Int 21 Fn 04  - DOS 1+ - Write Character To Stdaux                         [D]

   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

See Also: AH=03h,INT 14/AH=01h,INT E0/CL=04h