HP Vectra VL6 6 Technical Reference Manual page 69

The PCI interrupt lines A, B, C and D are spread across the four inputs of
the interrupt router (which is part of the PCI/ISA bridge, in the PIIX3 chip).
Since most PCI devices are single-function, this allows for an even
distribution of the lines. The distribution is shown in the following diagram.
In this, Slot 4 is present only on minitower models (and is omitted on
desktop models).
Integrated
Slot 1
graphics
A
A B C D
PCI interrupts are then mapped into ISA interrupts inside the PCI/ISA
Bridge (in the PIIX3 chip), by configuring registers 60h through 63h.
The possible choices given by the Setup program are 9, 10, 11, 15. If some of
these are unavailable due to ISA cards, some interrupts will have to be
shared.
The IDE controller (device 04h, function 01h) is configured in legacy mode,
and uses IRQ 14 (IRQ 15 for the secondary channel).
Slot 2
A B C D
Bit
7 Routing of interrupts: when enabled, this bit routes the PCI interrupt signal to the PC-
compatible interrupt signal specified in bits[3:0]. At reset, this bit is disabled (set to 1)
6:4 Reserved: read as 000
3:0 IRQx# Routing Bits: these bits specify which IRQ signal to generate.
Possible values are: 3, 4, 5, 6, 7, 9, 10, 11, 12, 14, 15.
Slot 3 Slot 4 (MT)
A B C D
A B C D
Description
4 Summary of the HP/Phoenix BIOS
BIOS Addresses
A
B
PCI/ISA
Bridge
C
D
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