Stack Frames and Local Variables

A procedure needs space for its own local variables and a stable way to find its parameters. A stack frame gives each call its own private region of the stack.

The base pointer (EBP in 32-bit, RBP in 64-bit) provides a fixed reference into the frame, while ESP keeps moving as you push and pop.

A procedure begins by saving the old base pointer and setting up a new frame.

Reserve space for locals by subtracting from the stack pointer. The locals then live at negative offsets from EBP.

Reference each local by a fixed offset below the base pointer.

With the classic cdecl layout, parameters sit above the saved EBP and return address, at positive offsets.

Before returning, restore the stack and base pointer so the caller frame is intact.

The LEAVE instruction performs mov esp, ebp then pop ebp in one step, a compact epilogue.

From high to low addresses in a typical frame:

• Parameters [ebp+8], [ebp+12]
• Return address [ebp+4]
• Saved EBP [ebp]
• Locals [ebp-4], [ebp-8]

Each call saves the callers EBP so nested calls can each have a valid frame and the chain can be unwound, which debuggers use to print stack traces.

Optimizing compilers may skip EBP setup to free a register, addressing locals relative to ESP instead. This is faster but harder to debug.

Test your understanding of stack frames.

You learned stack frames:

• Prologue: push ebp, mov ebp, esp
• Reserve locals with sub esp, access at [ebp-N]
• Parameters at [ebp+N]
• Epilogue restores frame; LEAVE is the shortcut