Write to character device (INT 21h, 4403h)

        Entry:
          AX = 4402h
          BX = Handle
          CX = Number of bytes to transfer
          DS:DX = Address of buffer
        Return:
          CF = 1
               AX = Error code
          CF = 0
               AX = Number of chars written

        Note: This call is not valid on block devices ("Invalid Function"
        will be returned).