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Windows 2000 IRQ's

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by adigman1999

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I recently had an end user trying to configure a modem and Win2000 configured it on IRQ17.
We all know that there are only 15 IRQ's so how is this possible?
Did Win2000 change something in its configuration that I didn't know about?
Thanks...

5 replies

by techd00d

Well, actually there are 16 IRQ's (numbered 0-15) in the systems that you are referring to ;)

And, this has more to do with the hardware than the particular OS used to drive it.

In the dark, dim days of Personal Computing there were only 8 IRQ's. Someone famous (was it Bill?) once said "Personal computers will never need more than 640K of memory" (but, alas, that's another story altogether) and likewise it was felt that 8 IRQ's would be plenty. To quote:

"The original 8-bit IBM PC andPC/XT used a single interrupt controller IC, which supported eight hardware interrupts (of which two were used on the CPU motherboard and are not brought out to the bus). Other interrupts were reserved for controllers and ports.

The later IBM PC/AT 16-bit ISA bus extended this to another eight interrupts by cascading a second interrupt controller to the first interrupt controller's IRQ 2 input (making IRQ 2 unavailable for use), though the new interrupt 9 was mapped to appear as interrupt 2."

As we start to see this is a question of how many Interrupt Controllers are built in to the system, with each controller able to handle 8 IRQ's.

The situation became even murkier when Intel introduced the APIC chipset (Advanced ProgrammableInterrupt Controller) and the concept of IRQ steering.

Bottom line here is that Interrupt Request Lines are really just electrical circuits on the mainboard, and as we noted earlier can be cascaded together in groups of 8.

Here's some more info if you want to learn more:

http://www.oreilly.com/reference/dictionary/terms/I/Interrupt_Request.htm

http://www.intel.com/design/chipsets/datashts/290566.htm

I'm sure with a little digging around on your favorite search engine you can find lots more ;)

by saihib

You can indeed have an IRQ with an assigned number greater than 15. Check out this Microsoft Knowledge Base article: DPT PCI-SCSI Fails on PPC if IRQ Greater Than 15 (Q154688)
It appears the the modem manufacturer enabled support for IRQs greater than 15 in the firmware and Win2K was just reading it.

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