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Chapter 7 Troubleshooting 77
77
interrupts have been user assigned. When a PCI card is installed, the BIOS checks for
available addresses and interrupt levels and automatically assigns them so that there are
no PCI address or interrupt conflicts. However, because the BIOS doesn't know about the
user-assigned ISA I/O address and interrupt level assignments, it is possible that a PCI
card will be assigned an address or interrupt that is already assigned to an ISA card. If
this happens, improper operation will result. Specifically, the problems could range from
erratic operation under specific conditions to complete system failure. If such a conflict
occurs, because the user has no control over the PCI address and interrupt assignments,
there will be no recourse but to examine the .ISA assignments and change them to values
which do not conflict. Most (but by no means all) ISA cards make provision for selecting
alternative I/O addresses and interrupt levels so that conflicts can be resolved. Software is
available to help identify specific conflicts.
The following example may serve to illustrate the problem. Suppose you had a system with
an ISA network card, a PCI video card and an ISA sound card. Further suppose that you
were then going to install a PCI Serial Buffer card. Before installing the PCI Serial card, the
I/O address and interrupt assignments for the installed cards might be as follows.
Slot Type
Status
I/O Address
Interrupt
1 (ISA)
ISA Network Card
200-210
11
2 (PCI)
PCI Video Card
FF00-FFFF
15
3 (ISA)
ISA Sound Card
300-304
9
4 (PCI)
Empty
N/A
N/A
Table 6. I/O Address & Interrupt Assignments
before Installing Serial Card
As shown, there are no conflicts, allowing the three peripheral cards to operate properly.
If the PCI Serial card were then installed, the BIOS would interrogate the PCI cards and
may reassign them new address and interrupt values as follows.
Slot Type
Status
I/O Address(s)
Interrupt
1 (ISA)
ISA Network Card
200-210
11
2 (PCI)
PCI Video Card
FE00-FEFF
11
3 (ISA)
ISA Sound Card
300-304
9
4 (PCI)
Princeton Instruments PCI
Serial Card
FF80-FFFF
15
Table 7. I/O Address & Interrupt Assignments
after Installing Serial Card
As indicated, there is now an interrupt conflict between the ISA Network Card and the
PCI Video card (both cards have been assigned Interrupt 11), causing the computer to no
longer function normally. This doesn't mean that the PCI Serial card is defective because
the computer stops functioning properly when the Serial card is installed. What it does
mean is that there is an interrupt conflict that can be resolved by changing the interrupt
level on the conflicting Network card in this example. It is up to the user to consult the
documentation for any ISA cards to determine how to make the necessary change.