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This page displays the COHERENT manpage for IRQ [Interrupts on the IBM PC].
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IRQ -- Technical Information Interrupts on the IBM PC The term IRQ stands for ``interrupt request''. The IBM PC has 16 interrupts channels built into it. Some are reserved for system hardware; most are available for cards and peripheral devices. The following gives the default assignments for IRQs: IRQ Device 0 System timer 1 Keyboard controller 2 Second IRQ controller 3 Serial port (COM) 2 4 COM1 5 Line printer (LPT) 2 or LPT3 6 Floppy-disk controller 7 LPT1 8 Real-time clock 9 Re-directed IRQ2 10 Available 11 Available 12 Motherboard mouse port (available if no mouse) 13 Mathematics coprocessor 14 Hard-disk (AT) controller 15 Available As you can see, there are two banks of interrupt controllers, each of which controls eight interrupts, with IRQ2 latched to the first port on the second chip, IRQ9. Channel 5 handles two parallel ports -- LPT2 and LPT3. If you install three serial ports onto your system, be careful on how you jumper the card, or you will confuse your system. Due to the design of the PC, IRQ 7 can display spurious interrupts when a device signals an IRQ line other than 7, then cancels the signal before the interrupt controller figures out which line the IRQ occurred on. Thus, you should not assign other devices to IRQ 7, if at all possible. Three interrupt channels are available for user hardware: channels 10, 11, and 15. Channel 12 is also available if you do not have a bus mouse. Only two interrupts are available for serial ports, COM1 and COM2. Note that COM3 and COM4 are ``linked'' to COM1 and COM2, respectively. For this reason, if you have both COM1 and COM3 your system, or both COM2 and COM4, only one of the pair can be interrupt driven; the other port of the pair must be polled. See Also Administering COHERENT