interrupt request
(IRQ) The name of an input found on many processors which
causes the processor to suspend normal instruction execution
temporarily and to start executing an interrupt handler
routine. Such an input may be either "level sensitive" -
the interrupt condition will persist as long as the input is
active or "edge triggered" - an interrupt is signalled by a
low-to-high or high-to-low transition on the input. Some
processors have several interrupt request inputs allowing
different priority interrupts.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)
IRQ
(Interrupt ReQuest) A hardware interrupt on a PC. There are 16 IRQ lines used to signal the CPU that a peripheral event has started or terminated. Except for PCI devices, two devices cannot use the same line. If a new expansion card is preset to the IRQ used by an existing board, one of them must be changed. This was an enormous headache in earlier machines.Starting with the Intel 286 CPU in 1982, two 8259A controller chips were cascaded together and bumped the IRQs from 8 to 16. However, IRQ 2 is lost because it is used to connect to the second chip. IRQ 9 may be available for general use as most VGA cards do not require an IRQ.
PCI to the Rescue
The PCI bus solved the limited IRQ problem, as it allowed IRQs to be shared. For example, if there were only one IRQ left after ISA devices were assigned their required IRQs, all PCI devices could share it. See Plug and Play and Win Technical details.
IRQ Assignment
0 System timer
1 PS/2 port
2 Connects to IRQ 9
3 COM2, COM4
4 COM1, COM3
5 Sound
6 Floppy disk
7 LPT1
8 Real-time clock
9 VGA, 3270 emulation**
10 **
11 **
12 PS/2 port
13 Math coprocessor
14 IDE primary
15 IDE secondary
** For general use.
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| IRQ Settings |
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| This shows typical IRQ settings in a PC. By the time Windows 98 came out, from which this dialog box was captured, technicians no longer had to deal with IRQs on a regular basis. |
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