If you had an Xservers file already in place, the SunForum software installation will have saved it as /usr/openwin/lib/X11/xdm/Xservers.nogfx.
By default, the installation will have added the following line, which assumes that the SunForum is the only frame buffer on which to start xdm:
:0 Local Local /usr/openwin/lib/xdm/StartOW \ :0 -dev /dev/fbs/gfxp0
You can add any other desired command line arguments to the end of this line. For example, you can start xdm on multiple displays.
The following example configuration displays xdm on the display named /dev/fbs/gfxp0 and uses the device named /dev/fbs/m640 (the built-in graphics device on the Sun Ultra 5 and Sun Ultra 10 systems) as a secondary frame buffer:
:0 Local Local_uid@console root /usr/openwin/bin/Xsun \ :0 -dev /dev/fbs/gfxp0 -dev /dev/fbs/m640