Knowledge of interrupt requests (IRQs) is not required of CompTIA A+ 2006 candidates but is still useful when working with legacy/non-PnP hardware.

Interrupt requests or IRQs are physical circuits on a motherboard that allow various devices to request the attention of the CPU and exchange data and/or instructions over I/O addresses in memory, assigned to those devices. On newer hardware the operating system assigns an IRQ number to each device during the OS boot sequence (Windows uses Ntdetect.com). The Basic Input/Output System (BIOS) will assign IRQ numbers on older hardware that doesn't support Advanced Configuration and Power Interface (ACPI).

Motherboards include a Programmable Interrupt Controller (PICs) which specify the limit of physical IRQ lines and enables the prioritization of interrupt signals being sent to the CPU.

Typical IRQ assignments in PCs are:

• IRQ 0: System Timer (main system clock)
• IRQ 1: Keyboard controller
• IRQ 2: Cascade to second PIC
• IRQ 3: COM 2 and 4 serial ports
• IRQ 4: COM 1 and 3 serial ports
• IRQ 5: Sound card (sometimes)
• IRQ 6: Floppy drive controller
• IRQ 7: LPT 1 parallel port
• IRQ 8: Real-time clock
• IRQ 9 - 11: Unreserved/open for other devices
• IRQ 12: PS/2 mouse port
• IRQ 13: Floating Point Unit (FPU)
• IRQ 14: Primary PATA/IDE controller
• IRQ 15: Secondary PATA/IDE controller