Why do we need a stackpointer register when we already have program counter register?

Question

I'm confused with it because if we already have a pointer then we can easily point to next instruction, we can easily point to get back previous function so then why we need another stack pointer register to get back to previous function or module?

Answer 1

The program counter tells the CPU where it is. The stack is used to (amongst other things) keep a record of where it has been.

If a function is called, that function needs to know where to return to once it has finished. This could indeed simply be an address passed in, but if that function itself calls other functions, or even calls itself recursively, then it needs to ensure that this address is not lost or overwritten - so it must be stored. The natural way of doing that, is to push the address onto a stack, and the pop it back off again when needed.

Of course it also stores the local state for a function, because that too needs to be pushed and popped as the program descends into or returns from functions.

Not all processors necessarily have a stack, but if you do anything remotely complex on one, you inevitably implement one.