Scroll Movieclip - stop at certain point

programming flash
1

Hi. I figured out the code below to make a movie clip scroll up and down. However, the problem is that it will keep scrolling forever. Does anyone know how I can get it to stop at a minimum and maximum vertical height?


var scrollUp = function() {
    test_mc.onEnterFrame = function() {
        this._y -= 5;
    }
};
var scrollDown = function() {
    test_mc.onEnterFrame = function() {
        this._y += 5;
    }
};
scrollup_mc.onRollOver = function() {
    scrollUp();
};
scrollup_mc.onRollOut = function() {
    delete test_mc.onEnterFrame;
};
scrolldown_mc.onRollOver = function() {
    scrollDown();
};
scrolldown_mc.onRollOut = function() {
    delete test_mc.onEnterFrame;
};