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The stack is, at best, a difficult concept to understand. However, understanding the stack is essential to reverse engineering code.
The stack register, esp, is basically a register that points to an arbitrary location in memory called "the stack". The stack is just a really big section of memory where temporary data can be stored and retrieved. When a function is called, some stack space is allocated to the function, and when a function returns the stack should be in the same state it started in. The stack always grows downwards, towards lower values.
The stack stores function parameters, local variables, and the return address of every function.
When a function is called, its parameters are typically stored on the stack before making the call. Here is an example of a function call in C:
func(1, 2, 3);
And here is the equivalent call in assembly:
push 3 push 2 push 1 call func add esp, 0Ch
The parameters are put on the stack, then the function is called. The function has to know it's getting 3 parameters, which is why function parameters have to be declared in C.
After the function returns, the stack pointer is still 12 bytes ahead of where it started. In order to restore the stack to where it used to be, 12 (0x0c) has to be added to the stack pointer. The three pushes, of 4 bytes each, mean that a total of 12 was subtracted from the stack.
call and ret Revisited
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