ActionScript 2.0 :: Setting Bounds For MovieClip On Stage In Flash?

I have a "circle_mc" movie clip and its registratoin is in the center but it moves and works fine but i dont want it to go off the stage half way I want it to stop before it goes off

ActionScript Code:
circle_mc.onRollOver = function() {
this.dx = Math.round(Math.random()*Stage.width);
this.dy = Math.round(Math.random()*Stage.height);
this.onEnterFrame = moveIt;
}; function moveIt() {
this._x += (this.dx-this._x)/10;
this._y += (this.dy-this._y)/10;
if (Math.round(this._x) == this.dx) {
this._x = this.dx;
}}