There are a number of functions in ADVC which are intended to be used
with a mouse.  In specific, they rely on a Microsoft-compatible mouse.
Most of the functions are self-explanatory, but the mouse cursor location
needs to be explained further.

     The mouse cursor location is automatically updated by the mouse
driver.  You don't need to write any control routines for that at all.  In
order to provide a generalized interface, though, the location is returned
in a somewhat unusual manner.  It may be helpful to go into that in a bit
more depth.

     The mouse cursor location is returned as a range of values equivalent
to the dimensions of a CGA screen.  That is, the columns range from 0-639,
and the rows from 0-199.  This holds true whether the display is in text
or one of the color modes.  To find where the cursor actually is, then,
requires conversion factors.

     If you're in hi-res color mode (640x200), there is no conversion
factor.  The values directly correspond to the screen location.

     If you're in lo-res color mode (320x200), you need to apply a
conversion factor of two to the columnar location.  That is, if you want
to position to the middle of the screen, you must set the mouse to column
160 * 2, or 320.  The row location is still accurate, however.

     If you're in text mode (80x25), you need to apply conversion factors
of eight to both the column and row locations, and add an offset of one.
For example, if the mouse locations were returned as column 80, row 24,
you would translate this to meaning text column 11, which is 80 / 8 + 1,
and text row 4, which is 24 / 8 + 1.  In effect, a text mode character is
equivalent to an 8x8 block of graphics pixels.  The offset is added
because the graphics screen has its origin at (0,0), whereas the text
screen has its offset at (1,1) in the usual notation.