The stack is one of the most significant things you must know when programming. Think of the stack as a deck of cards. While you put a card on the deck, it shall be the top card. Then you put another card, then another. While you remove the cards, you eliminate them backwards, the last card first and so on. The stack works the similar way; you put (push) words (addresses or register pairs) on the stack and then eliminate (pop) them backwards. That is called LIFO, Last in First Out. The 8085 use a 16-bit register to know where the stack top is situated, and that register is called the SP (Stack Pointer).