On the 16-bit ISA bus, to which higher-level interrupt line is IRQ2 cascaded (elevated) for system compatibility?

Introduction / Context:
Legacy PCs used Programmable Interrupt Controllers (PICs) where IRQ lines were mapped for hardware events. With two cascaded 8259 PICs, IRQ2 on the first controller was repurposed to cascade to the second controller, reappearing as IRQ9 for devices. Understanding this historical mapping is vital when interpreting old hardware manuals and configuring ISA-era devices.

Given Data / Assumptions:

• System uses classic 8259A dual-PIC design.
• Legacy 16-bit ISA interrupt mapping rules apply.
• We are identifying the remapped line for IRQ2.

Concept / Approach:
In AT-class systems, IRQ0–IRQ7 were on the master PIC; IRQ8–IRQ15 were on the slave PIC. The slave PIC connects to the master via IRQ2, effectively reserving IRQ2 for cascading. Consequently, devices that might have used IRQ2 are instead routed to IRQ9 on the slave, creating the conventional “IRQ2 = IRQ9” association for device usage.

Step-by-Step Solution:

Verification / Alternative check:
Motherboard diagrams and BIOS setup screens from ISA systems list devices that historically used IRQ9 where older references mention IRQ2, confirming the mapping.

Why Other Options Are Wrong:

• 11 / 13 / 15: These are valid IRQs but not the cascade mapping for IRQ2.
• None of the above: Incorrect because IRQ9 is the standard mapping.

Common Pitfalls:
Confusing DMA channels with IRQs and assuming IRQ2 can be freely assigned when it is reserved for cascading in AT systems.

Final Answer:
9