Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
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Int 21h Function 04h
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  Interrupt:   21h     Function:   04h
 
  Title:       Auxiliary Output
 
  See also:    Write Character to Communications Port, Auxiliary Input
 
  Description:
 
     Outputs a character to the standard auxiliary device. The default
     is the first serial port (COM1).
 
     Input                          Output
 
     AH = 04h                       None
     DL = 8-bit data for output
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