Problems with single CPU systems and Windows 2000/XP
On a dual CPU system Windows 2000 uses a special method to handle interrupts (higher than 15, APIC), but on single CPU systems all devices will have the same IRQ (9). While this seems to be no big problem on laptops, most desktop computers show significant performance problems. The timing critical access on the audio hardware is no longer guaranteed.
This one-interrupt-for-all is caused by the automatically chosen ACPI mode during installation. Normally this should be no problem, but here the IRQ sharing suffers from bad performance. The computer won’t crash, and everything works, but not as good as it should. Two examples: When using a Hammerfall USB/MIDI operation will cause audio stuttering even at highest latency. Data transfers via a network card in the background will disturb audio playback significantly.
The remedy is to change from ACPI to Standard-PC mode. Here’s how to do it:
* Check BIOS:
The entry Plug and Play OS in your mobo’s BIOS should be set to ‘NO’.
* Before installation:
The mode Standard-PC can be selected already prior to a W2k installation. Hit F6 when you are asked for updated SCSI etc. device drivers (blue screen). Then press F5 and choose Standard-PC.
* After installation:
Attention: users notified us that this method might lead to problems. Doing a clean install is also recommended by Microsoft!
Go to Device Manager (Control Panel/System/Hardware), click on ‘Computer’, then double click on ‘ACPI-PC’. Go on with ‘Driver’ and ‘Update Driver’. Select ‘Display a list of the known drivers for this device’, then ‘Show all hardware of this device class’. Now you can select ‘Standard-PC’ in the list shown in the right window.
Windows 2000 will re-start and re-install all hardware. After this the PC will use the BIOS’ IRQ assignments.
Under Windows XP Microsoft has significantly improved both interrupt handling and ACPI. In most cases the interrupts will not be sharing IRQ 9. Even when, problems like with the above mentioned simultaneous network transmission are no longer found. Furthermore XP operates more reliable and stable using the ACPI mode. Therefore changing to Standard-PC mode under Windows XP is not recommended.
The worst thing to do is to use Standard-PC mode with the latest generation of single CPU motherboards, having an advanced programmable interrupt controller (APIC). These boards offer 24 interrupts under Windows XP in ACPI mode (else found on dual CPU boards only) – but only 15 when using Standard-PC mode!
Note 1: The mode Standard-PC brings no advantage on dual CPU systems!
Note 2: For an optimized performance the system performance should be changed to ‘optimized for background tasks’ (see below).
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