something I'm working on.

var page1,page2,active_page;
var tim=0;

function draw()
{

}

function timer_proc()
{

}

function timer()
{
    tim++;
}

function main()
{
    allegro_init();
    install_keyboard();
    install_timer();
    install_mouse();
    install_sound();
    set_gfx_mode("example", 640, 480);
    page1 = create_bitmap(SCREEN_W, SCREEN_H);
    page2 = create_bitmap(SCREEN_W, SCREEN_H);

    install_int_ex(timer, BPS_TO_TIMER(60));
    active_page = page2;

    while(!key[KEY_ESC])
    {
      clear_to_color(active_page, makecol(255, 255, 255));

      while (tim >0)
      {
          timer_proc();
          tim--;
    }

      draw();

        blit(active_page,screen,0,0,0,0,SCREEN_W,SCREEN_H);

      if (active_page == page1)
            active_page = page2;
      else
            active_page = page1;

   }

    return 0;
}
END_OF_MAIN();