PC Interrupts

The following is a brief description of the IRQ's used on IBM compatible computers. On most PC's, without a lot of extra cards installed, IRQ's 5, 10, 11 and 15 are usually available. In addition, IRQ's 2 (or 9), 12 and 13 may be available.

IRQ 12 will be available if there is no bus mouse.

IRQ 15 may not be available if you have an IDE CD-ROM drive.

IRQ 2 (or 9) can be used for some cards. I have personally gotten it to work with some cards, but not others. I'm not quite sure why that is. Try using 2 and if that doesn't work, try 9. But, you will not be able to use both at the same time.

IRQ 13 will be available if there is no math co-processor.

If IRQ 5 or 7 is used for a printer port you may be able to disable the IRQ and use it for something else. For example, if LPT2 is used for digital IO and it has an IRQ assigned to it, you could disable IRQ 5 for that port and use IRQ 5 for COM3 or something else. Even if the port is used for a printer, you may still be able to disable the IRQ and still have the printer work.

In general, if you are already using COM1 and COM2, you will need to assign COM3 and COM4 to different IRQ's than their default since they use the same IRQ's as COM1 and COM2. This is done via the PORTS program in Control Panel in Windows 3.1x.


IRQ    Purpose

0:     Timer
1:     Keyboard
2:     video (cascade to IRQ 9)
3:     COM2 and COM4
4:     COM1 and COM3
5:     usually free (usually second printer port, LPT2)
6:     floppy
7:     LPT1 (not used by many printer drivers)
8:     real-time clock?
9:     cascade from IRQ2
10:    usually free
11:    usually free
12:    usually free (often a bus mouse)
13:    math co-processor (if present)
14:    Hard Drive (IDE channel 1)
15:    usually free (often IDE channel 2, CD-ROM)

Back to Dan's Home Automation Index

Back to Dan's Home Page

dhoehnen@infinet.com