The Stack and the PUSH and POP Instructions

What Do They Do?

Think of a pile of trays, each with a number on (this is a stupid example, but some programmers thought it sounded cute, so that's where the names come from). Then you take another tray with a number on it and PUSH it onto the top of the pile. Later on you come back and POP the same tray off the top of the pile.

Confused? OK, the stack is an area of memory. You can PUSH things onto the top of it and POP things off the top of it. The last thing you PUSH on is the first thing that is POPped off. Let's see an example:

» We start off with an empty stack.

» We then PUSH the number $1234 onto it.

» Later on we also PUSH the number $abcd onto it.

» And then the number $0fb3.

» We then decide to POP a number from the stack. As the stack is LIFO (Last In First Out) we get the 'top' value, $0fb3, leaving the stack looking like:

» We then PUSH another value onto the stack, $0003 (note that I am only expressing all these numbers in hexadecimal because I think that way...), getting:

» We then POP a value from the stack. As before, this is the last number that we PUSHed, $0003.

» We then POP from the stack again, geting $abcd.

» We then POP the final number from the stack, $1234. As this was the first number we PUSHed, it is the last value we get when POPping, leaving us with an empty stack.

The PUSH and POP Instructions

The PUSH instruction takes the general form push rr, where rr is a register pair (af, bc, de or hl). The POP instruction takes the general form pop rr. For example:

  push af
  push bc
  pop de
  push hl
  pop hl
  pop af

Of course you are allowed other instructions in between the PUSHes and POPs. The following are not allowed though:

  push a     ; Can't push single register
  pop be     ; be is not a register pair
  push $1234 ; Can't push a number

Note that when you PUSH a register pair onto the stack you can POP it back into a different register pair - you do not have to POP it back into the register pair it came from.

The SP Register

In my pictures above I used a red arrow to point to the top value of the stack. On the calculator there is a register which does the same thing as the arrow, called SP (short for Stack Pointer). You can't do much with it, but it is there.