Stack Pointer

The stack is a portion of consecutive memory in the data space which is used by the CPU to store return addresses and possibly register contents during subroutine and interrupt service routine calls. It is accessed with the commands PUSH (put something on the stack) and POP (remove something from the stack). To store the current fill level of the stack, the CPU contains a special register called the stack pointer (SP), which points to the top of the stack. Stacks typically grow “down”, that is, from the higher memory addresses to the lower addresses. So the SP generally starts at the end of the data memory and is decremented with every push and incremented with every pop. The reason for placing the stack pointer at the end of the data memory is that your variables are generally at the start of the data memory, so by putting the stack at the end of the memory it takes longest for the two to collide.

Unfortunately, there are two ways to interpret the memory location to which the SP points: It can either be seen as the first free address, so a PUSH should store data there and then decrement the stack pointer as depicted in Figure 2.2¹ (the Atmel AVR controllers use the SP that way), or it can be seen as the last used address, so a PUSH first decrements the SP and then stores the data at the new address (this interpretation is adopted for example in Motorola’s HCS12). Since the SP must be initialized by the programmer, you must look up how your controller handles the stack and either initialize the SP to the last address in memory (if a push stores first and decrements afterwards) or to the last address + 1 (if the push decrements first).

As we have mentioned, the controller uses the stack during subroutine calls and interrupts, that is, whenever the normal program flow is interrupted and should resume later on. Since the return address is a pre-requisite for resuming program execution after the point of interruption, every controller pushes at least the return address onto the stack. Some controllers even save register contents on the stack to ensure that they do not get overwritten by the interrupting code. This is mainly done by controllers which only have a small set of dedicated registers.