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<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE topic PUBLIC "-//OASIS//DTD DITA Topic//EN" "topic.dtd">
<topic xml:lang="en-us" id="changetscmode">
<title>Configuring the Guest Time Stamp Counter (TSC) to Reflect Guest
Execution</title>
<body>
<p>By default, <ph conkeyref="vbox-conkeyref-phrases/product-name"/> keeps all sources of time visible to the guest
synchronized to a single time source, the monotonic host time. This reflects the assumptions of many guest
operating systems, which expect all time sources to reflect "wall clock" time. In special circumstances it may be
useful however to make the time stamp counter (TSC) in the guest reflect the time actually spent executing the
guest. </p>
<p>This special TSC handling mode can be enabled on a per-VM basis, and for best results must be used only in
combination with hardware virtualization. To enable this mode use the following command: </p>
<pre xml:space="preserve">$ VBoxManage setextradata <varname>VM-name</varname> "VBoxInternal/TM/TSCTiedToExecution" 1</pre>
<p>To revert to the default TSC handling mode use: </p>
<pre xml:space="preserve">$ VBoxManage setextradata <varname>VM-name</varname> "VBoxInternal/TM/TSCTiedToExecution"</pre>
<p>Note that if you use the special TSC handling mode with a guest operating system which is very strict about the
consistency of time sources you may get a warning or error message about the timing inconsistency. It may also
cause clocks to become unreliable with some guest operating systems depending on how they use the TSC. </p>
</body>
</topic>
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