On legacy ISA-bus PCs, you are installing network adapters in 20 computers. What configuration rule must be followed regarding hardware interrupts (IRQs) when installing the adapters alongside other add-in cards?

Introduction / Context:
ISA-bus era PCs relied on manual configuration of hardware interrupts (IRQs), I/O port addresses, and DMA channels for add-in cards such as network interface cards (NICs), sound cards, modems, and SCSI adapters. This question checks whether you know the fundamental rule that prevents interrupt conflicts that can freeze systems or cause intermittent device failures.

Given Data / Assumptions:

• Each computer has multiple ISA adapters installed (NIC plus other cards).
• ISA cards commonly use fixed or jumper-selectable IRQ lines.
• Plug and Play support is limited or absent; the OS cannot reliably arbitrate conflicting IRQs.
• We are configuring each individual PC; cross-machine IRQ settings do not interact over the LAN.

Concept / Approach:

An IRQ line is a hardware signal a device raises to get CPU attention. On classic ISA systems, two devices sharing the same IRQ typically contend for service, causing missed interrupts and driver instability. Therefore, within a single PC, every active device must occupy a unique IRQ. Network-wide uniformity of IRQs is meaningless because interrupts are local to each host’s motherboard.

Step-by-Step Solution:

Verification / Alternative check:

Practical tests show that two ISA devices on the same IRQ lead to dropped packets (for NICs), lockups, or failure to initialize drivers. Unique IRQ assignments eliminate these symptoms.

Why Other Options Are Wrong:

All same IRQ (per PC): Causes immediate interrupt conflicts.

All NICs same IRQ across network: Irrelevant; IRQs are local per motherboard.

All NICs different IRQs across network: Also irrelevant between separate machines.

None of the above: Incorrect because unique per-PC IRQs are required.

Common Pitfalls:

Confusing per-machine configuration with network-wide standards; believing drivers will “share” ISA IRQs safely; ignoring serial/parallel defaults (COM1 IRQ 4, COM2 IRQ 3, LPT1 IRQ 7) when assigning the NIC.

Final Answer:

All adapters in a computer, including the network adapter, must be set to different IRQs