What function for stack pointer of the register?

A stack pointer is a register pointing to the top of a stack. It supports the fundamental stack manipulations (push and pop) in an efficient manner.

Most micro processor hardware has build-in hardware support for stack pointers, typically both in form of dedicated stack pointer registers and in form of addressing modes which support the creation and maintenance of stacks through general-purpose pointer registers.

In software, many programming languages feature constructs suited for implementation of stack pointers within the high-level language (such as post-increment and pre-decrement operators in C).