Slack Intro Part 2

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,