procedure TForm1.Button1Click(Sender: TObject);
var mode:TDeviceMode;
a:integer;
begin
mode.dmSize := sizeof (MODE);
mode.dmPelsWidth := 800;
mode.dmPelsHeight := 600;
mode.dmBitsPerPel := 16;
mode.dmFields := DM_PELSWIDTH or DM_PELSHEIGHT or DM_BITSPERPEL;
a := ChangeDisplaySettings(mode, CDS_TEST);
if a=DISP_CHANGE_SUCCESSFUL then ChangeDisplaySettings(mode, 0)
end;