(Interrupt ReQuest) A hardware interrupt on a PC. There are 16 IRQ lines used to signal the CPU that a peripheral event has started or terminated. Except for PCI devices, two devices cannot use the same line. If a new expansion board is preset to the IRQ used by an existing board, one of them must be changed. This was an enormous headache in earlier machines.

Starting with the 286 (introduced in 1982), two 8259A controller chips have been cascaded together for a total of 16 IRQs (the first PCs had only one chip and eight IRQs). However, IRQ 2 is lost because it is used to connect to the second chip. IRQ 9 may be available for general use as most VGA cards do not require an IRQ.

PCI to the Rescue

The PCI bus allows IRQs to be shared, which helps to solve the problem of limited IRQs available on a PC. For example, if there were only one IRQ left over after ISA devices were given their required IRQs, all PCI devices could share it. In a PCI-only machine, there cannot be insufficient IRQs, as all can be shared. See Win Technical details.

      IRQ  Assignment
      0    System timer
      1    PS/2 port
      2    Connects to IRQ 9
      3    COM2, COM4
      4    COM1, COM3
      5    Sound
      6    Floppy disk
      7    LPT1
      8    Real-time clock
      9    VGA, 3270 emulation**
     10    **
     11    **
     12    PS/2 port
     13    Math coprocessor
     14    IDE primary
     15    IDE secondary

   ** For general use.