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INT 21 - DOS 2+ - IOCTL - WRITE CHARACTER DEVICE CONTROL STRING
        AX = 4403h
        BX = device handle
        CX = number of bytes to write
        DS:DX -> buffer
Return: CF set on error
            AX = error code (see AH=59h)
        CF clear if successful
            AX = number of bytes written
SeeAlso: AX=4402h,AX=4405h,INT 2F/AX=122Bh