Function prologue

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.

Sometimes, function prologue and epilogue contain also buffer overflow protection code.

Prologue

A function prologue typically does the following actions if the architecture has a base pointer (also known as frame pointer) and a stack pointer (the following actions may not be applicable to those architectures that are missing a base pointer or stack pointer) :

• Pushes the old base pointer onto the stack, such that it can be restored later (by getting the new base pointer value which is set in the next step and is always pointed to this location).
• Assigns the value of stack pointer (which is pointed to the saved base pointer and the top of the old stack frame) into base pointer such that a new stack frame will be created on top of the old stack frame (i.e. the top of the old stack frame will become the base of the new stack frame).
• 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 variables (i.e. 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′s a typical IA-32 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	0, N

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. However, modern versions of these languages don′t use these instructions because they limit the nesting depth in some cases.

Epilogue

Function epilogue reverses the actions of the function prologue and returns control to the calling function. It typically does the following actions (this procedure may differ from one architecture to another):

• Replaces the stack pointer with the current base (or frame) pointer, so the stack pointer is restored to its value before the prologue
• Pops the base pointer off the stack, so it is restored to its value before the prologue
• Returns to the calling function, by popping the previous frame's program counter off the stack and jumping to it

The given epilogue will reverse the effects of either of the above prologues (either the full one, or the one which uses enter).

For example, these three steps may be accomplished in 32-bit x86 assembly language by the following instructions:

	mov	esp, ebp
	pop	ebp
	ret

Like the prologue, the x86 processor contains a built-in instruction which performs part of the epilogue. The following code is equivalent to the above code:

	leave
	ret

The leave instruction performs the mov and pop instructions, as outlined above.

A function may contain multiple epilogues. Every function exit point must either jump to a common epilogue at the end, or contain its own epilogue. Therefore, programmers or compilers often use the combination of leave and ret to exit the function at any point. (For example, a C compiler would substitute a return statement with a leave/ret sequence).

Further reading

• de Boyne Pollard, Jonathan (2010). "The gen on function perilogues". Frequently Given Answers. 
• Brian Tabone (2012). "A First Example Deconstructed". An Example. 

External links

• 80386 Programmer's Reference Manual - ENTER Instruction
• 80386 Programmer's Reference Manual - LEAVE Instruction