IRQs (or interrupt requests) are the means by which hardware components request computing time from the CPU. Today, there are 16 IRQ assignments (0-15), each one representing a different physical (or virtual) piece of hardware. For example, IRQ0 is reserved for the system timer, while IRQ1 is reserved for the keyboard. The lower the number, the more critical the function.
In the past, there were only 8 IRQ assignments (1-7). As the need for more hardware emerged, the need for more IRQs arose. The solution was another 8 IRQs, added by bridging IRQ2 to the new IRQ9.
In Pentium-class (and some 80486-class) computers the IRQs can be handled by the I/O APIC.
IRQs in the PC AT environment
IRQ0 = 8253/8254 interval timer (System Timer)
IRQ1 = keyboard
IRQ2 = reserved for the 8259B (Tied to IRQ 9)
IRQ3 = COM2 and COM4
IRQ4 = COM1 and COM3
IRQ5 = LPT2 or Soundcard
IRQ6 = Floppy drives
IRQ7 = LPT1
IRQ8 = Real-time clock (RTC)
IRQ9 = Bridged to IRQ2
IRQ10 = Not Assigned
IRQ11 = Not Assigned
IRQ12 = PS/2 Mouse
IRQ13 = Math coprocessor
IRQ14 = Primary IDE drives
IRQ15 = Secondary IDE drives
In the past, there were only 8 IRQ assignments (1-7). As the need for more hardware emerged, the need for more IRQs arose. The solution was another 8 IRQs, added by bridging IRQ2 to the new IRQ9.
In Pentium-class (and some 80486-class) computers the IRQs can be handled by the I/O APIC.
IRQs in the PC AT environment
IRQ0 = 8253/8254 interval timer (System Timer)
IRQ1 = keyboard
IRQ2 = reserved for the 8259B (Tied to IRQ 9)
IRQ3 = COM2 and COM4
IRQ4 = COM1 and COM3
IRQ5 = LPT2 or Soundcard
IRQ6 = Floppy drives
IRQ7 = LPT1
IRQ8 = Real-time clock (RTC)
IRQ9 = Bridged to IRQ2
IRQ10 = Not Assigned
IRQ11 = Not Assigned
IRQ12 = PS/2 Mouse
IRQ13 = Math coprocessor
IRQ14 = Primary IDE drives
IRQ15 = Secondary IDE drives
