Quick but deep look at local variables on the stack in x86-64 assembly (YASM). We cover allocation with sub rsp, accessing via offsets, why the stack must be 16-byte aligned when calling libc functions like printf, and two practical ways to fix alignment crashes. Includes live segfault debugging and a full working example with a local array.

Great for anyone studying systems programming, computer architecture, or just trying to figure out why their assembly program randomly crashes on a library call.

Like + subscribe if you want more clear, practical assembly tutorials!

00:00 Introduction to Local Variables on the Stack
00:28 C++ Example of Function and Local Variables
01:06 Incoming Arguments in RDI vs Stack Variables
03:24 Pointers as Local Variables on Stack
04:34 Why the Stack – Recursion and Multiple Calls
05:18 Visualizing Multiple Stack Frames
09:24 How Function Returns Adjust RSP
10:35 Stack Grows Downward in Memory
11:33 Program Setup – Hybrid C++/Assembly
12:46 Assembly Module Overview
14:00 Function Prologue – Register Push & Alignment
15:30 Allocating Stack Space for Local Array
17:45 Initializing Array in Loop
20:10 Printing Loop with printf
49:48 First Run – Segfault Observed
51:00 16-Byte Stack Alignment Requirement
51:55 Fix 1 – Extra Push/Pop in Prologue/Epilogue
53:15 Fix 2 – Push/Pop Around Each printf Call
55:04 Testing Different Array Sizes
56:52 Debugging Alignment Behavior
58:54 Summary – Creating Any Local Data on Stack
59:59 Closing Remarks & Subscribe Call

=-=-=-=-=-=-=-=-=

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Hi there! Today I’d like to talk to you about local variables on the stack

in an x8664 assembly program written in YASM.

If you don’t understand assembly or local variables or other things like that you might

want to see my other videos but I’m just going to give you a quick example. So what am I talking

about with local variables on the stack? For starters forget what you’re seeing here for a

blank code page and pretend that we’re coding in c plus plus just for a moment this is an assembly

video but pretend this is c plus plus so suppose you have c plus plus you have a main function here

and at the end of it you know this is your entry point for your program we return zero and maybe

above it there’s a function called f i’m putting it above because i don’t want to use prototypes

in this video but you should probably use prototypes um maybe main calls on f and so then

what happens? I don’t know. Maybe we have some arguments. We’ll call this integer argument A

that comes in. If you’ve watched my other videos, hopefully you have by now,

you’ll know that A comes into the function as RDI because that’s the first integer argument

register. But then when we start creating local variables, we’ll say integer B equals, let’s say,

a five and then integer C is equal to an eight for whatever reason. Maybe there will be an array.

So I’ll call this a int array, I guess.

We’ll say that we have 100 integers in our array.

We could also have a pointer, int pointer p,

and then allocate it to some kind of new memory just to prove a point.

And then, you know, later in your function,

you’re probably going to want to do something with your data.

So I don’t know, maybe a, how about like b is plus equal to a,

of C++ and then oh C++ C is plus equal to B for some reason I’m just making random nonsense up

honestly I’m just showing you that we use our variables and then maybe you want to build an

array so we just declared the array up here on line six but maybe you want to actually do something

with it you want to fill it with data maybe so I’ll do size type I is zero keep going until I

is less than as long as I is less than 100 I plus plus and now I’m going through every single index

array maybe I’ll say the array at index i is equal to c and then we’ll just say c is plus equal to b

and then we’ll do b plus plus just to have something in there okay so let me explain the

parts of your program real fast if you haven’t watched my other videos please do because this

should help a little bit anyway so a is an incoming argument like I said before that’s

usually coming to you in the RDI register because it’s the first integer argument so we don’t need

to worry too much about that we know that a is actually just a register in your CPU and b that’s

created on the stack and c is created on the stack also and the array is created on the stack by the

way I could name the array anything if I wanted to like just v I’m using the name array because

Notice also that we have a pointer here. I just wanted to make a point that the pointers we make in our function are considered local variables.

And that means they do sit on the stack because what I’m trying to say is that local variables that are not arguments, they sit on the stack.

But the memory we allocated and then gave to the pointer, that’s sitting in the heap somewhere.

So when you use the new operator or the malloc operator, or you’re just like making dynamic memory,

that dynamic memory sits in the heap, but the pointer itself, since we just declared it here,

that sits on the stack. That means later on when your function ends, the p pointer itself will get

cleaned up automatically and itself won’t be leaked memory. But if you forget to clean up your memory

here, like I did, you’ll have a memory leak. But I’m going to talk about memory leaks probably in

some other videos somewhere else at another time. Anyway, so then we have our local variables here.

remember B and C and array are all on the stack and well we just have our

for loop here where we just kind of start modifying data so why would we use

the stack let me just do a quick tutorial this is not a stack video I’m

gonna make a stack video in the future but I’ll just do like a quick little

rundown of what is the stack and why is it really good for function calls

consider that sometimes F might want to call itself maybe it’s a recursive

be like a long chain of function calls like f calls g and g calls h and h calls you know whatever

and it just goes on and on and on as long as it’s not infinite recursion like all the functions are

calling themselves in one circle that never ends you should be allowed to do that we should be

allowed to have a function f that sometimes calls itself or other functions or maybe it’s called

multiple times within our giant call stack our call graph and this will work because these

you know a b and c and array are sitting on the stack and the stack allows you to have sort of

different instances of variables so for example and this is just a quick thing i’m not this is

not supposed to be a full stack tutorial here but um you know if we have like the function f

and let’s say we call it at some point maybe uh maybe main calls f uh and then let’s let’s pretend

f and then f calls h and then maybe h calls f and um did i write an h there no i think that’s

supposed to be a parenthesis i’m sorry i have bad penmanship oh god it’s even worse hang on let me

let me do another h here i’ll just do it i’ll just do a g about that f g and then uh

F again. Let’s just pretend that we can have a call graph where F is sometimes called by something

and sometimes called something else or maybe sometimes F is called by a different thing. So

like I guess this was supposed to be the G function. So maybe that’s why down here was the H.

