Int 21 Fn 06  - DOS 1+ - Direct Console Input                              [D]

   AH = 06h
   DL = FFh

Return: ZF set if no character available
       AL = 00h
   ZF clear if character available
       AL = character read

Notes:  Break are NOT checked
   if the returned character is 00h, the user pressed a key with an
     extended keycode, which will be returned by the next call of this
     function
   this function reads from standard input, which is always the keyboard
     under DOS 1.x, but may be redirected under DOS 2+
   although the return of AL=00h when no characters are available is not
     documented, some programs rely on this behavior

See Also: AH=0Bh