Assembly and callstack - c

Question

I'm trying to get an understanding of assembly but unfortunately I have problems to understand the following C code in assembly:
void test_function(int a, int b, int c, int d) {
int flag;
char buffer[10]
flag = 31337;
buffer[0] = 'A';
}
int main() {
test_fuction(1,2,3,4);
}
The assembly of main() looks like this:
push ebp
mov ebp, esp
sub esp,0x18
and esp,0xffffffff0
mov eax,0x0
sub esp,eax
mov DWORD PTR [esp+12], 0x4
mov DWORD PTR [esp+12], 0x3
mov DWORD PTR [esp+12], 0x2
mov DWORD PTR [esp+12], 0x1
call <test_function>
The assembly for test_function(...) looks like this:
push ebp
mov ebp, esp
sub esp,0x28
mov DWORD PTR [ebp-12], 0x7a69 ;this is 31337 in hexadecimal
mov BYTE PTR [ebp-40], 0x41 ;this is the 'A' in ASCII
leave
ret
What is hard for me to understand is:
and esp,0xffffffff0
mov eax,0x0
sub esp,eax
Why are we operating an and with 0xffffffff0 on esp?
And why do we move a 0 to eax and sub the content of eax from esp?
Second:
Through sub esp,0x28 we are allocating 40 bytes of RAM. Why 40? The integer and the 10 chars of the array are altogether only 14 bytes, aren't they?
And why are we moving 0x7a69 to the position [ebp-12] and not to [ebp]? By operating mov ebp, esp I set ebp to the current ESP. Now ESP is pointing to the end of the stack. The last value I pushed on the stack was the ebp by operating push ebp. So EBP (= esp) points behind the saved ebp. So why couldn't I move 0x7a69 to [ebp] just directly behind the saved EBP?
And why is the 'A' moved to [ebp-40]?

Answer 1

This seems to be some standard compiler-generated assembler.
and esp,0xffffffff0
mov eax,0x0
sub esp,eax
The and will make esp a multiple of 16, i.e. alligns it on 16 bytes. Because the stack grows downward it is essentially a substraction, not an addition.
The next mov and add reserve space for the local variables. in main there are no local variables, so their total is 0x0. Because test_function has local variables, 0x28 is moved to eax and added to esp. Probably the compiler has also alligned this on some multiple. Lastly, [ebp-40] is the location on the reserved stack space the compiler has assigned to buffer.