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Upgrading single CPU system to multi CPU system

harrowc asked
Medium Priority
Last Modified: 2012-08-13

I realise that when windows is installed, the installer selects a HAL based on the hardware it detects.

My question is, if I install a dual core CPU or add an additional CPU to a motherboard that only had *one* (non hyperthreaded) CPU, does the HAL automatically change from a uniprocessor hal to a multiprocessor HAL.

If not, what steps are required to get the OS using both CPU's/cores.

Geek in training

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Start up Windows, shut down all unnecessary programs and TSR’s and insert the installation CD.  Run the setup Program, select upgrade and that will upgrade the HAL to run both processors.
From the windows support newsgroups (their answer was crisper than mine was going to be):

You can add a processor to a Windows 2003 server/SBS 2003. Actually when we
setup Win2003, we do have a choice to choose HAL to support single or dual
CPU. However this is not recommended. When you choose to support dual
processor while actually there is only 1 processor in the computer, you may
experience many potential problems. The support method is to let system
automatically choose the HAL when setting up Win2003 or SBS 2003. If you
want to add another CPU, you can just power off the server, plug the other
CPU and boot the system up. However there are two limitations:

1. You should not change the motherboard.

If you plan to upgrade to a dual CPU system, you should make sure your
motherboard supports multiple CPU at the first time (make sure the Dell
server hardware supports dual-CPU before setting up SBS 2003). If your
motherboard does not support multiple CPU and you want to replace the
motherboard when upgrading the system to a dual CPU system, you have to do
a reinstallation (in-place upgrade) or even a clean installation.

2. You should make sure the other CPU you plan to use is the SAME as the
existing one in the computer.

If you find that the other CPU is not recognized by Windows 2003
automatically, you can use the information in the following article to
manually upgrade to Multiprocessor PC:

309283 HAL Options After Windows XP or Windows Server 2003 Setup

More information:

234558 HOW TO: Add Support for Multiple Processors in Windows 2000

Please do not hesitate to let me know if you have any further concerns.

I will look forward to hearing from you.

Best regards,

Crina Li (MSFT)

Microsoft CSS Online Newsgroup Support

Note:  I'm not Crina Li...but they deserve credit for their words.

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I believe juanfermin's answer would not work, I think windows setup would say something like: "you are already running this version of windows, you can't upgrade".

To summarise TheCleaner's (copied) answer:
It will just work if you are using 2003, just add or upgrade the CPU and Windows will be able to use it.
If on 2000, do into device manager and change to the multiprocessor HAL first, then add the CPU.

You got it...thanks for the points.
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