Stack frame boundary

Question

I've disassembled some C/C++ codes and realized the stack boundary is specified at beginning of a procedure, somehow like this :

1. push ebp
2. mov ebp , esp
3. sub esp , <power> ; <power> is specified by mpreferred-stack-boundary=2^power

The above code is used to create stack frames, But What I need to know is why this subtract is used, Cause the stack grows down (by using push for local variables) and such the subtract causes :

---------------------------- ^
by sub esp,<power> <---- esp |
.                            |
.                            |
.                            |
---------------------------- |
esp <---- esp                |
---------------------------- |
ret address                  |
---------------------------- |

The above picture shows that.So when in the rest of code if you have such the following codes:

push var1

It should be taken at the top of it so it looks like this :

-----------------------------
var1 (4 byte)                
-----------------------------^
by sub esp,<power> <---- esp |
.                            |
.                            |
.                            |
---------------------------- |
esp <---- esp                |
---------------------------- |
ret address                  |
---------------------------- |

So the space between esp and var1 is going to be free without any use ? that's what I want to know.

Answer 1

So the space between esp and var1 is going to be free without any use ? that's what I want to know.

That stack space is used for the function's local variables (also known as "stack variables").

Here's a nice animated GIF that shows a function's stack frame being created and used to store local variables: