The stack may not grow up or down.
Each stack frame can potentially be allocated at random points inside the heap.
This is actually done in several OS to try and prevent stack smashing by malicious code.
The concept of a stack growing towards the heap is just an easy way to teach the concept of a stack (and of course early implementations did work this way as it was simple (no need to make something harder than you need when nobody is trying to break you)).
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…