MICINLAR SYSTEMS

What is "I/O APIC" ??

Need more IRQs in Windows XP ?

If you have an MSI 845 Ultra or other mainboards which support I/O APIC, "Advanced Programmable Interrupt Controller" (not to be confused with ACPI), you have the hardware to support at least 8 more IRQs, over the standard 16 (up to IRQ 23), with Windows XP.

XP will automatically use the additional interrupts ONLY if you have installed Windows XP with APIC enabled, otherwise XP will use the standard "PIC" (Programmable Interrupt Controller) with its 16 Interrupts.

NOTE: If you install XP with APIC disabled and then use the BIOS setup to enable it, XP will no longer run. In addition, if you have installed XP with APIC enabled, and then disable it, XP will again fail to run.

Many still hold that I/O APIC is only supported on Multiple CPU (Processor) Hardware. Actually, APIC is supported by Multiple CPU OPERATING SYSTEMS (such as Windows XP), which can control EITHER multiple or Single CPU's and CPU Hardware. Mainboard Hardware, such as MSI's Single processor 845 Ultra AR / ARU Mainboard, have both Hardware and BIOS support for APIC.

APIC is not supported by Windows 9X or ME operating systems. Enabling or Disabling APIC on a Windows 9X/ME operating system will have no effect, as in order to maintain DOS compatibility, those operating systems must use the 16 Interrupt "PIC" .

I am not advocating a switch if your system is operating well, since most systems with ACPI (now were NOT talking "APIC") will satisfactorily allocate Interrupts using Interrupt sharing (PCI IRQ Steering). In fact, I advocate just the opposite - if its working well - leave it alone.

Although, according to MS, IRQ Steering is by no means as efficient as I/O APIC, it's not necessarily bad. However, for those who have a relatively large amount of hardware and are plagued with IRQ conflicts, or for those advanced users who are just plain curious, APIC may be worth a try. Just be sure to back up your system / data first and be ready to contact either Microsoft or Intel (not me) if things go Awry.

Below is an example of the automatic IRQ allocation on an MSI Ultra 845 ARU mainboard running Windows XP, installed with APIC enabled. Both COM2 and MIDI Ports are disabled in the Bios. Both APIC and ACPI are enabled. All 6 USB ports are enabled. Legacy USB is disabled.

At this point, I do not know how the algorithm works which assigns the IRQs, nor can I explain why IRQs 10 & 11 appear unused, or why IRQ 21 has been assigned to three devices. That could be a challenge for some of you techies.

The cards are installed as follows: Video-AGP Slot, PCI 1-Unused, PCI 2-NIC, PCI 3-Video Capture, PCI 4-Modem, PCI 5-Unused.

Here are the automatic IRQ assignments: Keybd-01, COM1-04, Video Capture-05, Floppy-06, LPT1-07, ACPI Compliant System-09, Unused-10 & 11, PS2 Mouse-12, IDE Prim & Sec-14 & 15, GeForce 2 MX/400-16, NIC-17, Unused-18, PCI Modem-19, USB1 (1.1)-19, USB2 (1.1)-23, NEC USB1 (2.0)-20, NEC USB2 (2.0)-21, PCI On Board Audio-21, MSI USB Enhanced Controller-21, Raid Controller-22.

Be aware that once automatically assigned, most all of the IRQs are locked in and cannot be reassigned by simply using the Device Manager. Although it may rarely be necessary, it’s a relatively difficult process to attempt reassignment.

I've tried to provide some practical info on APIC in use, but much of the background was gleaned from the Microsoft Article on I/O APIC. For those interested in a more comprehensive discussion, here's the Microsoft link. [wlw/02272002/T5]

http://www.microsoft.com/HWDEV/PLATFORM/proc/IO-APICP.asp

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