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Function ChangeResolution(sizex,sizey,bpp: DWORD):Boolean;
var
  DeviceMode:TDeviceModeA;
  i:Integer;
Begin
  i:=0;
  Result:=False;
  while EnumDisplaySettings(nil,i,DeviceMode) do begin
    with DeviceMode do
      if (dmPelsWidth=sizex) and 
         (dmPelsHeight=SizeY) and 
         (dmBitsPerPel=bpp) then begin
        case ChangeDisplaySettings(DeviceMode, CDS_TEST) of 
          DISP_CHANGE_SUCCESSFUL: 
            Result:=True;
          DISP_CHANGE_RESTART:
            Showmessage('You have to restart'); 
          DISP_CHANGE_BADFLAGS:
            Showmessage('Bad display mode flags');
          DISP_CHANGE_FAILED:
            Showmessage('Failed in changing resolution');
          DISP_CHANGE_BADMODE:
            Showmessage('Display mode not supported');
          DISP_CHANGE_NOTUPDATED:
            Showmessage('Registry could not be updated');

        else Result:=true;
        end;

        if Result then 
          ChangeDisplaySettings(DeviceMode, CDS_FULLSCREEN)
      end;
    Inc(i);
  end;
End;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ChangeResolution(1024,786,16);
end;