Legacy PC Interrupts: IRQ2 on 16-bit ISA On the 16-bit ISA bus, IRQ2 is cascaded and effectively mapped to which higher-numbered interrupt line?

Introduction / Context:
Classic PCs used two cascaded 8259A Programmable Interrupt Controllers (PICs) to expand available hardware interrupts. Understanding the mapping helps interpret device configuration and troubleshoot conflicts in legacy systems.

Given Data / Assumptions:

• System uses dual PICs (master and slave) for IRQ0–IRQ15.
• Slave PIC is connected to the master via a cascade line.
• We are considering the standard IBM PC/AT mapping conventions.

Concept / Approach:

In the PC/AT design, IRQ2 on the master PIC is reserved for cascading to the slave PIC. Devices that would have used IRQ2 are instead serviced through IRQ9, which is tied into the cascade mechanism. Therefore, OS/BIOS typically report such devices as using IRQ9.

Step-by-Step Solution:

Verification / Alternative check:

Review BIOS setup and OS device manager on vintage systems; devices expected on IRQ2 are listed under IRQ9. Technical references for the PC/AT confirm this cascade mapping.

Why Other Options Are Wrong:

IRQ11, IRQ13, and IRQ15 are separate lines on the slave PIC and not the canonical remap target for IRQ2. “None of the above” is incorrect because 9 is the established mapping.

Common Pitfalls:

Assuming IRQ2 is freely available on AT-class machines, or forgetting that some adapters labeled “IRQ2/9” simply acknowledge this mapping.

Final Answer:

9