Define text cursor

        Entry:
          AX = 000Ah
          BX = hardware/software text cursor
              0 software
                  CX = screen mask
                  DX = cursor mask
              1 hardware
                  CX = start scan line
                  DX = end scan line

        Note:   when the software cursor is selected, the char/attribute data
                at the current screen position is ANDed with the screen mask
                and then XORed with the cursor mask.