var top = 0;
function scrool(item, direction, step)
{
    step = parseInt(step, 10);
    el = document.getElementById(item);
    height = parseInt(el.offsetHeight) - 100;
    if (direction == 'up'){
        if (top - step < -height)
            return;
        top = top - step;
    }else{
        if (top + step > 0)
            return;
        top = top + step;
    }
    el.style.marginTop = top + 'px';
}