cdecl | Together C & C++ | Page 1

glossy crow Jan 23, 2024, 7:33 PM

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What do these operations do ? Dont they cancel each other

mov DWORD PTR [ebp-4], eax
mov eax, DWORD PTR [ebp-4]

obsidian vaporBOT Jan 23, 2024, 7:33 PM

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thorny prism Jan 23, 2024, 7:39 PM

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they don't cancel each other out

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its just not necessarily important

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they second instruction is irrelevant

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well so its the first one but technically it does something

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you copy a value from one place to another. Therefore both locations contain the same value
Therefore copying in the reverse is pointless since we already know both values contain the same value

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this is unoptimized code

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the first store is storing to sum

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the load is loading from sum

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into eax, which is the return register

glossy crow Jan 23, 2024, 7:41 PM

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i guess its a trick operation

thorny prism Jan 23, 2024, 7:42 PM

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but the value was in eax originally so this is pointless

glossy crow Jan 23, 2024, 7:42 PM

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the next question ask the state of the stack before leave instruction

thorny prism Jan 23, 2024, 7:42 PM

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glossy crow i guess its a trick operation

no this is just what unoptimized code looks like
it does silly things like this because it doesn't know

glossy crow Jan 23, 2024, 7:42 PM

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i still dont understand how it comes to this

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thorny prism Jan 23, 2024, 7:42 PM

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what are you confused by?

glossy crow Jan 23, 2024, 7:42 PM

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ebp+8 is where the x is

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but dont we push y to the stack first

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by cdecl

thorny prism Jan 23, 2024, 7:43 PM

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no?

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usually they go left to right in order

glossy crow Jan 23, 2024, 7:44 PM

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in the CDECL calling convention the following holds:

Arguments are passed on the stack in Right-to-Left order, and return values are passed in eax.

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okay lets say its saved like that

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when we do

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add eax,edx than the block where y is should have the value 12 right?

thorny prism Jan 23, 2024, 7:47 PM

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hmm

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okay maybe they are right to left

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AH

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wait yes I misphrased

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right to left and left to right depends on your perspective

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they are pushed in right to left order

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which means the most right will be deepest on the stack

glossy crow Jan 23, 2024, 7:49 PM

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you mean on the highest adress?

thorny prism Jan 23, 2024, 7:50 PM

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it will be the highest address yes because the stack grows downwards

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hence why y is at the top

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personally I think of that as left-to-right because of how Ive implemented it before, hence why I may have confused you

glossy crow Jan 23, 2024, 7:50 PM

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mov edx, DWORD PTR [ebp+8] so this saves x

thorny prism Jan 23, 2024, 7:50 PM

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yes

glossy crow Jan 23, 2024, 7:51 PM

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why is return address right before saved frame pointer(ebp?) is it the norm?

thorny prism Jan 23, 2024, 7:51 PM

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how could it not be?

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the call instruction is what places that on the stack

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before you even get to the current stack frame

glossy crow Jan 23, 2024, 7:51 PM

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call instruction

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oh okay

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so its always hhere

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what did mov ebp,esp do

thorny prism Jan 23, 2024, 7:52 PM

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call pushes the return address onto the stack
ret pops it off the stack

glossy crow Jan 23, 2024, 7:52 PM

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they point to different spaces

thorny prism Jan 23, 2024, 7:53 PM

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glossy crow what did mov ebp,esp do

ebp and esp are the stack and base pointers

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ebp points to the bottom of the stack

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esp points to the top

glossy crow Jan 23, 2024, 7:53 PM

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oh

thorny prism Jan 23, 2024, 7:53 PM

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(which is actually below ebp but nevermind)

glossy crow Jan 23, 2024, 7:53 PM

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the bottom?

thorny prism Jan 23, 2024, 7:54 PM

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in a single stack frame btw, not necessarily the entire stack

glossy crow Jan 23, 2024, 7:54 PM

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oh the more you learn

thorny prism Jan 23, 2024, 7:54 PM

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well

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when you enter a function you know 1 thing about the stack

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esp points to the top of it

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(which is now the bottom of your current stack frame)

glossy crow Jan 23, 2024, 7:55 PM

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yep

thorny prism Jan 23, 2024, 7:55 PM

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so you copy that to ebp so now ebp contains the bottom of your stack frame

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and then you modify esp to grow your stack frame

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(see the sub instruction)

glossy crow Jan 23, 2024, 7:56 PM

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so the boxes are called a stack frame i always wondered what a frame is

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im a hopeless case

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thanks a lotttt

thorny prism Jan 23, 2024, 7:56 PM

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then at the end you want to restore the stack frame when you return (because you don't want the stack frame messed up every time you call a function)
so you move the value in ebp back into esp moving all the values back to their original locations as if the call never happened

thorny prism Jan 23, 2024, 7:57 PM

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glossy crow so the boxes are called a stack frame i always wondered what a frame is

stack frame is just a conceptual thing. The space of the stack a single function is currently using

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but assembly stack management is entirely based on stack frames in modern programming

thorny prism Jan 23, 2024, 7:58 PM

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thorny prism then at the end you want to restore the stack frame when you return (because you...

btw this what the leave instruction does

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there is also an enter instruction

glossy crow Jan 23, 2024, 7:58 PM

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so many words..

thorny prism Jan 23, 2024, 7:58 PM

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which does some of the stuff you see at the top, but in 1 instruction

glossy crow Jan 23, 2024, 7:58 PM

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thank you ill read all that tomorrow morning i cant take this anymore

thorny prism Jan 23, 2024, 7:59 PM

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https://en.wikipedia.org/wiki/X86_calling_conventions#cdecl see this for more information

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wikipedia has a nice annotated assembly example

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helps explain what each instruction is doing

obsidian vaporBOT Jan 23, 2024, 8:04 PM

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@glossy crow Has your question been resolved? If so, type !solved :)