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 _twM10( nB, nC, nD )
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     There are two types of mouse cursor in text mode, a hardware and a
     software cursor. The hardware cursor is based on the number of
     scan lines displayed in a pixel, much like the normal cursor. If
     you choose the hardware cursor, you should shape it differently
     from the normal text cursor in order to distinguish between the
     two. The software cursor is depends on a screen and cursor mask
     passed to it in nC and nD.

     The software cursor is most common since it may be defined in more
     ways than the hardware cursor. The attribute and character bytes
     at the mouse cursor's location are ANDed with the screen mask and
     XORed with the cursor mask to determine the form it will take. The
     standard default is to reverse the character's attributes:


                Bit         15      8 7      0
                Screen Mask: 01110111 11111111  - 77FFh
                Cursor Mask: 01110111 00000000  - 7700h
                                |        |
                                |        +----- ASCII character
                                +-------------- Attribute byte

     For the attribute byte, bit 15    sets blinking or non-blinking.
                                 12-14 sets background colour.
                                 11    sets high or medium intensity.
     For the character byte,     0 - 7 is th ASCII character code.

     To leave the ASCII character unchanged,

                the screen mask must be xxFF, and
                the cursor mask must be xx00.

       Arguments:

     nB - 'N' 0 for software cursor.
              1 for hardware cursor.

     nC - 'N' Screen mask for software cursor or starting scan line for
              hardware cursor.

     nD - 'N' Cursor mask for software cursor or ending scan line for
              hardware cursor.

       Return:

     Nothing.

       Example:

     _twM10( 0, ( 119 * 256 ) + 255, ( 119 * 256 )) yields  the default
     mouse cursor definition.