Computers that are equipped with multiple processors that support processor power management features, such as Advanced Configuration and Power Interface (ACPI) processor performance states, require Microsoft Windows XP Service Pack 2 (SP2). Additional updates are available to optimize performance and behavior on computers that are running Windows XP SP2. Without these updates, computers that are equipped with these power management-capable, mobile, dual-core processors may experience decreased performance or unexpected behavior.

This problem also applies to x64-based versions of Microsoft Windows Server 2003.


The following file is available for download from the Microsoft Download Center: Download the WindowsXP-KB896256-v4-x86-ENU.exe package now

You must restart the computer after you apply this update.


loadTOCNode(1, ‘moreinformation’);Windows XP SP2 is required on computers that have multiple CPUs that support ACPI processor performance states. This requirement includes computers that support the following items:

  • Multiple physical sockets
  • Multiple-core designs
  • Multiple logical threads, such as Intel hyper-threading technology

Because Windows XP was not originally designed to support performance states on multiprocessor configurations, changes are required to correctly realize this support on multiprocessor systems. Windows XP Service Pack 2 includes the required changes to the kernel power manager. These changes make sure that Windows XP correctly functions on multiprocessor systems with processor performance states.

This hotfix also addresses the following issues on computers that have multiple processors that support processor performance states:

  • A possible decrease in performance on single-threaded workloads when processor performance states are using demand-based switching.
  • The synchronization of the processor Time Stamp Counter (TSC) registers across processors when you use the ACPI Power Management timer on multiprocessor systems.
  • ACPI C-state promotion and demotion issues in the kernel power manager.

Update details

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Possible decrease in performance during demand-based switching

loadTOCNode(3, ‘moreinformation’);Demand-Based Switching (DBS) is the use of ACPI processor performance states (dynamic voltage and frequency scaling) in response to system workloads. Windows XP processor power management implements DBS by using the adaptive processor throttling policy. This policy dynamically and automatically adjusts the processor’s current performance state in response to system CPU use without user intervention.

When single-threaded workloads run on multiprocessor systems that include dual-core configurations, the workloads may migrate across available CPU cores. This behavior is a natural artifact of how Windows schedules work across available CPU resources. However, on systems that have processor performance states that run with the adaptive processor throttling policy, this thread migration may cause the Windows kernel power manager to incorrectly calculate the optimal target performance state for the processor. This behavior occurs because an individual processor core, logical or physical, may appear to be less busy than the whole processor package actually is. On performance benchmarks that use single-threaded workloads, you may see this artifact in decreased performance results or in a high degree of variance between successive runs of identical benchmark tests.

This hotfix includes changes to the kernel power manager to track CPU use across the processor package. These changes enable visibility into the true activity level of a CPU complex and therefore help correctly calculate an increased target performance state.

This solution favors performance gains over power savings. Although benchmark performance scores may improve, battery life could be negatively affected. Accordingly, this kernel policy change may be disabled by a registry key to allow for maximum flexibility.

How to disable the new performance state policy behavior

loadTOCNode(3, ‘moreinformation’);Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.

After you install the hotfix that is described in this article, you may use registry settings to disable the new performance state policy behavior. To do this, follow these steps:

  1. Click Start, click Run, type regedit, and then click OK.
  2. Right-click HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager, point to New, and then click Key.
  3. Type Throttle for the new key name.
  4. Right-click Throttle, point to New, and then click DWORD Value.
  5. Type PerfEnablePackageIdle for the value name.
  6. Right-click PerfEnablePackageIdle, and then click Modify.
  7. In the Value data box, type 0. Make sure that Hexadecimal is selected in the Edit DWORD Value dialog box, and then click OK.
    • You can type 1 in the Value data box to enable the new performance state policy behavior.
  8. Quit Registry Editor.

Correct TSC synchronization

loadTOCNode(3, ‘moreinformation’);On some operating systems, the processor TSC may change the rate at which it counts. Additionally, the processor TSC may stop counting when specific processor power management features are used. On computers that have multiple processors, the TSC is typically the operating system hardware timer that supports calls to the kernel KeQueryPerformanceCounter function. When TSC does not increment monotonically, system components that use the kernel KeQueryPerformanceCounter function may not work correctly. To address this problem, Microsoft makes it possible for the ACPI Power Management Timer to be used as the operating system timer that supports the kernel KeQueryPerformanceCounter function. However, some programs may directly access the TSC by bypassing the Windows timer APIs. The multiple-processor Hardware Abstraction Layer (HAL) makes sure that the TSC registers on all processors on a multiple-processor computer remain closely synchronized. Therefore, access by system software that may be directed to different processors does not return different results. This change makes sure that the multiple-processor HAL continues to correctly synchronize the TSCs across all processors on a computer, even if the ACPI power management timer is used as the operating system hardware timer.