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PCI to ISA Bus Interrupt Mapping
The ISA bridge provides the sixteen conventional ISA interrupts, plus four interrupt request
pins for PCI peripheral interrupts (PIRQ0 through PIRQ3). For PC-AT architecture
compatibility reasons, the PCI interrupts are routed to the ISA interrupts within the ISA
bridge. The assertion of a PCI interrupt concludes in an ISA interrupt being asserted.
Bit 7 of each PIRQ registers enable (Low) or disable (High) the routing of the PIRQ to an ISA
interrupt. The lowest four bits (3:0) of each PIRQ register determines to which ISA interrupt
the PIRQ is routed. You can map the PIRQs to the following ISA interrupts:
IRQ 5, 10, 11, 15
You can also map to IRQ 3, 4, 7, 12, and 14, but other devices are disabled if you do so.
Each PCI slot has four available interrupt lines: INTA, INTB, INTC, and INTD. These are
connected to the PCI interrupts, PIRQ0 through PIRQ3, as shown below:
Interrupt Line
Slot 1
Slot 2
Slot 3
Slot 4
Slot 5
Slot 6
Slot 7
INTA
PIRQ0
PIRQ1
PIRQ2
PIRQ3
PIRQ0
PIRQ1
PIRQ2
INTB
PIRQ1
PIRQ2
PIRQ3
PIRQ0
PIRQ1
PIRQ2
PIRQ3
INTC
PIRQ2
PIRQ3
PIRQ0
PIRQ1
PIRQ2
PIRQ3
PIRQ0
INTD
PIRQ3
PIRQ0
PIRQ1
PIRQ2
PIRQ3
PIRQ0
PIRQ1
