.TH IRQ "" "" "Technical Information"
.PC "Interrupts on the IBM PC"
.PP
.II interrupts
The term
.I 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:
.sp \n(pDu
.nf
	\fIIRQ	Device\fR
	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\fR
.PP
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.
.PP
Channel 5 handles two parallel ports \(em 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.
.PP
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.
.PP
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.
.PP
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.
.SH "See Also"
.Xr "Administering COHERENT"
