_twClock( mode, row, col, attr )
 ----------------------------------------------------------------------------
     Display a digital clock on the screen in text mode.

                     *** NOTE ***

     This routine uses Interrupt 1C. CARE MUST BE TAKEN TO TURN THE
     CLOCK OFF WHEN LEAVING THE APPLICATION, EITHER ON PURPOSE OR IN AN
     ERROR CONDITION.

       Arguments:

     mode - "N" 0 turns display OFF
                1 turns display ON

     row  - "N" Screen row.

     col  - "N" Screen column.

     attr - "N" DOS colour attribute ( 0 - 255 )

       Return:

     1 if on, 0 if off or -1 if something is wrong.

       Example:

     _twclock( 1, 1, 5, 31 )

     Turns the clock on at row 1, column 5 in white on blue.