Interrupt Request : Computer Hardware Buyers’ Glossary

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©1997-2017 Roedy Green of Canadian Mind Products

IRQ
IRQ (Interrupt Request). Whenever you install new hardware, improperly assigned IRQs (Interrupt Requests) are nearly always the cause of failure. The computer has 16 shoulders that I/O devices can tap to get the CPU (Central Processing Unit) ’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 IRQ s, 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 also 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 itself 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 (Local Area Network) cards
10 free often used by sound cards
11 free often used by SCSI (Small Computer System Interface) 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 (Extended Integrated Drive Electronics) hard disk controller for hard disk, CD-ROM (Compact Disc — Read Only Memory).
15 E: F: Secondary EIDE hard disk controller for extra hard disk, CD-ROM.
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