A stack is a linear array or list of locations. It is sometimes referred to as a pushdown list or last-in-first-out queue. A stack is a reserved block of locations. Items are appended to the top of the stack so that, at any given time, the block is partially filled. Associated with the stack is a pointer whose value is the address of the top of the stack. The stack pointer is maintained in a register. Thus, references to stack locations in memory are in fact register indirect addresses.
The stack mode of addressing is a form of implied addressing. The machine instructions need not include a memory reference but implicitly operate on the top of the stack.