When programming realtime interfaces and computergames, you  need a precise timing signal.

The game engine must run at the same rate, no matter what your framerate are.
One way of doing this is to calculate how many moves your game engine must have done since last frame was drawn.

But if you have all the clock cycles in the world (common when doing smaller games), you can create 2 timers: One with high priority that controls all the game movements and one with low priority doing the screen updates.

This example shows a game timer implementation using a thread and messaging.

unit gametimer;
// -----------------------------------------------------------------------------
//
// Very precise timer running in its own thread.
//
// Usage: Add the following line to the public section of your game form:
// procedure MoveGame( var Msg : TMsg ); message WM_MOVEGAME;
//
// MoveGame is the code where you do all the moves for your game.
//
// You could then create a timer that draws your game. This timer must
// have a lower priority than your game timer. Your game will then have the same
// game timer, but a slower framerate if the PC lacks resources.
//
// -----------------------------------------------------------------------------

// -----------------------------------------------------------------------------
   interface
// -----------------------------------------------------------------------------

uses
  Windows, Messages, Classes, MmSystem, Sysutils, forms;

const
  WM_MOVEGAME = WM_USER + 203;
  TIMERVALUE  = 20; { 10 = 100 fps / 20 = 50 fps / 40 = 25 fps / 50 = 20 fps }

Type
  TGameTimer = class( TThread )
    private
      Msg        : TMsg;
      FHeartbeat : UINT;
      fisBeating : boolean;
      FGameBeat  : integer;
      fForm      : TForm;
      fCount     : longword;
      procedure  Execute; override;
    public
      constructor Create( AForm : TForm ); overload;
      constructor Create( AForm : TForm; AFps : integer ); overload;
      destructor  Destroy; override;
      procedure   StartBeat;
      procedure   StopBeat;
      procedure   PauseBeat;
      property    GameBeat : integer read fGameBeat write fGameBeat;
      property    isBeating : boolean read fisBeating;
      property    Count : longword read fCount;
  end;


// -----------------------------------------------------------------------------
  implementation
// -----------------------------------------------------------------------------

var
  gSelf : TGameTimer;

procedure GameUpdate(uTimerID, uMessage: UINT; dwUser, dw1, dw2: DWORD); stdcall;
begin
  gSelf.Resume;
end;

// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
// -----------------------------------------------------------------------------
constructor TGameTimer.Create( AForm : TForm );
begin
  inherited Create( TRUE );
  inherited priority := tpTimeCritical;
  fForm := AForm;
  fGameBeat := TIMERVALUE;
  fCount := 0;
end;

// -----------------------------------------------------------------------------
constructor TGameTimer.Create( AForm : TForm; AFps : integer );
begin
  Create( AForm );
  fGameBeat := round( 1000 / AFps );
end;

// -----------------------------------------------------------------------------
Destructor TGameTimer.Destroy;
begin
  try
    gSelf := nil;
  finally
    inherited Destroy;
  end;
end;

// -----------------------------------------------------------------------------
procedure TGameTimer.Execute;
begin
  while not Terminated do
  begin
    PostMessage( fForm.Handle, WM_MOVEGAME, 0, 0 );
    inc( fCount );
    Suspend;
  end;
end;

// -----------------------------------------------------------------------------
procedure TGameTimer.PauseBeat;
var
  r : integer;
begin
  if FHeartbeat <> 0 then
    begin
      r := TimeKillEvent(FHeartbeat);
      if r <> TIMERR_NOERROR then
        raise Exception.CreateFmt('Cannot stop heartbeat: %d', [r])
      else
        fisBeating := FALSE;
    end;
end;

// -----------------------------------------------------------------------------
procedure TGameTimer.StartBeat;
begin
  gSelf := self;
  FHeartbeat := TimeSetEvent(FGameBeat, FGameBeat, GameUpdate, UINT(Self), TIME_PERIODIC);
  if FHeartbeat = 0 then
    raise Exception.Create( 'Cannot start heartbeat' )
  else
    fisBeating := TRUE;
end;

// -----------------------------------------------------------------------------
procedure TGameTimer.StopBeat;
begin
  PauseBeat;
  FHeartbeat := 0;
  fCount := 0;
  gSelf := nil;
end;

end.


<<-- Here is an example of a usage -->>

  TfrmMain = class(TForm)
  private
    { Private declarations }
    fGameTimer : TGameTimer;
  public
    { Public declarations }
    procedure MoveGame( var Msg : TMsg ); message WM_MOVEGAME;
  end;

procedure TfrmMain.MoveGame( var Msg : TMsg );
begin
  { Do your game movements here }
end;