Should I delete this video? I don’t know. You know what? I’m going to start. I’m going to delete this

and then G calls H and then H calls F and then F calls I I guess and then

eventually these functions start returning right oops how come my greens

not working green there we go too late now so we can have like a really

complicated call graph and F might appear in there multiple times and the

Each call to F along the call graph should have its own copy, unique copy of local variables.

So the B for this first F call, let me just do an arrow here, will have a B.

And you can imagine it as B subscript 1.

And down here when F executes, you can imagine that it’s got its own copy of B.

So we can imagine this as B subscript 2.

So these are two totally different Bs.

If you tried to use global variables for this, it would be really, really, really hard to get the code to work.

get the code to work and it would be really, really hard to debug.

So that’s kind of why we have local variables.

The stack allows us to do this.

So what is the stack itself?

Again, this is not a full tutorial on the stack,

but I just want you to kind of see what’s happening.

Imagine a data structure that kind of grows upward.

I’ll say that here’s the floor at the bottom.

And when you put an item onto the stack,

the items kind of stack on top of each other.

So imagine that this is the call to F.

is the call to f and embedded within this little stack frame area we’ll call it a stack frame but

actually the stack continues to grow as you create and destroy local variables within a function call

we’ll just imagine that there’s i don’t know a mini stack sitting inside of the stack that

contains all the local variables i’ll just put i’m not going to put a because a is an argument so

it’s sitting in a register but we could do dude i got to learn how to how to draw with this tablet

We got to do B and C and the array.

I’ll put AR for array.

Sitting in, you know, their own little spots on the stack within the major portion of the stack that is designated for that first F call.

And then, you know, maybe F calls G and some other stuff happens.

I’ll just pretend that a bunch of other stuff happened.

And then eventually F is called again.

But there’s a different, this is a different instance of the call to F.

it’s got its own little area that is separate from the the previous call to f and again we’ll

also have variables that we can create locally that are supposed to be separate from the original

variables oh god that’s awful and ugly i need to maybe decrease the size of my eraser or something

but imagine this is uh you know these are two separate copies so that’s like what i said before

when we have like a b1 and a b2 basically they’re not going to be called b1 and b2 but they’re just

They’re not going to be called B1 and B2, but they’re just two separate instances.

And I just want you to know.

So then when we start returning from functions, like when this F eventually returns, and by

the way, I know that what I’m drawing on the right doesn’t match the code because this

is not a function that calls itself, but just suppose that your code is a lot more complicated

than what I drew up.

When it eventually returns, all that happens is those items on the stack just sort of get

not necessarily deallocated, but ignored.

We’ll just say that they’re gone.

They’re still sitting as junk data in system RAM somewhere.

And in assembly talk, we know that we have a stack pointer called RSP.

We have a register called RSP that just sort of points to the location in the

stack that is considered the top of the stack, like the most recent piece of data

that we have available.

So all the other data is actually still kind of above, but we’re not pointing to it

anymore, so we consider that it doesn’t exist.

So then when G eventually returns, you know, we just change the stack pointer,

Rsp, to point to that other piece of data.

The G data frame and the other F data frame are still sitting above somewhere in memory,

but we just ignore them, right?

So that’s how the stack works.

And that’s how we have local function call copies of all of our local variables.

Something to note.

Something to note, this is not a stack video, but you know, just something to note that

even though I draw the stack visually as growing vertically up, when you actually manipulate

the stack in assembly or just like in any language, the stack grows downward in terms

of memory locations.

So you can imagine, I’m trying so hard not to make this like a huge stack video.

Imagine this is a memory location 80, we’ll say.

You would think that memory location 81 would be the next item of the stack, or I guess

the stack or i guess if you if you’re considering the fact that the items on the stack are quad words

we would say it goes up to 88 but that’s not true it goes down to 72 so the memory location goes down

even though we imagine the stack growing uh upward vertically just so you know that’s the kind of

thing we’re going to do so what i’m going to do is just show you an assembly program where we can

create local variables and i’m just going to show you how to create an array because this array is

just like a bunch of integers and you can imagine it would be really easy to create only one integer

by just imagining an example where the array is a size of one so keep that in mind and i’m not going

to show you malloc or anything like that we’re just going to look at the local variables okay

so for starters i have a make file here that’s just going to compile a hybrid program if you don’t

know make files or you don’t know hybrid programs that’s okay just see my other videos i’ve explained

The first source code file here is just driver.cpp.

Again, this is a hybrid program,

so I’m going to mix my C++ modules with my assembly modules,

which is pretty cool.

The whole point of the driver is just to contain the entry point, you know, main.

And I’m just going to print a hello message.

And then I’m going to call the real function that I’m interested in,

which I’ve named local underscore varrs.

And that’s going to be all the assembly stuff that we talked about.

block so that C++ can call an assembly module that’s explained in other videos. And then

here’s the real heart of what we got to do. Let’s write up an assembly module

that can do local variables. Okay. So again, if you don’t know assembly, that’s okay,

but you need to watch my other videos before you can understand this one. So I’m going to just

copy paste some starter code here. This is Yasm assembly in x86-64. So I’ve got a data section up

So I’ve got a data section up top and I’m just going to define some messages.

So, you know, I’ve got like an intro message that just says, hello, I’m so and so.

And that’s not my name, but I like those kinds of names.

And then over here, I’m going to do some printf formatted strings.

That’s why I’m using a hybrid program for this video.

I don’t want to import my own personal library.

