13
PCI Interrupts
The PCI interrupts (INT A ~ D) are routed to free interrupts in the PnP interrupt pool by the
BIOS. PCI cards all support PnP, so the BIOS knows what system resources they require.
The A,B,C,D Interrupt Request lines are mapped to physical devices in the following way:
PCI slot 1 PCI slot 2 PCI slot 3 PCI slot 4 PCI slot 5 AGP port USB
INT A
X X
INT B
X
INT C
X
INT D
X X X
USB and PCI slot 1 and PCI slot 3 use the same interrupt, and AGP, PCI slot 5 use the same
interrupt. Because PCI interrupts are shareable this is OK. The BIOS contains the following
items related to PCI interrupts:
BIOS item
Settings
Explanation
Assign IRQ to
USB
Enabled/disabled If the USB ports are not used, the user can set this item
to disabled. This will make sure that PCI INT D is only
granted to PCI slot 1, PCI slot 3.
Assign IRQ to
VGA
Enabled/disabled Most modern VGA cards require an interrupt to function.
If yours does not, disable this item to free the PCI
interrupt.
ACPI Interrupt
The ACPI specification defines that devices that support ACPI (Advanced Configuration
and Power Interface) can issue interrupts to the OS to notify it of an ACPI event. This
interrupt is called the SCI (System Control Interrupt), and it is always mapped to IRQ 9. If
ACPI is enabled (see the FAQ chapter, ACPI section), PCI devices will be able to make use
of IRQ 9, because PCI interrupts are shareable. But ISA PnP cards can not use IRQ9 if
ACPI is enabled.
Therefore the BIOS contains an item that allows the user to disable ACPI. This will release
IRQ 9 to the PnP interrupt pool for use by ISA PnP cards.
