To describe a viewport fully, you need to set the following parameters:
height, width, depth and display mode.

In addition to these parameters, you must tell the system the location in
memory from which the data for the viewport display should be retrieved
(by associating with it a bitmap structure) and how to position the final
ViewPort display on the screen. The ViewPort will take on the user's
default Workbench colors unless otherwise instructed with a colormap.  see
the section called "preparing the colormap structure" for more information.