Device Mapping
C–2
Interrupt Map
The IRQ (interrupt request) assignments for the Digital AlphaStation 255 system are
shown in Table C–2 .This information is useful when adding or reconfiguring options on
your system. Avoid conflicts when assigning IRQs by associating one IRQ to one source.
PCI interrupt levels are set either by the operating system (as in the Windows NT
operating system) or by the SRM console (as in the OpenVMS and Digital UNIX operating
systems).
The Windows NT operating system preassigns the PCI devices specific IRQs, as
shown in Table C–2. If a PCI device is not installed in a PCI slot, the IRQ
associated with that slot remains unavailable for ISA devices. For example, if
PCI slot 2 is empty, IRQ9 is not available for any other device.
Table C–2: Default System Interrupt Assignments
IRQ#
Source
Windows NT-Specific
Preassignments
0
Timer/counter.
1
Keyboard.
2
Internal cascade of IRQ8-15.
3
Serial port 2 or ISA slot.
4
Serial port 1 or ISA slot.
5
Parallel port, PCI interrupt, or ISA slot.
6
Floppy controller or ISA slot.
7
Parallel port or ISA slot.
8
Reserved.
9
PCI interrupt or ISA slot.
PCI slot 2 (top slot).
10
PCI interrupt or ISA slot.
Embedded Ethernet
controller and PCI slot 0.
Shared IRQ.
11
SCSI controller or ISA slot.
PCI SCSI controller.
12
Mouse or ISA slot.
13
Reserved.
14
ISA slot.
15
PCI interrupt or ISA slot.
PCI slot 1.
