14
Assignment Sequence
The interrupts are assigned to the devices in the following sequence:
1.
The fixed interrupts are hardwired and are left out of the assignment process (IRQs
0,1,2,6,8 and 13)
2.
The BIOS determines what legacy ISA devices have been disabled in the BIOS setup
and releases these IRQs into the PnP interrupt pool
3.
The PnP interrupt pool now contains the free interrupts that can be assigned to PnP
devices.
There are 2 possible ways of assignment now, depending on the setting in the BIOS for the
PnP OS installed item. (On the PnP/PCI Configuration Setup page). If set to no, the A
sequence applies, if set to yes the B sequence.
A
PnP OS Installed: No
The BIOS will now assign all interrupts
1.
The BIOS first assigns the interrupts to the ISA PnP cards from the interrupt pool.
The interrupts (if available) are assigned in the following order: 15, 11, 10, 12, 14, 3,
4, 5, 7, 9. As mentioned, if ACPI if enabled, IRQ 9 will not be available.
2.
Now the BIOS assigns the PCI interrupts from the remaining interrupts in the pool.
The interrupts are assigned in the A, D, B, C order. (With other words, the AGP IRQ
is first assigned, then the USB IRQ with the others trailing). The USB, VGA and
ACPI IRQs can be disabled in the BIOS as mentioned before.
B
PnP OS Installed: Yes
The BIOS will only assign the PCI interrupts. The
interrupts are assigned in the same order as mentioned above.
After assigning the interrupts control is passed to the OS. The OS will now assign the ISA
PnP interrupts from the remaining IRQs in the pool. However, since ISA PnP cards pass the
OS a list of IRQs that can be assigned to it, if the OS finds that all the IRQWs on the list are
already assigned, it may reassign an IRQ that was assigned to PCI by the BIOS. This means
concretely that the OS can change BIOS settings, and that settings made in the BIOS are
not necessarily correct when using a PnP OS.
