Switching from ACPI mode to Standard PC Mode (Win2000/XP)
Written by: pcAudioLabs.com

Problems with single CPU systems and Windows 2000 and XP
If not indicated otherwise, this text refers to Windows 2000 and Windows XP.

NOTE: The following is just one solution to fix problems assciociated with Digital Audio playback and recording on a PC. This solution is for a specific problem which causes all of your devices to share the same IRQ. Please make sure that this is your problem before you attempt this solution. If you are performing a fresh installation of Windows 2000/XP, we recommend a Standard PC installation as discussed in the following paragraphs.

The main problem is the automatically chosen ACPI mode (*1) during installation. Windows 2000 will use only one IRQ (9) for all PCI devices. Normally this should be no problem, but in the case of IRQ sharing, the computer suffers from bad performance. The computer won't crash, and everything works, but not as well as it should. Two examples: When using a Sound card, USB or MIDI device, operation will result in 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 2 ways how to do it:

1. If you are installing Windows 2000/XP for the first time:

Check the Motherboard BIOS: the entry Plug and Play OS in your motherboards BIOS should be set to 'NO'.

During installation of Windows 2000:

The mode Standard-PC can be selected in the beginning of a Windows 2000 installation. Hit F5 when you are asked to press F6 to install SCSI RAID device drivers (blue screen). After 5-10 seconds you will be given the option to set up your computer as a Standard PC.

2. If Windows 2000 is already installed:

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. After the successful reorganization your device manager should look like this:



Disclaimer: The assignment to Standard PC bears risks and is not recommended by Microsoft. Eventually you will have to reinstall Windows and re-select the Standard PC mode during the installation. Further information you can find in the Microsoft knowledgebase (support.microsoft.com).

The ACPI problem is not as severe with Windows XP, as Microsoft has introduced a number of beneficial changes. However, it might still occur that ACPI mode has to be deactivated manually with some system configurations. The procedure here is in principle the same as with Windows 2000.

Please go through the following steps to further optimize your system (for Windows 2000 and XP):

For an optimized performance the system performance should be changed to 'optimized for background tasks':
To use ASIO at lowest latencies under Windows 2000/XP single CPU systems, the 'system performance' has to be optimized for background tasks. Go to Control Panel > System > Advanced > Performance Options. Change the default 'Applications' to 'Background tasks'. The lowest usable latency will drop from 23 ms to around 3 ms.


(*1) ACPI = Automatic Configuration Power Interface

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