Pointers

Pointers are very easy. Remember, pointers are simply the address that a value resides at. Let's start by taking a look at global variables. For example:

 int global_data = 30; 

In assembly language, this would be:

  .section .data global_data:  .long 30 

Taking the address of this data in C:

  a = &global_data; 

Taking the address of this data in assembly language:

  movl $global_data, %eax 

You see, with assembly language, you are almost always accessing memory through pointers. That's what direct addressing is. To get the pointer itself, you just have to go with immediate mode addressing.

Local variables are a little more difficult, but not much. Here is how you take the address of a local variable in C:

 void foo() {  int a;  int *b;  a = 30;  b = &a;  *b = 44; } 

The same code in assembly language:

 foo:  #Standard opening  pushl %ebp  movl  %esp, %ebp  #Reserve two words of memory  subl  $8, $esp  .equ A_VAR, -4  .equ B_VAR, -8  #a = 30  movl $30, A_VAR(%ebp)  #b = &a  movl $A_VAR, B_VAR(%ebp)  addl %ebp, B_VAR(%ebp)  #*b = 30  movl B_VAR(%ebp), %eax  movl $30, (%eax)  #Standard closing  movl %ebp, %esp  popl %ebp  ret 

As you can see, to take the address of a local variable, the address has to be computed the same way the computer computes the addresses in base pointer addressing. There is an easier way - the processor provides the instruction leal, which stands for "load effective address". This lets the computer compute the address, and then load it wherever you want. So, we could just say:

  #b = &a  leal A_VAR(%ebp), %eax  movl %eax, B_VAR(%ebp) 

It's the same number of lines, but a little cleaner. Then, to use this value, you simply have to move it to a general-purpose register and use indirect addressing, as shown in the example above.