PC hardware interrupts What is the standard IRQ line used by the legacy PC floppy disk controller (FDC)?

Introduction / Context:
On legacy ISA/AT-class PCs, hardware devices signal the CPU via Interrupt Request (IRQ) lines. Knowing the canonical IRQ assignments helps diagnose conflicts and misconfigurations on older systems or embedded setups derived from them.

Given Data / Assumptions:

• Legacy hardware model: ISA-based IRQ mapping.
• Device in question: floppy disk controller (FDC), handling drives like A: and B:.
• We assume default PC/AT assignments without custom remapping.

Concept / Approach:

Historically standardized IRQ usage assigns IRQ 6 to the floppy controller, IRQ 14 to primary IDE, IRQ 4 and 3 to COM1/COM3 and COM2/COM4 respectively, and IRQ 1 to the keyboard controller, among others. These defaults informed BIOS and OS drivers for decades.

Step-by-Step Solution:

Verification / Alternative check:

Motherboard manuals and OS hardware lists consistently show the floppy controller on IRQ 6 unless explicitly changed.

Why Other Options Are Wrong:

IRQ 9 commonly supported ACPI or cascaded PIC functions; IRQ 10 was often free or used by expansion cards. “All of the above” contradicts the single-standard assignment for the FDC.

Common Pitfalls:

Confusing IRQ 6 (FDC) with IRQ 7 (LPT1) or IRQ 5 (often sound/LPT2) and mixing serial-port IRQs (3/4).

Final Answer:

6.