Stacks - why PUSH and POP?

Question

I was wondering why we use the terms "push" and "pop" for adding/removing items from stacks? Is there some physical metaphor that caused those terms to be common?

The only suggestion I have is something like a spring-loaded magazine for a handgun, where rounds are "pushed" into it and can be "popped" out, but that seems a little unlikely.

A second stack trivia question: Why do most CPUs implement the call stack as growing downwards in memory, rather than upwards?

Answer 1

Re your "second trivial question": I've seen considerable inconsistency in defining what "up" and "down" mean! From early days, some manufacturers and authors drew memory diagrams with low addresses at the top of the page (presumably mimicking the order in which a page is read), while others put high addresses at the top of the page (presumably mimicking graph paper coordinates or the floors in a building).

Of course the concept of a stack (and the concept of addressable memory as well) is independent of such visual metaphors. One can implement a stack which "grows" in either direction. In fact, I've often seen the trick below (in bare-metal level implementations) used to share a region of memory between two stacks:

+---+---+--------   -------+--+--+--+
|   |   |   ->   ...   <-  |  |  |  |
+---+---+--------   -------+--+--+--+
^                                   ^
Stack 1      both stacks      Stack 2
base        "grow" toward        base
              the middle

So my answer is that stacks conceptually never grow either "downward" or "upward" but simply grow from their base. An individual stack may be implemented in either direction (or in neither direction, if it's using a linked representation with garbage collection, in which case the elements may be anywhere in nodespace).