Null pointer and the address that is null in c++ visual studio and gcc compiler

Null pointer and the address that is null in c++ visual studio and gcc
compiler

I have tried many tutorials and got more confused. So please teach this
noob with simplest answers. If possible then just stick to the
question......i am having information overload :)
first the main function
main(){
int y=0;
display(&y);
}
Now the function
int display(int* x){
//do something depending on different values of x
}
One of the case is following: "if no variable is pointed to, that is, if
the address of the variable is NULL, your function sets the value of the
variable pointed to zero."
Now my understanding in display function i need to do
if (x==NULL)
*x=0;
Now here is where I am getting stuck with...... - if i call the function
with display(NULL); I get following error in Visual "Unhandled exception
at 0x00C84036 in BTP 300 A1.exe: 0xC0000005: Access violation writing
location 0x00000000."
How do i store some value in y from a function if its address is null?
or the question is just wrong and it should have said value stored at the
address, to which pointer variable points, is null i.e y=Null ???
If int* z is a null pointer than what is the value of the address to which
z points to and what is the value that is stored in address that is
pointed???
thanks a lot guys in advance?