I want you to be able to do this at home with just the GCC libraries.

link a hybrid library you know linking against gcc instead of linking against ld again if you don’t

know that stuff check my other videos then we’re allowed to call c functions in this case we’re

going to call printf and we’re just going to give it the string percent lu meaning i would like you

to print just you know an unsigned long integer so i’m going to give it a value at some point on the

stack representing a local variable and then i want it to print as just like a long like a string

along like a string that a human can read then after that this is the carriage return line feed

the crlf printf won’t flush its output unless that is sitting at the very end of the string so

i’m just going to use printf to also print my new lines and then i’m going to null terminate the

string so that printf doesn’t freak out and try to print a bunch of stuff after the crlf and uh

oh i this was from another video let me get rid of that we don’t really need crlf in this video

CRLF in this video because we’re just putting it directly inside of the printf string we’re not

making our own function to do that so then I’m going to make some defines I’m going to define

that we’re going to have 50 integers so I’m calling this define 50 I’m calling it num integers

and I’m saying that it has a value of 50 so I want to make an array that has 50 integers I don’t

know maybe if you want to imagine 100 you know like the example that I just showed I’m going to

going to define what is the integer size so i’m going to use quad words which are 64 bit integers

so i’m just going to say that there are eight bytes per integer that will help me multiply later

and then i’m going to decide to fill up the array on the stack with just some numbers just to prove

that i can just to prove that we can like uh you know manipulate and and fetch the values

of all this stuff going on in the stack and i’m going to say that the starting value is seven so

with this we should expect to see like an array of numbers that starts with seven and it just

kind of increases somehow then i’m going to do some system call codes we talked about that in

a different video and then some file descriptors i don’t think we actually need anything but standard

output but i put it in there anyway then the next thing we’re going to add is the text section

let me just do copy paste on my solution here so here’s the text section section text

section text in Yasm and I’m going to let my module know that I want to be able to call printf

which is a function sitting in the GCC libraries when I link against a GCC I have the ability to

do that that way I don’t have to come up with like a complicated printing method or use one

of my own shared libraries or something so we can just ask printf to do everything so now here’s the

entry point for the module it’s just a function called local VAERS I mark it as global so it’s

it’s accessible to outside modules ie or eg driver dot cpp and then so here’s the label saying the

function starts and here’s the return statement saying that we’re done with the function i’m not

going to manipulate any registers inside of the function so i don’t really need to do any push pop

to preserve them first thing i’m going to do is call a welcome let me comment this part out by

the way i’m going to call a welcome a function and the whole job of the welcome function is just to

is just to, you know, print a welcome message to the user.

So nothing that I haven’t talked about before in other videos.

So it’s just we’re using a system call to print a string.

Okay, so with that in mind, let me open this up here and see if this is going to work.

I just want to basically print the welcome message at this point.

Clear and make run.

And again, if you don’t know make files or anything like that, see my other videos.

So this is the driver, I think, that prints.

think that Prince maybe I should change the driver’s message to make it more

clear hello about this is the driver and my name is whatever I’m gonna do it again

and now it says hello this is the driver okay so that’s the CPP module and then

here is the assembly module and then finally the driver gets control back and

then the program exits so nothing really happened so now let’s upgrade the

So now let’s upgrade the assembly module a little bit.

Next thing I want to add is the actual demo function, which is going to be absolutely huge.

So first I’m going to start off with, how about just the signature here?

Let’s go right after the welcome module.

And I’m just going to copy paste the signature, put a return at the end of it.

So we’ll consider this a function that can be called.

instruction on line 47 of the entry point and then now we have a demo

function that is being called but it does nothing for starters you know I in

my comments I like to put the signature of the function and I like to remind

myself of how I’m using my registers hopefully I’m not using the same

register in two different ways but you know sometimes it happens if it does and

I’m able to break my function up into multiple parts I’ll probably do it

you know with modular thinking modular programming but in this case I’m just

using these registers it’s fine to use them they are all designated per the ABI

as callee saved so that means the callee which is the demo function is

responsible for preserving them so if you don’t remember that or if you don’t

know about that see my other videos this is not an ABI video so I’m just going to

push all of them to make sure that they’re preserved I’m gonna call this

call this the prolog then at the end of my function i’m going to call this the epilog where

i just restore them in reverse order because that’s the way the stack returns data to you

the stack returns data to you in reverse order so i have to pop in the reverse register order to

un-reverse the reversal if that makes sense pop r13 pop r12 okay so i think 15 14 13 okay

so i got that next thing we’ll do is um let’s remember where the stack pointer started because

started because we have our register here that we’re going to mess with let me just type rsp

real fast so this is the stack pointer register rsp this helps all programs know where where

they’re looking at in the stack all of your functions have to be really really careful about

messing with the stack pointer if you do it wrong you will crash the entire program because not only

will your local function not really know where its local variables end and begin it probably also

It’s return address when you try to return from the function because that is also sitting on the stack

And even if you were lucky enough to be able to jump back correctly to whoever called you

If you messed up the stack pointer then you’ve also messed it up for any caller of you and any of their callers

So the whole program is ruined. So we’ll start off by trying to remember where the stack pointer was

We’ll move

The stack pointer into the base pointer

didn’t do here that I that I want to do we should since we’re messing with the

base pointer and other programs sorry other functions or modules might also

rely on the base pointer and it’s considered callie saved we probably also

want to preserve that too so I’m going to do push RBP to basically say I would

like to restore I would like to preserve the base pointer so I don’t mess it up

for my callers so that means I have to restore it with the pop so RBP the base

The base pointer isn’t necessarily a pointer to the stack, but it’s often used as kind of like a bookmark.

So we have RBP at the front and the back there.

Let’s see.

Next thing I want to add is, so now that we’ve restored it, we’re allowed to just overwrite it because, you know, we’re kind of like keeping its value at the top.

Then we’re restoring its value at the bottom.

