Microsoft KB Archive/811840

= How to Change the PromoteLimit Value for the Transition from C2 to C3 Power States =

Article ID: 811840

Article Last Modified on 1/29/2007

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APPLIES TO


 * Microsoft Windows XP Home Edition
 * Microsoft Windows XP Professional

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IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

256986 Description of the Microsoft Windows Registry



SUMMARY
This article describes how to increase the time that must elapse before Windows XP promotes the processor from a C2 to a C3 power state. You may have to change this setting if your computer does not work correctly when Windows XP dynamically changes the processor power state to a deeper idle state (from the C2 state to the C3 state). For example, you may experience intermittent errors when you try to access your floppy disk drive. For additional information about this issue, click the following article number to view the article in the Microsoft Knowledge Base:

811839 An I/O Device Error May Occur When You Access the Floppy Disk Drive in Windows XP Service Pack 1



MORE INFORMATION
WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk.

The steps that are listed in this article increase the time that must elapse from 100 milliseconds to 500 milliseconds before Windows XP promotes the processor from a C2 to a C3 power state. Note that the actual value that you must use depends on your computer hardware. Contact your computer manufacturer for information about the appropriate setting for your computer.

Note The PromoteLimit value is measured in microseconds. A microsecond is one millionth of a second (or one thousandth of a millisecond).  Click Start, click Run, type regedit, and then click OK. Locate and then click the following key in the registry:

 In the right pane, click the  binary value. On the Edit menu, click Modify. In the Value box, change the first four bytes of the line that begins with &quot;0030&quot; and the last four bytes of the line that begins with &quot;0078&quot; from &quot;A0 86 01 00&quot; to &quot;20 A1 07 00&quot;.

For example, change

0030 A0 86 01 00 28 3C 00 00

0078 A0 86 01 00 A0 86 01 00

to:

0030 20 A1 07 00 28 3C 00 00

0078 A0 86 01 00 20 A1 07 00

Note These values are stored in the registry in hexadecimal format and in reverse-byte order. Therefore, &quot;A0 86 01 00&quot; is &quot;0x0186a0&quot; or 100,000 microseconds (100 milliseconds); &quot;20 A1 07 00&quot; is &quot;0x07a120&quot; or 500,000 microseconds (500 milliseconds). Repeat steps 2 through 5 for each of the following keys:

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Computer processor power policy settings apply to all power schemes on the computer if the processor supports performance control and C-states. For additional information about processor power policies, visit the following Microsoft Web site:

http://msdn2.microsoft.com/en-us/library/aa373187.aspx

Keywords: kbpowerman kbpolicy kbhowto KB811840

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