INT 21 - DOS 1+ - WRITE CHARACTER TO STDAUX
        AH = 04h
        DL = character to write
Notes:  keyboard checked for  Break, and INT 23 executed if detected
        STDAUX is usually the first serial port
        if STDAUX is busy, this function will wait until it becomes free
SeeAlso: INT 14/AH=01h,INT E0/CL=04h