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I'm using this Windows XP computer at work right now and I realized it's poor performance may be due to it only using one core, details follows:

  • Processor is a Pentium E5400
  • The multi-core feature is enabled in BIOS
  • Device Manager shows both cores under "Processors"
  • Intel Processor Identification Utility shows both cores
  • CPU-Z only detects one core
  • Task Manager's performance tab only shows one graph ("One graph per CPU" option is enabled).
  • While doing heavy operations CPU goes to 100%, I would expect it to go to 50%
  • Process Explorer (by sysinternals) also shows only one graph.
  • msconfig has no option checked under "BOOT.INI"->"Advanced options" (and if I try to check /NUMPROC I only have the "1" option)

I'm puzzled, it looks like the second core is detected as hardware but it's not being used. Any help?

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  • Have you tried putting in another hdd, installing Windows XP, and verifying what it shows?
    – Ramhound
    Nov 29, 2012 at 15:20
  • That's my company's pc at work, sadly I'm not allowed to play that much with it (actually I'm surprised I'm allowed to install application and access the BIOS). I take from your answer that you suggest something went wrong with the OS installation, right?
    – capitano666
    Nov 29, 2012 at 15:35
  • I'll post a comment rather than an answer, as not sure it will work in your case. See incore.net/winxp-multicpu. If your XP installation was ghosted from a single CPU machine (as can happen in corporate environments) you might need to enable multi-core support.
    – DaveP
    Nov 29, 2012 at 15:49

3 Answers 3

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Short answer -- wrong HAL installed.

Long answer -- in Device Manager under Computer it should say ACPI Multiprocessor PC. If it doesn't then OS was probably installed while multi-core was disabled in the BIOS.

To fix the problem, run the following command:

rundll32 syssetup,SetupInfObjectInstallAction ACPIAPIC_MP_HAL 128 %windir%\inf\hal.inf

You should have all cores working after reboot.

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I have seen this happen with working with VM's and XP. Have you ever replaced the CPU of this computer from a single core CPU to a dual core CPU?

The easiest solution for this when I encountered this with a VM where I added an additional core is doing a "Repair Install" of XP and it will refresh the internals of XP to use all of the available cores.

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Before ACPIAPIC_MP_HAL installs, the corresponding Hardware ID may have to be set in the registry. From an admin cmd use sysinternal's psexec to start regedit with system permissions:

psexec -i -s regedit

In the registry editor navigate to:

HKLM\SYSTEM\CurrentControlSet\Enum\ROOT\ACPI_HAL\0000

and change:

HardwareID=acpiapic_mp

Mind there is a newline after the string, leave it as it is.

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