Slack Intro Part 2

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Course
Time
13 hours 15 minutes
Difficulty
Beginner
CEU/CPE
14
Video Transcription
00:01
Additionally, we have the operations like call we can call a function name. So what call does is it pushes the address of the next instruction onto the stack
00:10
and then a cop. Easy, I p to that new function, location or address.
00:16
And then the reverse of that is the return. The return is going to allow us to exit a function so or return from a function.
00:24
So what that does is it pops off the return address from the stack and then moves I p e i p to that new address.
00:30
So we end up always having a call to a function, and then inside the function when it's done, will have a return. And so that's how we can write a program with functions. Now you've done this in class. We have called a lot of different functions. So, for example, call read into call print end culprit, NL.
00:48
All of those we are calling a function, and then we assume that it's going to resume
00:52
from the location that we were at before.
00:58
We also have the ability to allocate space using, add and subtract.
01:03
So, for example, we can say sub e s p X and that'll subtract X bytes from E S P.
01:08
So, typically, when you enter a function, you're going to do a subtract on E S p to allocate some temporary space that we can use for local variables or buffers.
01:18
And then at the end of the function, we can add back E S P so well typically add and subtract the same number of bytes.
01:26
And so this will unallocated that space. And we'll do that right before we leave a function.
01:30
And then if you ever go through and you are reverse engineering some code, you might see that when a function enters, it might do something like Andy S. P. And then you got a bunch of efs and a zero,
01:42
and this is because it is trying to make sure that yes, p is on an even boundary. And this is a security protection that some operating systems and some compilers implement to make it harder to attack these type of systems.
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