IRQ Questions (IRQ 16 and Up)

[puma1824]

Why does my OS report IRQs 16 and up? I thought there is only 0-15?

How can I explain IRQ 16 and up showing up in Device Manager?

Any links to articles?

Thanks,
Puma

 

[ScrapSilicon]

the OS is ...? <edit>heh...I have 17 devices showing under WinXPPro..

 

[thorin]

Originally posted by: ScrapSilicon
the OS is ...? <edit>heh...I have 17 devices showing under WinXPPro..

17 Devices != 17 IRQs . Most modern OS allow up to 4 devices to share 1 IRQ.

Thorin

 

[hjo3]

In WinXP, one IRQ is set aside for "steering" to other IRQs. You can see it in the System Information screen.

 

[Jeff7]

I think mine has up to IRQ 22, I think it goes higher. I remember seeing a bit of info on this awhile ago - something about new PC's and WinXP supporting some new specification or something. Perhaps someone else can provide something less sketchy?

 

[redhatlinux]

yep, 0-15 are the only 'real IRQ's' the higher numbered ones are virtual.

 

[puma1824]

Originally posted by: redhatlinux
yep, 0-15 are the only 'real IRQ's' the higher numbered ones are virtual.

Would you have any links documenting this?

Thanks,
Puma

 

[AndyHui]

IRQs higher than 15 are, on modern/recent motherboards, the result of support for the IO-APIC. The IO Asynchronous Programmable Interrupt Controller. These are real IRQs that can only be addressed through the ACPI or MPS HALs for Windows NT, Windows 2000 and Windows XP (of course other non-MS multiprocessor supporting OSes can address the IO-APIC as well). Normally the IO-APIC extends IRQs out to 24 or 48. Multiprocessor motherboards may provide more.

Windows 2000 and Windows XP can provide virtual IRQ support for up to 255 though, which is slightly different to IO-APIC. If the IO-APIC is not in use, all of your devices usually end up on IRQ9 or IRQ11. If the IO-APIC is in use, you will see devices on IRQs higher than 15. Obviously, using the IO-APIC is faster than virtual IRQ support.

 

[ScrapSilicon]

the man !! :Q

You have been FAQ'ed by the AT FAQ MAN

thanks Andy

 

[puma1824]

Originally posted by: AndyHui
IRQs higher than 15 are, on modern/recent motherboards, the result of support for the IO-APIC. The IO Asynchronous Programmable Interrupt Controller. These are real IRQs that can only be addressed through the ACPI or MPS HALs for Windows NT, Windows 2000 and Windows XP (of course other non-MS multiprocessor supporting OSes can address the IO-APIC as well). Normally the IO-APIC extends IRQs out to 24 or 48. Multiprocessor motherboards may provide more.

Windows 2000 and Windows XP can provide virtual IRQ support for up to 255 though, which is slightly different to IO-APIC. If the IO-APIC is not in use, all of your devices usually end up on IRQ9 or IRQ11. If the IO-APIC is in use, you will see devices on IRQs higher than 15. Obviously, using the IO-APIC is faster than virtual IRQ support.

Thanks!

You would have you any links for further info? I need to present this to a class.

Thanks,
Puma