IRQ

(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 card 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 Intel 286 CPU in 1982, two 8259A controller chips were cascaded together and bumped the IRQs from 8 to 16. 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 solved the limited IRQ problem, as it allowed IRQs to be shared. For example, if there were only one IRQ left after ISA devices were assigned their required IRQs, all PCI devices could share it. See Plug and Play and 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.