Microsoft KB Archive/811366

= Cannot Select MPS Uniprocessor or ACPI Uniprocessor HAL =

Article ID: 811366

Article Last Modified on 12/3/2007

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APPLIES TO


 * Microsoft Windows Server 2003, Standard Edition (32-bit x86)
 * Microsoft Windows Server 2003, 64-Bit Datacenter Edition
 * Microsoft Windows Server 2003, Enterprise x64 Edition
 * Microsoft Windows Server 2003, Datacenter Edition (32-bit x86)
 * Microsoft Windows Server 2003, Enterprise Edition (32-bit x86)
 * Microsoft Windows Server 2003, Web Edition
 * Microsoft Windows XP Professional
 * Microsoft Windows XP Home Edition
 * Microsoft Windows Small Business Server 2003 Premium Edition
 * Microsoft Windows Small Business Server 2003 Standard Edition

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SYMPTOMS
If you remove a processor from a multiprocessor computer or turn off hyperthreading support in the BIOS, and then try to change your Hardware Abstraction Layer (HAL), you only have the following options:

If the MPS Multiprocessor HAL is currently installed you can change to the following options:
 * Standard PC
 * MPS Multiprocessor

If the ACPI Multiprocessor HAL is currently installed you can change to the following options:
 * Advanced Configuration and Power Interface (ACPI) PC
 * ACPI Multiprocessor
 * MPS Multiprocessor

You do not have the option to switch to MPS Uniprocessor or ACPI Uniprocessor.



CAUSE
This behavior occurs because Windows Server 2003 and Windows XP only permit you to change to reliable HAL types.



WORKAROUND
To switch to a uniprocessor HAL, reinstall the operating system.



MORE INFORMATION
In Windows 2000, you can change to any listed HAL type. However, if you select an incorrect HAL, the computer may not start correctly. Therefore, only compatible HALs are listed in Windows Server 2003 and Windows XP. If you run a multiprocessor HAL with only a single processor installed, the computer typically works as expected, and there is little or no affect on performance.

Additional query words: hyper-threading

Keywords: kbprb KB811366

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