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function Herz : Double;  {$O-}
var
  TimerHigh, TimerLow: DWORD;
begin
  SetPriorityClass(GetCurrentProcess, REALTIME_PRIORITY_CLASS);
  SetThreadPriority(GetCurrentThread, THREAD_PRIORITY_TIME_CRITICAL);
  asm
    push    10
    call    Sleep;
    dw      310Fh
    mov     TimerLow, eax
    mov     TimerHigh, edx
    push    500
    call    Sleep
    dw      310Fh
    sub     eax, TimerLow
    sbb     edx, TimerHigh
    mov     TimerLow,  eax
    mov     TimerHigh, edx
  end;
  Result := TimerLow / 500000.0;
end;        {$O+}

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(FloatToStr(Herz));
end;