Read from block device (INT 21h, 4404h)

        This call allow arbirtary control strings to be received from
        a block device.

        Entry:
          AX = 4404h
          BX = Drive (0=Current, 1=A, 2=B, ...)
          CX = Number of bytes to transfer
          DS:DX = Address of buffer
        Return:
          CF = 1
               AX = Error code
          CF = 0
               AX = Number of chars read

        Note: This call is not valid on character devices
        ("Invalid" will be returned). If the drive is invalid,
        "Access Denied will be returned