Assigning Interrupt Requests (IRQ)

Some expansion cards may require an interrupt request (IRQ) to operate.  Generally an IRQ must be exclusively assigned to only one device. In a standard design there are sixteen IRQs available, but most of these are used by the system.

Hardware devices gain the attention of the processor by using interrupt requests.   The sixteen hardware interrupts available are numbered 0 to 15 and they are prioritized where interrupt 0 will be attended to first, interrupt 15 last.  Most of the interrupts are available to the expansion cards through the bus slot, however a few are not.  The interrupts and their assignments are listed in the table below.

Its not uncommon for Plug-and-Play (PnP) to misdetect or even fail to detect new hardware, especially the first time around.  If you have already clicked on Device Manager and selected the category and offending device (flagged with a red X or yellow exclamation point ! and removed the item, then rebooted and windows still not detected it correctly, you may have to resolve an IRQ or I/O port conflict.

First click on Device Manager's hardware list

Second double-click a mis-installed device's entry. 

Next click on the resources tab and clear the "Use automatic settings" checkbox.

In the Resource Type list, double click the IRQ's, I/O ports and upper memory blocks (UMB) Device Manager tells you conflict with other devices.  Then make new entries in the Edit Resource dialog.

The dialog should restrict your choices to values your device supports, and advise you of any conflicts between the current resource assignments and those of other devices.  Once you have made your changes, click OK in all open dialogs and reboot.

Here is an example:  your system BIOS may assign a PCI slot an IRQ that is already taken by an ISA card.  If so, try turning off or reassigning motherboard resources through the BIOS.  If your motherboard lets you use the BIOS to assign IRQ's to PCI slots, this may also help you untangle things.

Poorly constructed PCI cards may also cause problems.  If they don't adhere to the PCI 2.1 specs, they may play games with IRQ's or do other unorthodox things.

If you suspect your hardware of being flaky, test it on another machine.  No-name brand motherboards with lower-rent PCI controllers or BIOs can also be at fault.  In such case, replacement is the only solution.