How to change from ACPI Multiprocessor HAL back to ACPI Uniprocessor HAL

it is quite easy to add one or more vCPU to your Virtual machine in vSphere, you sometimes needs to do the opposite. To change the configuration of a vCPU from two to one for example. Because finally you figured out that you don’t need a second vCPU present in your VM.

If you’re using Windows 2003 or legacy as an Operating system in your VM, the “downgrade” to one CPU does not change the ACPI Multiprocessor HAL back to Uniprocesor automatically…. You must do it manually.

NOTE: it is always good practice to start with only 1 vCPU when creating virtual machines and only increase the number of vCPUs if it’s necessary and if the VM has an application that can benefit from multiple processors. Otherwise it’s just wasting of resources.

What do you need to change from ACPI Multiprocessor HAL to Uniprocessor?

You must run Windows 2003 with SP2.


  • Shutdown the VM -> Go to “VM Settings” and Change the Number of Virtual processors to “1”


  • Power on the VM. after power on login to the machine.
  • Right Click on the “My Computer” -> Go to “Properties”


  • In System Properties ->”Hardware” Tab -> Click “Device Manager”


  • Right-click “ACPI Multiprocessor PC” and select “Update Driver“.


  • In “Hardware Wizard”. Select “No, not this time” -> Click “Next”


  • In Next Screen choose “Install from a list or specific location” -> Click “Next”


  • In Next Screen ChooseDon’t search. I will choose the driver to install” -> Click “Next”


  • Select “ACPI Uniprocessor PC.”


  • Reboot the virtual machine. After reboot verify your processor should change from “Multiprocessor” to “Uniprocessor”

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