A Windows 2000 Server has a non–Plug and Play ISA modem fixed at IRQ 5. After adding a PCI modem and rebooting, Device Manager shows an IRQ 5 conflict (both modems want IRQ 5). What is the correct way to resolve this resource conflict?

Introduction / Context:
Legacy ISA devices often require fixed, jumper-set resources. PCI devices typically use Plug and Play to auto-assign interrupts—unless the BIOS allows that line to be reserved for legacy ISA hardware. Here, an ISA modem already occupies IRQ 5, but the new PCI modem grabbed the same IRQ, producing a conflict.

Given Data / Assumptions:

• ISA modem is non-PnP and hard-set to IRQ 5.
• PCI modem is PnP and can use alternative IRQs.
• Windows reports both devices on IRQ 5.

Concept / Approach:

The proper fix is to prevent the PCI enumerator from using IRQ 5 at all, by marking it “Reserved for ISA/Legacy” in BIOS/CMOS. Then, on the next boot, the PCI bus will reassign the PCI modem to a different, available interrupt (for example, IRQ 10 or 11), while the ISA device legitimately keeps IRQ 5. Changing the ISA modem via Device Manager is ineffective because it is not Plug and Play.

Step-by-Step Solution:

Verification / Alternative check:

After reserving IRQ 5 for legacy devices, Device Manager should display the PCI modem on another IRQ. Test dial-up to confirm stable operation and no sharing conflicts.

Why Other Options Are Wrong:

Changing the ISA modem's IRQ in Device Manager is not possible for non-PnP hardware.

Reserving IRQ 10 for legacy does not protect the ISA device on IRQ 5 and can cause further conflicts.

None: An appropriate BIOS reservation fix exists.

Common Pitfalls:

Attempting to force PCI device IRQs manually in Windows; moving the PCI card to another slot can work, but BIOS reservation is the reliable method.

Final Answer:

Edit the BIOS/CMOS to reserve IRQ 5 for non–Plug and Play devices so the PCI modem selects another IRQ