GetMainDevice
GetMainDevice can be used to show which device is listed as the monitor with
the menu bar. GetMainDevice gets this value from the MainDevice global
variable.
Returns: a GDHandle; a handle to the gDevice that carries the menu bar.

Notes: Contains information on the size and depth of the gDevice of the
monitor containing the menu bar. Unless a different screen has been
called out in parameter RAM as the main device, this is the screen with
its video card in the lowest- numbered slot.