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Technical Reference Guide
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition - September1997
4-9
4.2.5 PCI INTERRUPT MAPPING
The PCI bus provides for four interrupt signals; INTA-, INTB-, INTC-, and INTD-. These
signals may be generated by on-board PCI devices or by devices installed in the PCI slots. In
order to minimize latency, INTA-..INTD- signal routing from the PCI slot to the system board is
distributed by the riser card (backplane) as shown below:
System Board PCI Slot
INTA- INTD-
INTB- INTA- [1]
INTC- INTB-
INTD- INTC- [2]
NOTES:
[1] Shared with network interface controller
[2] Shared with graphic controller
Interrupts generated by PCI devices can be configured to share the standard AT (IRQn) interrupt
lines. Two devices that share a single PCI interrupt must also share the corresponding AT
interrupt. Example: If a PCI card is installed in slot 5 and wants to use INTA- then it must share
INTA- as well as the corresponding AT interrupt with the on-board network interface controller.
Three PCI configuration registers of the 82C586 are used to route the INTA-..INTD- signals to
the IRQn signal lines (refer to section 4.3.4.1 for information on IRQn routing). The power up
(default) configuration has PCI interrupt redirection disabled.
PCI Configuration Register 55h, IRQ Routing Reg. 1
Default Value = 00h
Bit Function
7..4 INTD- Routing:
0000 = Reserved 1000 = Reserved
0001 = IRQ1 1001 = IRQ9
0010 = Reserved 1010 = IRQ10
0011 = IRQ3 1011 = IRQ11
0100 = IRQ4 1100 = IRQ12
0101 = IRQ5 1101 = Reserved
0110 = IRQ6 1110 = IRQ14
0111 = IRQ7 1111 = IRQ15
3..0 MIRQ0- Routing (Same as PIRQD-)
PCI Configuration Register 56h, IRQ Routing Reg. 2
Default Value = 00h
Bit Function
7..4 INTA- Routing: (Same as PIRQD-)
3..0 INTB- Routing (Same as PIRQD-)
PCI Configuration Register 57h, IRQ Routing Reg. 3
Default Value = 00h
Bit Function
7..4 INTC- Routing: (Same as PIRQD-)
3..0 MIRQ1 Routing (Same as PIRQD-)