Legacy IRQ assignments – What is IRQ6 typically reserved for?
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AThe floppy disk controller
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BThe keyboard controller (uses IRQ1)
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CLPT2 (parallel port)
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DAn internal modem (commonly IRQ3/4)
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ENone of the above
Answer
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.