An update is available that allows for a potential power saving on a multicore processor that is running an x64-based version of Windows Vista SP2 or of Windows Server 2008 SP2

This article describes an update that enables the operating system to take advantage of the Enhanced C1 state (C1E) for lower power consumption offered by multicore processors. This update applies to the x64-based versions of the following operating systems:

• Windows Vista Service Pack 2 (SP2)
• Windows Server 2008 Service Pack 2 (SP2)

Note To install the update for Windows 7 or for Windows 2008 Server R2, use the hotfix that is provided in the following Microsoft Knowledge Base article:

Update Information

The local Advanced Programmable Interrupt Controller (APIC) timer is programmed to count even when it is not being used. Therefore, the local APIC timer runs in the C1 sleep state continuously at a constant rate regardless of performance state (P-state) transitions. This behavior causes the computer to consume unnecessary power. However, some multicore processors support an inactive state for when the CPU APIC timer is not being used. Therefore, these processors can enter the low-power C1E state. This low-power state saves up to an additional 10 percent in power.

If your processor supports the C1E state, you can apply this update to allow for the potential power savings of the C1E sleep state.

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing the problem described in this article. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.

Prerequisites

To apply this hotfix, the computer must be running an x64-based version of one of the following operating systems:

• Windows Vista Service Pack 2 (SP2)
• Windows Server 2008 Service Pack 2 (SP2)

Restart requirement

You must restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace any other hotfix.

File information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

For all supported x64-based versions of Windows Server 2008 SP2 and of Windows Vista SP2

For more information, click the following article number to view the article in the Microsoft Knowledge Base:

Additional file information for Windows Server 2008 and for Windows Vista

Additional files for all supported x64-based versions of Windows Server 2008 SP2 and of Windows Vista SP2