If you remove processors from your multiprocessor computer, so that only one processor is left, you have a limited choice when you attempt to change your HAL (Hardware Abstraction Layer).
If the MPS Multiprocessor HAL is installed, you can switch to:
If the ACPI Multiprocessor HAL is installed, you can switch to:
Advanced Configuration and Power Interface (ACPI) PC
You cannot switch to the MPS Uniprocessor HAL, or the ACPI Uniprocessor HAL.
The only way to switch to these HALs is to reinstall the operating system.