creating a BECK button

Hello!

I'm trying to create a BACK button that leads to the last played frame. In order to do so, I tried to use a variable that equals to the current frame number, and another variable that equals the number of the last frame played.

The problem is that the variable is not being update by "_currentframe", unless there is a KEYFRAME.
How can I make the variable update without keyframes on every frame of the movie?

I created a movie clip with AS:

onClipEvent (enterFrame) {
_root.frameNum = _currentframe;
}

I would highly appreciate your assistance.