Get Input status (INT 21h, 4406h)

        This function allow you to check if a handle is ready for input.

        Entry:
          AX = 4406h
          BX = Handle
        Retry:
          CF = 1
               AX = Error code
          CF = 0
               AX = Status

        This status is coded this way:
          If handle is a file:
            00h = At End of file
            FFh = Not at End of file
          If handle is a device:
            00h = Not ready
            FFh = Ready