PC IRQ conventions: Historically, which hardware interrupt request (IRQ) line was most commonly assigned to ISA Sound Blaster–compatible sound cards?

Introduction / Context:
Legacy ISA devices required unique IRQ lines to signal the CPU. Classic Sound Blaster–compatible sound cards needed a dedicated IRQ for audio events. Knowing the common defaults helps resolve conflicts on vintage systems.

Given Data / Assumptions:

• Context is ISA-era PCs (DOS/Win9x), not modern PCI/PCIe plug-and-play.
• Common peripherals also occupied well-known IRQs (e.g., keyboard on IRQ 1).
• We are asked which IRQ was typically used by Sound Blaster–type cards.

Concept / Approach:

Sound Blaster cards commonly defaulted to IRQ 5 (sometimes configurable to IRQ 7). IRQ 7 was often used by LPT1 (parallel printer), so IRQ 5 avoided conflicts while remaining a standard SB setting. Thus, IRQ 5 is the best answer.

Step-by-Step Solution:

Verification / Alternative check:

Sound card manuals and SET BLASTER environment strings (e.g., A220 I5 D1 H5) corroborate IRQ 5 as a frequent default.

Why Other Options Are Wrong:

• IRQ 6: generally floppy disk controller.
• IRQ 15: typically secondary IDE or available for other devices.
• IRQ 1: reserved for keyboard.
• It does not use an IRQ: incorrect for ISA sound cards.

Common Pitfalls:

Forgetting to change printer port IRQ or disable LPT when using IRQ 7 for the sound card; mixing DMA/IRQ settings leading to no audio in DOS games.

Final Answer:

IRQ 5