The $$pushdown store,$$ or $$stack$,$ is a storage structure that operates much like a set of plates stacked ona spring in a cafeteria. All storage locations are initially empty. An item of data is added to the top of thestack by a $$push$$ instruction, which pushes any previously stored items farther down in the stack. Onlythe topmost item on the stack is accessible at any moment, and it is fetched and removed from the stackby a $$pop$$ instruction.