And that means we can actually mess it up in the middle if we want to, and it’ll be fine.

let me show you real fast what happens what happens is nothing the program is

still okay because we we restored it so now we’re using the base pointer to

remember where the stack was now we’ve got to sort of calculate how much room

we want to make on the stack let me show you what I’m talking about here

remember all of our local variables are going to be on the stack and before we

drew this thing where it was like well we’ve got like a stack sitting here and

let’s just pretend that there’s some kind of data sitting on the stack data

data data right if the stack pointer dude green green there we go oh I erased my

green RSP if the stack pointer is currently pointing to this frame then

in order for us to make room on the stack to hold our array well if the

whole array is going to be sitting on the stack that just basically means

have five integers suppose five suppose that we want to make five integers on the stack that just

means we need to do five extra slots let me draw it in red here well let’s see can we get a green

no how about a yellow my green is just having a hard time we can do it in red even though this is

not a bad thing to do so i’ll just draw like five extra frames on top of on top of the stack here

imagine these are all 64-bit integers and so they take eight bytes on the stack even though in our

previous example we were using just regular ints which are 32 we’ll just say we’re going to make

five 64-bit integers because that’s easier so they’re quad words so every frame is actually

eight bytes and not just four bytes and it’s definitely not one byte so we make five of those

how do we make five slots it’s pretty easy we we literally just move the stack pointer let’s see

we just move the stack pointer to or the you know the rsp register to just say let’s let’s point you

know further out and how do we get that number we’re just going to multiply the size of one

integer so you know the size of one integer here we know it’s going to be eight bytes we just

multiply that by the number of integers that we want you know that’s going to be 40 so we’ll just

increase uh sorry decrease the stack pointers value by 40 because remember again when the

vertically, it’s actually growing downward in memory. So we’re going to decrease by 40 there,

at least in this drawing example. And that gives us a bunch of junk data, you know, because there

is always going to be some kind of a value sitting at every memory location in your computer.

It’s impossible that there is literally nothing at some memory location,

unless you’re trying to go beyond your RAM stick. But then the system will still acknowledge that,

you know, you’ve done that and it won’t just give you back nothing if you try to read

nothing if you try to read.

So there’s going to be junk data sitting on there and then we’ll loop

through all those slots on the stack and we’ll just modify the data one by one

so that we can control what it is instead of just printing whatever junk data we end

up with.

So really, we’re just moving the stack pointer, just making room and then just

remembering where our array is.

We could put, you know, another frame on top if we wanted to make just like one

integer as a local variable.

You just got to remember where it is.

You know, what is it?

What is its offset?

is it what is its offset okay so i’m going to erase this because we’re going to do a lot more than

than five uh integers on the stack but i just want you to understand what we’re doing before we do it

okay so the next thing i’m going to grab is a move instruction and i’m going to put it right here so

to move the stack pointer rsp so the first thing i’m going to do is i’m going to use a temporary

register we don’t need to preserve this in the push and pop because it’s marked as temp so we’re

not responsible for preserving it and so i’m going to say r10 is just going to be the number of

integers that we want to create if you recall at the top of our program here num integers is just

50 okay so then the next thing that i’m going to grab is well i’m not going to grab it i’m going

it I’m going to straight multiply by integer size so again if you look at integer size that’s going

to be eight because we’re using quad words for our integers so we’re really just going to take

50 times 8 whatever number that is is that 400 tell me in the comments if that’s a right or wrong

and so uh you may or may not know if you don’t see more videos see more textbooks you may or may not

instruction just multiplies two numbers.

If you use the three operand format,

then the last two operands get multiplied

and the results stored in the first operand.

But if we use the two operand format like I’ve done here,

then both of those operands get multiplied

and then the result gets stored in the first operand.

So basically at this point,

R10 should hold the number of memory locations

that we should move the stack pointer

in order to make room for all those integers

like I showed you a second ago.

showed you a second ago. So then we’re going to move the stack pointer.

And maybe I’ll leave a little comment here. Remember, the stack grows downward

in memory. And so all I’m doing is subtracting the stack pointer. Remember the stack pointer

register, it just holds a number, which is a memory location. So if you subtract some numbers

from it, you’re really having it go downward in memory. And that’s what we want to do

memory and that’s what we want to do to you know grow the stack for a local variable so I’m going

to say well I should also say that we’re using the two operand version so just like I’m all if we

had the three operand version then the last two operands would have one subtracted from the other

and the result will be stored in the first one but since I’m using the two operand version

basically it’s taking RSP minus R10 and then storing it in RSP so this this instruction just

this instruction just says let’s move the stack pointer downward in memory

enough times that we have room for all of our integers okay no problem next thing that we’re

going to do is um we’re going to move r12 we’re going to move into r12 the current value of rsp

word in memory right less okay so the first integer you know it’s up to you

how you style this because once we do the subtraction then RSP is actually

going to be pointing towards an integer you could consider that to be the first

integer or the last integer because all we have is an array of integers so you

you know rsp wherever it’s sitting when we’re finished we could say that’s pointing to the

first integer or we could say it’s pointing to the last integer but um if we decide to say that it is

pointing to the let’s see yeah if we’re deciding that it points to the first integer let me just do

a little comment here first integer it just makes it a little bit easier for me to write our loop

You could start by pointing to the one that RSP is pointing to,

or you could start by pointing to the other one that was like the first one that you added onto the stack.

You could call either one of those the first integer, as long as you remember where you started.

So you can increase or decrease the memory location to get to the next integer.

So I’m just going to do it in this style.

But keep in mind, as long as the only thing that you modify and read is within that range, it’s okay.

So let’s remember where is RSP.

So that’s like the top of the stack.

We’re going to say R12 holds the stack pointer so that we can use R12 as sort of a running

pointer.

I think that’s the way I’m going to use it.

Let me just double check my solution.

Yeah.

Okay.

a calculation where we remember where the last integer is so we know where the first integer

we’re just going to call this the first integer i’ll put it in quotes just to remind you that

this is just the style i happen to be using so here we’re going to say that the first integer

is wherever rsp is pointing and then in r13 we’re going to remember where the other side of the

array is in this case we’re calling it the last integer and pretty much it’s just r12 which is

And then we add to it a memory location.

