Keep.PAS
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  { Example for Keep }

  {$M $800,0,0 }   { 2K stack, no heap }
  { This program causes a click each time
   a key is pressed.}
  uses Crt, Dos;
  var
    KbdIntVec : Procedure;
  {$F+}
  procedure Keyclick; interrupt;
  begin
    if Port[$60] < $80 then
      { Only click when key is pressed }
      begin
      Sound(5000);
      Delay(1);
      Nosound;
      end;
    inline ($9C); { PUSHF -- Push flags }
    { Call old ISR using saved vector }
    KbdIntVec;
  end;
  {$F-}
  begin
    { Insert ISR into keyboard chain }
    GetIntVec($9,@@KbdIntVec);
    SetIntVec($9,Addr(Keyclick));
    Keep(0); { Terminate, stay resident }
  end.