Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
Int 21h Function 0Bh
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  Interrupt:   21h     Function:   0Bh
 
  Title:       Check Input Status
 
  See also:    Get Keyboard Status, Character Input
 
  Description:
 
     Checks whether a character is available from the standard input
     device. Input can be redirected.
 
     Input        Output
 
     AH = 0Bh     AL = 00h (if no character is available)
                     = FFh (if at least one character is
                            available)
                                    -♦-