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Function 4403h (68-3)    IOCTL: Write to Character Device

    Writes a control string to a character device.

       On entry:      AH         44h
                      AL         03h
                      BX         Device handle
                      CX         Number of bytes to write
                      DS:DX      Data or buffer

       Returns:       AX         Error code, if CF is set

       Error codes:   1          Device doesn't support control strings

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    This subfunction writes CX bytes of data to the character device's
    control channel from either DS:DX or from memory pointed to by DS:DX.
    How data is written is defined by the device driver.  Data can be
    written only if the device can process control strings.