The dheight field of the viewport structure determines how many video
lines will be reserved to show the height of this display segment.  The
size of the actual segment depends on whether you define a non-interlaced
or an interlaced display.  An interlaced ViewPort displays twice as many
lines as does a non-interlaced ViewPort in the same physical height.

For example, a complete view consisting of two viewports might be defined
as follows:

  * viewport 1 is 150 lines, high-resolution mode (uses the top
    three-quarters of the display).

  * viewport 2 is 49 lines of low-resolution mode (uses the bottom
    quarter of the display and allows the space for the required
    blank line between ViewPorts).

Initialize the height directly in DHeight.  Nominal height for a
non-interlaced display is 200 lines for NTSC, 256 for PAL.  Nominal height
for an interlaced display is 400 lines for NTSC, 512 for PAL.

To set your viewport to the maximum supported (displayable) height, use
the following code fragment (this requires Release 2):

    struct DimensionInfo querydims;
    struct Rectangle *oscan;
    struct ViewPort viewport;

    if (GetDisplayInfoData( NULL,(UBYTE *)&querydims,
                            sizeof(struct DimensionInfo),
                            DTAG_DIMS, modeID ))
    {
            /* Use StdOScan instead of MaxOScan to get standard   */
            /* overscan dimensions as set by the user in Overscan */
            /* Preferences                                        */
            oscan = &querydims.MaxOScan;
            viewPort->DHeight = oscan->MaxY - oscan->MinY + 1;
    }