IRQ's

I have noticed that XP has 32 IRQ's. How did this happen? What did I miss? I do know about IRQ steering, but when did they add the additional 16 interrupts? Is it because XP is a 64-bit operating system?

 

XP is still a 32-bit OS, though a 64-bit version is available.

The higher IRQ numbers (I've seen up to 128) proceed from IRQ-sharing rather than a change in bus architecture. Sharing results in "virtual IRQ's ".

Though ACPI and IRQ-sharing can both exist alone, the use of ACPI allows for system-wide power management by the OS. This results as a side-effect in improved performance of shared IRQ's by reducing the likelihood of 2 devices requiring interrupts at the same time.

System Information shows your shared IRQ's in the format of the 16 absolute IRQ numbers.

 

Thank you

Thank you very much for your prompt and great reply. I had no idea these were virtual IRQ's. I have just ordered an XP book by Peter Norton.

 

Great. If the book explains this all in more detail, perhaps you can come back and share the ideas with us?

 

OK, will let you know!!

 

Well Abraxas answered my most confusing question...I was also wondering where the extra IRQs came from but...in the resource summary report print out it still lists the additional IRQs. For example the modem is listed as being on IRQ 18. From your post I assumed it would give the real IRQ instead of the virtual IRQ. Did I misunderstand that part of your answer? And also...IRQ 2 is not listed as being assigned. In fact there is no Programable Interrupt Controller listed at all. Does that mean there isn't one or that it just isn't allowing me to "see" IRQ 2 because I can't have access to it?

Thanks in advance!

 

You'll only see the IRQ's that are assigned to something. If nothing is using IRQ2, then it's not visible. If the modem is a PCI winmodem, then it IS on IRQ 18, or at least that's where XP has it, no matter what the hardware says. It's because of ACPI and APIC that this works so well. Without those, you'd be limited to the nominal 16, and would have to manually turn off the computer, instead of it shutting down automatically.
FYI, XP can have up to 255 IRQ's, 16 (0 through 15) normal ones, and 239 virtual ones (17 through 255).