Function prologue and epilogue

In assembly language programming, the function prologue is a few lines of code at the beginning of a function, which prepare the stack and registers for use within the function. Similarly, the function epilogue appears at the end of the function, and restores the stack and registers to the state they were in before the function was called. The prologue and epilogue are not a part of the assembly language itself; they represent a convention used by assembly language programmers, and compilers of many higher-level languages. They are fairly rigid, having the same form in each function. Function prologue and epilogue also sometimes contain code for buffer overflow protection. A function prologue typically does the following actions if the architecture has a base pointer (also known as frame pointer) and a stack pointer: Pushes current base pointer onto the stack, so it can be restored later. Value of base pointer is set to the address of stack pointer (which is pointed to the top of the stack) so that the base pointer will point to the top of the stack. Moves the stack pointer further by decreasing or increasing its value, depending on whether the stack grows down or up. On x86, the stack pointer is decreased to make room for the function's local variables. Several possible prologues can be written, resulting in slightly different stack configuration. These differences are acceptable, as long as the programmer or compiler uses the stack in the correct way inside the function. As an example, here is a typical x86 assembly language function prologue as produced by the GCC push ebp mov ebp, esp sub esp, N The N immediate value is the number of bytes reserved on the stack for local use. The same result may be achieved by using the enter instruction: enter N, 0 More complex prologues can be obtained using different values (other than 0) for the second operand of the enter instruction. These prologues push several base/frame pointers to allow for nested functions, as required by languages such as Pascal.