When you switch a virtual machine into full screen mode, GSX Server changes the full screen display resolution to better match the resolution set in the guest operating system. On a Linux host, GSX Server uses the VidMode or DGA2 extension from the XFree86 Project or Xfs from Xig to match the host resolution to the one requested by the guest running in the virtual machine.

In a few cases, GSX Server may not find the best resolution.

When GSX Server switches into full screen mode, it can choose only those resolutions that are already configured on your host.

If a virtual machine runs at a resolution that does not match a mode listed in the X server configuration, then for full screen mode GSX Server chooses the closest larger mode (and uses black borders) or else simply does not offer full screen mode at all.

It is possible to have bad modes configured in the XF86Config file on your host. If your host's X server configuration was automatically generated, or if you never tested all modes with your current monitor and video card, it is possible that some enabled modes do not work with your monitor. However, the mode-switching code in GSX Server has no way of knowing this and a virtual machine that tries to use a resolution with a bad mode line can cause your monitor to fail to display correctly.

If this happens, immediately leave full screen mode by pressing Ctrl-Alt, then fix your X server configuration and restart the X server. However, if the only problem is that the image is off center or is not quite the right size on the monitor, you can usually correct it using the controls on your monitor. Note that most modern monitors are capable of storing separate settings for each resolution, so changing the settings for a new mode should not impair the settings for the host resolution.