Why does the stack address grow towards decreasing memory addresses?

Question

I read in text books that the stack grows by decreasing memory address; that is, from higher address to lower address. It may be a bad question, but I didn't get the concept right. Can you explain?

Answer 1

First, it's platform dependent. In some architectures, stack is allocated from the bottom of the address space and grows upwards.

Assuming an architecture like x86 that stack grown downwards from the top of address space, the idea is pretty simple:

===============     Highest Address (e.g. 0xFFFF)
|             |
|    STACK    |
|             |
|-------------|  <- Stack Pointer   (e.g. 0xEEEE)
|             |
.     ...     .
|             |
|-------------|  <- Heap Pointer    (e.g. 0x2222)
|             |
|    HEAP     |
|             |
===============     Lowest Address  (e.g. 0x0000)

To grow stack, you'd decrease the stack pointer:

===============     Highest Address (e.g. 0xFFFF)
|             |
|    STACK    |
|             |
|.............|  <- Old Stack Pointer (e.g. 0xEEEE)
|             |
| Newly       |
| allocated   |
|-------------|  <- New Stack Pointer (e.g. 0xAAAA)
.     ...     .
|             |
|-------------|  <- Heap Pointer      (e.g. 0x2222)
|             |
|    HEAP     |
|             |
===============     Lowest Address    (e.g. 0x0000)

As you can see, to grow stack, we have decreased the stack pointer from 0xEEEE to 0xAAAA, whereas to grow heap, you have to increase the heap pointer.

Obviously, this is a simplification of memory layout. The actual executable, data section, ... is also loaded in memory. Besides, threads have their own stack space.

You may ask, why should stack grow downwards. Well, as I said before, some architectures do the reverse, making heap grow downwards and stack grow upwards. It makes sense to put stack and heap on opposite sides as it prevents overlap and allows both areas to grow freely as long as you have enough address space available.

Another valid question could be: Isn't the program supposed to decrease/increase the stack pointer itself? How can an architecture impose one over the other to the programmer? Why it's not so program dependent as it's architecture dependent? While you can pretty much fight the architecture and somehow get away your stack in the opposite direction, some instructions, notably call and ret that modify the stack pointer directly are going to assume another direction, making a mess.