Installation guide

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Using the Real-Time Clock with Windows Server 2003 and Windows XP guests

Windows uses the both the Real-Time Clock (RTC) and the Time Stamp Counter (TSC). For Windows

guests the Real-Time Clock can be used instead of the TSC for all time sources which resolves guest

timing issues.

To enable the Real-Time Clock for the PMTIMER clock source (the PMTIMER usually uses the TSC)

add the following line to the Windows boot settings. Windows boot settings are stored in the boot.ini

file. Add the following line to the boot.ini file:

/use pmtimer

For more information on Windows boot settings and the pmtimer option, refer to Available switch

options for the Windows XP and the Windows Server 2003 Boot.ini files

Using the Real-Time Clock with Windows Vista, Windows Server 2008 and Windows 7

guests

Windows uses the both the Real-Time Clock (RTC) and the Time Stamp Counter (TSC). For Windows

guests the Real-Time Clock can be used instead of the TSC for all time sources which resolves guest

timing issues.

The boot.ini file is no longer used from Windows Vista and newer. Windows Vista, Windows

Server 2008 and Windows 7 use the Boot Configuration Data Editor (bcdedit.exe) to modify the

Windows boot parameters.

This procedure is only required if the guest is having time keeping issues. Time keeping issues may

not affect guests on all host systems.

1. Open the Windows guest.

2. Open the Accessories menu of the start menu. Right click on the Command Prompt

application, select Run as Administrator.

3. Confirm the security exception, if prompted.

4. Set the boot manager to use the platform clock. This should instruct Windows to use the PM timer

for the primary clock source. The system UUID ({default} in the example below) should be

changed if the system UUID is different than the default boot device.

C:\Windows\system32>bcdedit /set {default} USEPLATFORMCLOCK on

The operation completed successfully

This fix should improve time keeping for Windows Vista, Windows Server 2008 and Windows 7

guests.

http://support.microsoft.com/kb/833721