问题
i used stack in assembly but i didn't got idea about push ebp and pop ebp.
.intel_syntax noprefix
.include "console.i"
.text
askl: .asciz "Enter length: "
askb: .asciz "Enter breadth: "
ans: .asciz "Perimeter = "
_entry:
push ebp # establishing stack-frame
mov ebp, esp
sub esp, 12
Prompt askl
GetInt [ebp-4] # length
Prompt askb
GetInt [ebp-8] # breadth
mov eax, [ebp-4] # eax = l
add eax, [ebp-8] # eax = l + b
add eax, eax # eax = 2 * (l + b)
mov [ebp-12], eax
Prompt ans
PutInt [ebp-12]
PutEoL
mov esp, ebp
pop ebp # unwinding stack-frame
ret
.global _entry
.end
回答1:
Maybe you're wondering about this:
push ebp
mov ebp, esp
sub esp, 12
These lines are known as the assembly function prologue. The first 2 instructions save the previous base pointer (ebp) and set EBP to point at that position on the stack (right below the return address). This sets up EBP as a frame pointer.
The sub esp,12
line is saving space for local variables in the function. That space can be addressed with addressing modes like [ebp - 4]
. Any push/pop of function args, or the call
instruction itself pushing a return address, or stack frames for functions we call, will happen below this reserved space, at the current ESP.
At the end you have:
mov esp, ebp ; restore ESP
pop ebp ; restore caller's EBP
ret ; pop the return address into EIP
This is the inverse the prologue does (i.e. the epilogue), so the previous context can be restored. This is sometimes called "tearing down" the stack frame.
(EBP is non-volatile aka call-preserved in all standard x86 calling conventions: if you modify it, you have to restore your caller's value.)
The leave instruction does exactly what these two instructions do, and is used by some compilers to save code size. (enter 0,0
is very slow and never used (https://agner.org/optimize/); leave
is about as efficient as mov + pop.)
Note that using EBP as a frame pointer is optional, and compilers don't do it for most functions in optimized code. Instead they save separate metadata to allow stack unwinding / backtrace.
回答2:
ebp
is known as the base pointer or the frame pointer. On entry to your function, you push it (to save the value for the calling function). Then, you copy esp
, the stack pointer, into ebp
, so that ebp
now points to your function's stack frame. At the end of your function, you then pop ebp
so that the calling function's value is restored.
For some clarification on exactly what is going on - the push
instruction puts the value from the specified register (ebp
in this case), onto the stack, and decrements the stack pointer by the appropriate amount. The pop
operation is the opposite - it increments the stack pointer and takes a value from the stack and puts it in the specified register.
来源:https://stackoverflow.com/questions/3638075/explanation-about-push-ebp-and-pop-ebp-instruction-in-assembly