IRQ planning: given a PC with COM1 on IRQ 4 (mouse), a modem on COM2 using IRQ 3, a SCSI host adapter on IRQ 5, and a sound card on IRQ 11, which supported IRQ could you assign to the new network interface card without conflicts?

Introduction / Context:
Legacy PCs required careful manual assignment of IRQs to avoid conflicts between ISA devices. This scenario asks you to choose a free interrupt for a NIC when several common IRQs are already taken by serial ports, a SCSI controller, and a sound card.

Given Data / Assumptions:

• COM1 (mouse) uses IRQ 4.
• COM2 (modem) uses IRQ 3.
• SCSI host adapter uses IRQ 5.
• Sound card uses IRQ 11.
• NIC supports IRQs: 2, 3, 4, 5, 9, 10, 11.
• Standard LPT1 uses IRQ 7; IDE uses 14/15 (not conflicting here).

Concept / Approach:

Pick an IRQ not already used. 4 and 3 are busy (serial ports), 5 is busy (SCSI), and 11 is busy (sound). IRQ 2 historically cascades to IRQ 9 on AT-class machines; many configurations prefer avoiding 2/9 unless planned carefully. IRQ 10 is commonly free and a traditional choice for NICs, ensuring no overlap with existing devices.

Step-by-Step Solution:

Verification / Alternative check:

Historical configuration guides list IRQ 10 as a common, conflict-free selection for network cards when 3/4/5/7/11 are taken.

Why Other Options Are Wrong:

2: Often tied to 9; may create hidden conflicts.

4: Already used by COM1.

5: Used by SCSI adapter.

None of the above: Incorrect because IRQ 10 is suitable.

Common Pitfalls:

Overlooking the IRQ 2↔9 cascade; forgetting default serial/parallel assignments; choosing an IRQ used by Plug and Play devices.

Final Answer:

10