So remember we said before that the stack grows downward in memory.

So if we consider the top of the stack to be the first integer,

then that means previous items, maybe I should draw this,

are going to be increased memory locations.

So that’s kind of like the backwards of what you imagine a stack is doing,

but it’s kind of the way that I like it sometimes.

So imagine just in a very simple stack,

let’s pretend that the stack has one byte values,

one byte of values, which it doesn’t, but let’s pretend if you, if you do it,

I got to learn how to draw. I can’t get that five right. Okay. I’m defeated. So let’s pretend

that that address on the thing sitting at the bottom of the stack is five. So then that means

the next address would be four, right? But if we decided, let’s say that the, well, let’s do like

like a few more just to make it a little bit more interesting.

Let’s do a total of five.

So we’ll say four, three, two, one.

Maybe I should just add some numbers in front of those values.

So it doesn’t feel like we’re hitting zero.

But so just pretend we have a memory location of 15.

That’s not going to be the case in real life.

Pretend that we have one byte integers on the stack, which is not going to be the case.

We’re going to have quad words.

But you can see the memory locations go downward, right?

to the stack pointer is pointing to the topmost location we have to remember that anything up here

might exist in system ram but it’s not considered valid data because we didn’t

you know make it part of the stack by by growing the stack pointer so that means uh these

these items well maybe i’ll do a check instead of an x because x looks bad i’ll do a check

check and a check and a check and a check these items are okay to use so if I have

RSP or in this case we just remembered where RSP was by storing r12 if we have

that memory location 11 and we want to get an additional integer like somewhere

else like the next integer well that would be you know this one down here we

wouldn’t go in the other direction we wouldn’t go you know up we would just go

know up we would just go down in the stack but down visually is actually growing upwards in memory

because remember when we grow upwards visually we’re growing downward in memory so and you can

see here too if we’re if we’re increasing 11 to 12 that means we’re adding memory locations to get

to the next uh integer that we have in the stack so that’s why here my green is just like frustrating

fix this yeah so that’s why here we’re adding a little formula instead of subtracting because

the rsp started there we’re saying that the top of the stack is the first integer

just so we can add in a more convenient nice way so what are we adding to it we’re just adding to it

the number of integers minus one and then so that’s that’s the number of slots that

size. So if you imagine, you know, if we had 10 integers, then, you know, 10 minus one is nine

slots. So if you imagine that zero is the first integer, let’s say the memory location again,

then that means one, two, three, four, five, six, seven, eight, nine, 10 minus one is nine. So if we

added the number of integers minus one from the start, which we’ll consider zero here, that means

directly so that’s just like a little math because sometimes when you when you

think of adding two numbers together or taking the difference or including the

first number or not including the first number it’s a little confusing right so

keep in mind for this particular calculation we are adding to it the

number of integers minus one so that will be sitting on the last integer

rather than going past it so start with the first integers memory location and

the number of integers minus one or size minus one times the integer size,

because remember, every integer is going to be eight bytes.

And that will give us the memory location of the last integer in R13.

And then I’ve said this in other videos,

but basically you’re only allowed to make these sorts of calculations in Yasm

when you have the calculation inside of brackets,

but brackets will automatically dereference the value on the inside.

It’ll consider it as a pointer that needs to be dereferenced,

but we don’t want to dereference anything.

anything we don’t want to go to a memory location and take a value the memory location is the value

so these dereferencing brackets which are required for the formula are kind of bad so that’s why we

use the lea instruction instead of the move instruction if i put move there it would

definitely dereference the memory location and give me a value in r13 so r13 wouldn’t actually

be a pointer it would just be the value of the junk data of the last integer keep that in mind

keep that in mind okay so uh where’s RSP so we got that so now let’s do a loop

I’m gonna start off with a label called demo loop init so for me personally when

I’m looping you know I like to make my labels inside of my loops start with a

prefix that matches the function so it’s like demo and then everything else is

gonna be demo underscore something so uh init loop init I’m gonna make a loop

to make a loop where i initialize the values of the array so i’m calling this loop the init loop

and then the last part is just like this is the initialization part of the loop this is where we

sort of like set up the initial values to loop so we have r12 and 13 that point to the first

and last integers now we’re going to set up r14 and 15 where r14 is the running pointer i think

before i might have accidentally said that r13 is the running pointer it just points to the last

it just points to the last integer but if you look back up at my comments r14 is the running pointer

to the current integer what is a running pointer it’s just a pointer that runs it’s just a pointer

that just keeps increasing so we can look at different data values so i’m going to start it

by looking at the first integer so now r14 is pointing to the first integer and then r15 is

going to be the value that i want to put into that position in the array so like the first integer

integer I want to put some kind of a starting value into it you can put the

number zero or whatever you want I just wanted to have a start value so that it

sort of looks more like I’m putting data and less like I have a loop counter so

remember the integer start value up here is just seven so I’m just going to start

at the number seven and now I’m done initializing my loop then the next thing

I’m going to add is the top of my loop so you can imagine this as the top of a

top of a while loop where you start comparing some sort of an expression maybe i’ll say uh

you know expr to say that we’re comparing some sort of an expression

and if that expression evaluates to true the loop continues if it evaluates to false

then the loop does not continue let’s see so i’m going to compare r14 with r13 inside of the

and 13 remember R14 is the running pointer and R13 is the last integer so basically I’m trying to

figure out am I looking at or like let’s compare the running pointer with the pointer of the last

integer then I’m going to say if the running pointer has a greater memory location than than

the last integer that means I’ve gone beyond the last integer and again the way I arranged the first

last integers just makes it easier for me to think of them as having increasing

memory locations so I’m going to jump if the running pointer has already

surpassed the last integer by saying let’s jump if it’s greater than so you

can imagine maybe in the expression here I should probably say while not

r14 is greater than r13 not a great expression but it’ll do so that’s what

up there at the top let me just put that into my notes too so that my notes match

