Interrupt Request : Computer Hardware Buyers’ Glossary

by Roedy Green ©1997-2008 Canadian Mind Products
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IRQ

Interrupt Request. Whenever you install new hardware, improperly assigned IRQs are nearly always the cause of failure.The computer has 16"shoulders" that i/o devices can tap to get the CPU’s attention. For example, every time you hit a key, the keyboard controller taps the CPU on shoulder number 1. This interrupts the CPU from what it was doing, and makes it run a little program to save the keystroke. In general, every device needs its own private shoulder (IRQ), not used by any other device. When you install new hardware, sometimes two devices accidentally get assigned the same IRQ. Under some circumstances IRQ 3 and 4 can be shared by COM2:/COM4: and COM1:/COM3: respectively. Sometimes, especially when you install something like a sound card that needs 3 or more IRQs, there simply are not enough IRQs to go around. You have to remove some device to make room. Cheap 8-bit cards can only use IRQs 0..7. More expensive 16-bit ones can alse use so some of 8..15. Plug & Play devices attempt to assign IRQs automatically. However, 50% of the time it does not work perfectly, and you have to assign them manually, by setting switches and jumpers, by running configuration programs, or by assigning them in the Windows-95 Control Panel / System / Device Manager / Properties / Resources. By clicking on Device Manager / Computer you can see how the various IRQs are currently allotted. Getting these right is tricky and usually requires someone with experience.

IRQ	Device	Use
0	Timer	allows computer to say to iself "interrupt me in 30 milliseconds",a sort of reminder alarm clock.
1	CON:	keyboard
2	cascade	not available. A kludge used hook the two interrupt controllers together.
3	COM2: or COM4:	serial port. Often the modem
4	COM1: or COM3:	serial port. Often a mouse
5	LPT2:	parallel port for the second printer. Printers can work without any IRQ. IRQ5 is often used by sound cards or bus mice.
6	A: B:	floppy disk drives
7	LPT1:	parallel port for the printer. Printers can work without any IRQ.
8	real time clock	time of day
9	free	sometimes used by network Ethernet LAN cards
10	free	often used by sound cards
11	free	often used by SCSI cards
12	PS/2 mouse	Mouse with tiny round connector
13	numeric coprocessor	for the chip that does scientific calculations and trigonometry.
14	C: D:	EIDE hard disk controller for hard disk, CD-ROM.
15	E: F:	Secondary EIDE hard disk controller for extra hard disk, CD-ROM.

IRQ
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