PC resource conflicts — Why are IRQ conflicts historically more common than I/O address conflicts on legacy PCs?

Introduction / Context:
Legacy ISA/AT hardware required manual configuration of device resources. Understanding why certain conflicts were common helps when supporting older systems and embedded designs that retain similar concepts.

Given Data / Assumptions:

• Environment: ISA/AT-era PCs with fixed IRQ lines and configurable I/O ranges.
• Devices compete for limited, sharable resources.
• Plug and Play was limited or absent.

Concept / Approach:

IRQ lines were a scarce resource (typically 15 usable on AT systems, with several reserved), while I/O address space was much larger and easier to partition to avoid overlap. Therefore, IRQ conflicts were statistically and practically more frequent.

Step-by-Step Solution:

Verification / Alternative check:

Old motherboard manuals show limited IRQs and many default claims by popular devices, confirming typical conflicts on installation.

Why Other Options Are Wrong:

BIOS does manage IRQ routing where possible, but scarcity is the real reason. I/O addresses can and do conflict if misconfigured. Software cannot automatically solve all I/O overlaps on non-PnP systems.

Common Pitfalls:

Assuming modern PCI/PCIe interrupt sharing rules apply to pure ISA systems; forgetting reserved IRQs (e.g., 2 for cascade).

Final Answer:

There are far more I/O addresses available than IRQ lines.