the video okay so we are comparing and then we’re jumping to the end of the

loop if we end up you know going beyond the last integer so that labels not

created yet I’ll create that in a moment but pretty much that’s a label that’s

just going to be below the loop just to say like we’re finished with the loop

the loop body I like to put comments here to help myself remember oh this is

the part of the while loop that I’m currently inside of just makes things a

little bit easier to understand you know you put a block comment up top of every

label or every you know chunk of instructions just to let you know the

general idea and then you know sometimes you put comments also on the right side

to help you remember what each instruction is actually doing so then

what am I going to do here remember R14 is the running pointer if I D ref R14

ref r14 that means i want to move an actual value into that memory location rather than changing the

memory location that r14 points to so r15 is going to be the uh the integer that we want to

write into the array so all i’m doing here is i’m saying let’s take that value seven which is what

it starts as and just move it in to you know the ram stick at that memory location so i’m setting

seven right now then I’m going to increase our 15 so that means every time

we iterate the loop we should see that the value increases so the first integer

should be seven the second one should be eight next one should be nine and so

forth so just a simple loop where I’m just writing data into my array so now

that we’ve ended the loop body let’s write the bottom of the loop which is

just going to increase the running pointer and jump back up to the top and

And this is not necessarily the only style for translating while loops.

I’m just doing it.

And, you know, I’m going to make another video in the future where we talk about, you know, for loops and while loops and all that stuff.

But this video is just really about local variables on the stack.

So I’m not going to go over all the different ways you can do it.

Anyway, so R14 is the running pointer.

So I’m going to just make the running pointer jump to the next integer.

And we can do that by increasing its memory location by the size of one integer.

Again, this is another benefit of the first and last pointers that I chose at the beginning.

I can just increase to go to the next integer.

So we’re going to increase by 8 bytes to just go to the next integer.

If you increase by 1 byte, you’ll probably have a huge corrupted mess because you’re

messing with 8 byte integers but you’re only increasing by 1 byte.

And then after we increase, we’re just going to jump to the top of the loop.

So notice how I have a jump statement here.

It’s going to go just to loop top.

loop top. So now this part is here is basically just going to execute over and over and over again

until we finally scan through all of the integers in our array. So that’s the bottom of the loop.

And then I’ll make the label for the loop being done. It’s not really going to do anything

except just be done. And do you, you know, I don’t know, depending on your style, maybe you can let

drop through if the loop’s done rather than always jumping to the top but i’m just going to say when

we’re done we jump to the loop done label and therefore there’s no more looping of that

initialization loop okay so we got that done let me just run the program real fast to make sure

that i haven’t screwed it up we actually should not see anything right now oh what did i do

must have done something naughty maybe if i finish

uh this program then everything will be okay oh my gosh what did i even do

well i’ve got a working solution in the other window so hopefully when i paste all the extra

steps everything will be fine you never know subtract the stack pointer oh did i forget to

oh did i forget to restore something at the very end print body move the base pointer into the

RSP stack pointer oh yeah okay that’s definitely what you got that’s why I

crashed okay so um remember I said you got to be very careful about the stack

this is a great lesson so I did preserve the base pointer but I didn’t actually

preserve the stack pointer notice how right here I subtracted from the stack

pointer but I did not restore the stack pointer anywhere so that means I

corrupted the stack for anyone that called me and also for my return address

address. So I’m kind of trying to copy paste my instructions from top to bottom, but I think I’m

just going to copy paste something else to make sure that we can actually run this. So I’m going

to copy paste into the epilog a restoration of the stack pointer. And that’s why we saved the

stack pointer in the base pointer, just to remember where it was when we originally started our

function. So now on line 128, it should be restored and the program should work. Let me just double

yeah it worked okay nothing happened that we can see but it did write values

into the array now let’s do another loop where we just print the array so let’s

see loop in it done looping it bottom and looping it done okay so now there’s

like gonna be another loop here we’re gonna call this the print loop and so

the print loop is gonna be kind of the same pattern we’re just gonna loop

going to loop through all the integers in the array but instead of modifying them we’re just

going to print them so now you know the first thing we’ll do is we’ll set r12 we’ll store that

inside of r14 in order to start the loop at the first integer what was r14 again that was the

running pointer remember let’s see where is that yeah it was the running pointer so now we’re

resetting the r14 running pointer to the very beginning of the array and we know where the

thing we’ll do is we will set up the top of the loop and the body so I’m just

going to copy paste again this stuff right here

right there okay so the top of the loop we’re asking ourselves you know we’re

gonna compare I’m not gonna put all the extra while stuff that I put in the

previous loop because hopefully by now you understand loops a little bit better

and if we’re beyond the last integer because the running point of r14 is beyond the memory

location of r13 that means we’re totally done so we should jump if it’s greater than

now i feel bad let’s uh let’s put a comment in here on the top let’s go uh

basically that you know if uh keep going as long as r14 is not greater than r13 so if it is

in the body and in the body all i’m going to do is use r14 the running pointer to print uh you know

whatever value is sitting in that particular integer so how do we do that i’m just going to

use the printf statement or sorry the printf function which is provided by the c libraries

that’s why we’re doing you know a modular or a hybrid program with multiple modules and c linking

very quickly there is a function called printf which I can call it it takes multiple arguments

but the first two arguments that I can give it are the string that represents the formatting

that I want to print like I could do like a regular string message I could do tokens

to format some data inside of them and then the second argument is going to be the piece of data

that I actually want to use let me see if I can just type that up for you real fast

So, you know, the printf instruction, or sorry, this is not an instruction.

This is a function in C.

We would typically, you know, give it some kind of string.

The string should be null terminated, and it should have a new line at the very end of it

to make sure that printf actually flushes.

It won’t flush if you don’t have a new line, so the program will look really weird.

