Why do we "PUSH EBP" and "MOV EBP, ESP" in the CALLEE in Assembly?

assembly x86 x86-64 cpu-registers
Artur Grigio picture Artur Grigio · Mar 16, 2016 · Viewed 13.8k times · Source

Why do we push ebp as the first action in the Callee of an Assembly function?

I understand that then we use mov edi, [ebp+8] to get the passed in variables, but our esp is already pointing to return address of the Caller function. We can easily access the passed in variables with mov edi, [esp+4] or if we pushed the Callee registers, then mov edi, [esp+16].

So, why have that extra register in the cpu (the ebp) which you later have to manage in functions? i.e.

push ebp
mov ebp, esp

...

mov esp, ebp
pop ebp

Answer

Remy Lebeau picture Remy Lebeau · Mar 16, 2016

It is establishing a new stack frame within the callee, while preserving the stack frame of the caller. A stack frame allows consistent access to passed parameters and local variables using fixed offsets relative to EBP anywhere in the function, while ESP is free to continue being modified as needed while the function is running. ESP is a moving target, so accessing parameters and variables using dynamic offsets relative to ESP can be tricky, if not impossible, depending on how the function uses the stack. Creating a stack frame is generally safer, at the cost of using a few bytes of stack space to preserve the pointer to the caller's stack frame.

Related questions

What are the names of the new X86_64 processors registers?

Where can I find the names of the new registers for assembly on this architecture? I am referring to registers in X86 like EAX, ESP, EBX, etc. But I'd like them in 64bit. I don't think they are the same …

Read More
Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?

In the x86-64 Tour of Intel Manuals, I read Perhaps the most surprising fact is that an instruction such as MOV EAX, EBX automatically zeroes upper 32 bits of RAX register. The Intel documentation (3.4.1.1 General-Purpose Registers in 64-Bit Mode in …

Read More
What does the R stand for in RAX, RBX, RCX, RDX, RSI, RDI, RBP, RSP?

The x86 assembler language has had to change as the x86 processor architecture has changed from 8bit to 16bit to 32bit and now 64bit. I know that in 32bit assembler register names (EAX, EBX, etc.), the E prefix for each …

Read More