Alt stack

The Alt-Stack is an additional abstracted data structure employed in some stack-based systems such as Script. As a stack has only push and pop operations performed in a Last In First Out (LIFO) order, an alt-stack can enable much more functionalities by allowing elements to be pushed or popped to the alt-stack to allow intermediate recursive operations on the elements.

Alt stack systems are similar to Two Stack Push Down Automata which can be considered equivalent to a Total Turing Machine.