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 Low-Level Mouse Functions
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     TSDWIN Low-Level mouse functions all have the prefix:

                _twM....

     Each is a call to DOS Interrupt 33h. The characters following the
     'M' indicate the number of the Interrupt  33h  function called
     plus the register returned, if any. For example:

        _twM3B() calls Interrupt 33h Function 3 and
                 returns the value in register BX.

     If there is a parameter list, the arguments indicate the registers
     to be loaded on executing the interrupt call. For example:

        _twM4( nC, nD ) calls Interrupt 33h Function 4,
                        passing nC to register CX and
                                nD  to register DX then
                        returning nothing.

        _twM5C( nB ) calls Interrupt 33h Function 5,
                     passing nB to register BX, the
                     returning the value in register CX.

     The virtual screen used by the mouse is NOT the same size as the
     display screen. It is, by default, eight times wider and longer
     than the display screen. All TSDWIN Low-Level mouse functions and
     some high level functions return values based on the virtual
     screen. It may be necessary to turn them into screen coordinates
     with:
                nScreenX := INT( nMouseX / 8 )
                nScreenY := INT( nMouseY / 8 )

     For both the screen and the mouse, the coordinate origin is the
     top, left corner of the display area. Therefore, if you are using
     row, column coordinates, please remember that:

                Y denotes the ROW.
                X denotes the COLUMN.

     It is easy to become confused.

     Please see any DOS Programmers' Reference for more information.

     While TSDWIN does not support all Interrupt 33h function calls it
     uses function calls which are supported by Microsoft Mouse Driver
     version 7, except for four which are found in Version 8 and call
     function 49. TSDWIN mouse support also works  with equivalent
     Logitech drivers.

     All TSDWIN functions which include mouse support assume the mouse
     cursor is off on entry. You may be assured that in most cases, you
     will not have to worry about the trail in the display area left by
     overwriting the mouse cursor if you use TSDWIN functions
     exclusively. If you design your own functions, you will have to
     take the mouse into account.

     ALWAYS turn off the mouse cursor before writing to the screen,
     then turn it on again after.

     If you are using the software for both mouse/keyboard and keyboard
     only environments, be sure to check that you can call a mouse
     function by checking for the presence of a mouse with twOKMouse()
     before calling the function. twOKMouse() is true if you have
     initiated mouse support with twMouseInit() at the start of your
     application, and there is a mouse present.