But I guess it’s more performant if you have a way to delay the flushing,

flushing and you know that you can flush it later at the very end but for now i’m just going to

flush every time and then every argument after that is some sort of you know data that we can

print so imagine we have a long and we’ll call it a and we’ll say that it has like some giant value

so that means we would give that long as the next argument the rsi argument and then for the string

what I’m using right here %lu so you can imagine instead of this string it is this

string right here whoops too many too many quotes it’s just this string right

here and then instead of a 10 13 that’s the same thing as just doing an or if

you want to be you know more of a windows windows person slash r slash n

it’s all good and the zero is not needed because the string if you put a string

literal it’s automatically going to be null terminated which means there’s just

null terminated which means there’s just a zero at the end of the string in memory so this is

basically what I’m doing I’m making a an integer in the case of the assembly program it’s going to

grab an integer from that position in the array that we’re looking at and it’s going to give it

as an argument and then the first argument is going to say let’s just print this as a unsigned

long so that’s why I have that string here let me search for it and go down a little bit again so

I’m saying first argument is this the format that I want to be printed second argument is the actual

is the actual value and then I’m gonna make a call to printf why do I have this

weird push and pop pair sitting around printf so this is not a video about

stack alignment in GCC but basically the GCC libraries expect that your stack is

aligned to I think 16 bytes but since we use 8 byte integers every single time we

address to the stack which is eight bytes and then every time we do one single push or pop

we’re modifying the alignment of the stack by eight bytes so if you think about it when we’re

programming in assembly for the most part the stack is going in and out of alignment because

every time we modify it by eight bytes it it might line up with a 16 byte alignment or it might not

it’s just kind of like oscillating right so when I first wrote this solution I wasn’t doing the

what happened oh actually maybe i guess i don’t need to jump to the top right now i don’t need

to finish the loop let’s see if this prints just one number well let me let me let me see if this

prints one number certainly and if it’s an okay assembly program just to print one number is it

going to work okay we got to do the done symbol okay so i’ll show you in a minute why we need

that for stack alignment but i guess i’ll just finish the loop so demo print loop done so we

just did print loop top and that means we need the bottom and the done so i’m just going to copy paste

into program here i’ll just say nada because we’re not really doing anything and then at the bottom

you know that’s the epilogue that’s separate from the the other label so basically now let me finish

know we already know the loops but I’ll just I’ll just say it at the bottom of

the loop we do the same thing that we did with the initialization loop we just

increase the running pointer you know we move it along to the next integer and

then we jump to the top of the loop that’s it and then the done label we

don’t really do anything we’re just letting execution drop through down to

that point so that the loop doesn’t continue so now we should be able to run

the program don’t get excited oh actually you know what get excited I

was gonna say don’t get excited because it was gonna totally work now but now

because it was going to totally work now,

but now I think we can just say that it’s going to crash.

So if I run it, notice how there’s a segpult.

So the GCC libraries, many functions expect your stack

to be aligned to 16 bytes.

So if you see mysterious crashes

and you are absolutely sure that you’re not ruining

the stack pointer or ruining something else,

you’re doing everything correctly,

but the program still crashes, it might be stack alignment.

So one way to get around stack alignment

is just to move the stack pointer.

move the stack pointer like at the top here we could have said oh we’ve got one two three four

five we’ve got five pushes and then here we’re moving the stack by I don’t know how many other

addresses the stack might be out of alignment somehow so we could add an extra push up here

and then add a corresponding pop down at the bottom like we could easily do this let me just

show you real fast we could push r15 twice for no reason I acknowledge and then at the bottom

and then at the bottom we pop R15 twice,

that would change the alignment because that’s one more 8 byte push.

But in my case, and actually that would be a little bit smarter

because if you have our loop where it’s constantly calling on printf,

this is a lot of hits to memory, right?

This is like 100 hits to memory because every single time we do a push-pop pair

around a call to printf, we’re like touching memory.

Whereas if I did it at the beginning and the end,

Maybe I should just do it this way.

I want to do it both ways so you understand, but it’s more efficient, I think, if we do

it this way.

Anyway, so we’ll do pop twice at the bottom and then push twice at the top.

And so then we don’t really need to surround it with a push-pop pair.

I think I haven’t tested this.

We’ll hope now that the stack is in alignment at all times in our functions so that it doesn’t

crash.

Yeah, so now see how the program works.

So I’m going to do it the other way now, which is the less efficient way, because here

now which is the less efficient way because here we just have one extra push

pop pair but if we do it the other way it’ll still work but we’ll be hitting

memory much more often so I’m just gonna do it this way just to show you you can

surround any call because sometimes in your programs you might have the stack

like you know modified throughout the function many different times so it

wouldn’t make too much sense for you to add an extra push pop pair in the prologue

and epilogue because that might not solve it for all of your calls to all of

to all of your seed library functions.

So in that case,

where you can’t really predict the stack well enough,

you can just surround your call with a push-pop pair.

It hits memory more, but it’ll work.

So this is basically, you can imagine,

by the time we get to line 139,

the stack is out of alignment.

So I just do a push that puts it into alignment.

And then after the call comes back,

I just pop it so it’s back out of alignment again,

but I don’t have extra erroneous data sitting on the stack.

Because if I only had push and not pop,

then it’s going to push it more and more out of alignment.

It’s going to push it in and out and in and out,

but it’s going to add a bunch of junk data to the stack

that I’ll never recover from.

Or actually, I guess I will recover at the very end

when I restore the stack pointer, but it’s pointless.

It’s going to consume too much memory.

What if I was writing like a billion

or like a million items on the stack, right?

a billion iterations of the loop would probably be a bad idea to start adding onto the stack

we’ll probably end up stack overflowing probably with far less than a billion items so anyway i’m

going to surround the call with the push pop pair and then at the bottom we just do the same thing

you know increase the integer and then go to the top and so now we you know you already can see it

it starts off printing well it starts off initializing the array but then the thing that we

can see starts off printing the first integer and then the next iteration of the loop prints the

second integer and the third and the fourth and so forth until we get all the way down just to

prove to you maybe we can increase it by two instead of one each time I’ll just modify that

Oh no, that’s the print loop. Let’s do the init loop.

