it, otherwise uncheck Show compatible hardware, then select Standard PC from the list. Click Next.

You must do this so as to force installation of the Standard PC HAL.