As you probably know, the stack is made up of stack frames, and a stack frame is added each time you go 'deeper' by calling another function. A stack frame is r

As you probably know, the stack is made up of stack frames, and a stack frame is added each time you go 'deeper' by calling another function. A stack frame is removed every time you get to the end of a function and return. So, yes, the stack is closely linked to the depth of function calls. However, in modern systems (or basically anything you'll ever see today, including the cheapest microcontrollers), the stack doesn't just keep track of where to return to after each function completes. It also stores all the local variables used by the function. This is vital because it allows the variables to be restored after you return from a 'deeper' function. A convenient side effect is that when the function completes and the frame is popped off the stack, the references to the local references are destroyed and the data can be garbage-collected. In this sense, local variables are directly related to stack frames. If your system doesn't store local variables in the stack frame, it almost certainly doesn't store them anywhere else (stack frames are the natural place for them), and you've only got globals to work with.