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Legacy IRQ assignments – What is IRQ6 typically reserved for?

Difficulty: Easy

Correct Answer: The floppy disk controller

Explanation:


Introduction / Context:
Understanding classic IRQ mappings is helpful when maintaining or studying legacy PCs. Exams and interviews often ask which peripherals used which IRQs in the pre-APIC era.



Given Data / Assumptions:

  • Standard ISA/PIC interrupt scheme.
  • No Plug-and-Play reassignments; default legacy layout.
  • Typical IBM-PC compatible hardware set.


Concept / Approach:

Common defaults: IRQ1 = keyboard; IRQ3/4 = serial ports; IRQ5/7 = LPT/sound variations; IRQ6 = floppy controller; IRQ14/15 = IDE. Therefore, IRQ6 is reserved for the floppy controller.



Step-by-Step Solution:

Recall mapping table for legacy hardware.Identify IRQ6 → floppy disk controller.Eliminate distractors: keyboard (IRQ1), LPT2 (IRQ5/7), modem (IRQ3/4).


Verification / Alternative check:

Vintage diagnostics (MSD, Device Manager in early Windows) list the floppy controller on IRQ6.



Why Other Options Are Wrong:

They correspond to other well-known IRQ assignments, not IRQ6.



Common Pitfalls:

Mixing serial port IRQs with floppy; assuming “None of the above” because modern systems virtualize interrupts.



Final Answer:

The floppy disk controller.

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