Init loop bottom.

Oh, right there, when we’re in the body.

So I’m just going to increase R15 twice.

Just to show you, you know, we can kind of control what’s going inside of the local array.

So see how it goes from 7 to 9 to 11 instead of 7, 8, 9.

So I’m going to take that out and then just show you that we can control

how many integers we have in our local array with just the number of integers.

just the number of integers so I’m going to change the 50 to a 5 and run it again and you can see

what the dang did I do number of integers is 5 and then it was 50 did I put the stack out of

alignment no I I have a push put pair there number 50 is not hard-coded anywhere anywhere

there. Oh, I have a bug that I can debug. That’s nice. I guess the bug debugging is not for this

video. I wonder if some other value would work for that. There’s a number of things that could

be the problem. It could be like stack alignment somehow. It could also be, let’s see, am I making

any calls here inside of my function demo? I’m saying sub and LEA and then start and then jump

and start and then jump and increase and jump and then RDI.

Not really doing anything else.

So I don’t think it’s stack alignment.

I must have miscalculated somewhere somehow for changing that.

All right.

Well, if I figured it out, then I will release another video.

But basically, this is the idea.

100 works, 50 works.

What about 99?

Would that work?

Save faults on 99.

faults on 99 and then 98. 98 works so like every two seems to work but that is

two quad words or 16 oh right so like a hundred integers if we assume that

that’s in alignment then 99 would be eight bytes less that we’re moving the

98, notice how it is okay.

And then 97, it’s going to segfault again.

Watch.

This is a great stack alignment video, I guess.

96, it won’t segfault.

So it’s going in and out of alignment.

I think I figured it out.

So if we use the number 100, it’s in alignment.

Sorry, it’s out of alignment.

If we use the number 100, the program works because the number 100 throws it out of alignment.

number 100 throws it out of alignment but then we have this push pop pair here around the call to

printf which puts it back in alignment so if I change the number of integers I’m actually

changing the number of memory locations that I modify the stack pointer so I have to do it by

twos if I wanted to do 99 here then that means the stack is in alignment by the time I call

by the time I’m getting ready to call printf which means the push pop pair around it throws

push pop pair around it throws the stack out of alignment so just watch here I’ll

prove it okay so it’s sake false I comment that out now it won’t throw it

out of alignment okay should have known that before I recorded the video it’s

fun to guess sometimes though you know I get a little nervous oh no my program

broke on camera can I debug it live well I guess I can but kind of slowly

anyway so we I think we’ve gone over every single part that I wanted to show you

we know how to create a local array on the stack and and therefore you also know how to create like

any other data type on the stack if you want you want to create a long a 64-bit integer

just move it by eight bytes instead of moving it by eight times however many integers we were doing

in this video you want to I don’t know put a character on the stack you can do that if you

Just, you know, move it by one memory location instead of eight memory locations.

So, you know, one byte instead of eight bytes.

You want to store a short, you know, a two byte integer?

Well, just move it by two bytes instead of eight bytes, right?

So you can do this as many times as you want.

You want to have several local variables?

Just move it one time for every local variable.

Same thing for accessing.

You just have to remember where everything is.

offset of the of the first variable local variable and then the offset of the second

local variable you can store those in globals or store those in registers if you can you just

got to remember somehow where everything starts but it’s all sitting on the stack if it’s a local

variable okay I guess that’s everything that I really have to say I hope you enjoyed this video

I hope you learned a little bit of stuff and had a little bit of fun I’ll see you in the next video

have a good one

you

Hey everybody, thanks for watching this video again from the bottom of my heart. I really appreciate it

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Thank you.

The post x86-64 Assembly: Local Variables, Stack Frames & Alignment Explained appeared first on NeuralLantern.com.

Learn pointers & dereferencing in x86-64 YASM assembly and how to pass them correctly between assembly and C++ in a hybrid program. We build a small working example that sends strings, longs, and doubles both directions using pointers, modifies values across module boundaries, and explains why pointer-to-double still uses general-purpose registers. Includes a quick demo of stack misalignment crash + fix.

Great for assembly beginners moving to real programs, systems programming students, or anyone curious how low-level code talks to C/C++.

00:00 Introduction to Pointers and Dereferencing in x86-64 Assembly
00:28 Pointers explained in C++
01:02 Changing values via pointers in C++
01:43 Pointers in assembly basics
02:09 Defining variables and pointers in YASM data section
03:23 Pointers are always integers even to doubles
04:20 Function arguments are pointers treated as 64-bit integers
05:00 Driver C++ code overview
05:58 Marking extern “C” functions
06:40 Local stack variables and passing pointers
07:51 Stack lifetime warning
08:34 Assembly data section strings and numbers
09:39 Print null-terminated string helper functions
10:38 External symbols and hey_driver_print_this
11:29 Point function prologue and stack alignment
13:04 Extra push for 16-byte alignment
14:20 Printing welcome message from assembly
16:00 Driver sees initial long value
16:58 Printing received string from C++
18:20 Using received char pointer without dereference
20:21 Modifying incoming long via dereference
21:46 Driver sees modified long value 101
22:43 Calling back to C++ to print assembly-owned data
23:48 Passing pointers to assembly string long and double
25:08 Driver prints assembly-owned values and addresses
26:14 Summary of pointer passing between modules
26:36 Stack alignment crash demonstration
27:39 Adding extra push/pop fixes segfault
28:00 Closing remarks and call to subscribe

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Hey there! In this video we’re going to talk about pointers and dereferencing in a YASM x8664

assembly program, also as a hybrid program so that assembly and C++ can talk to each other

and send each other pointers and send each other data and things like that.

for what pointers are.

I’m going to write in C++ for a second.

Suppose you have a pointer for an integer.

We’ll call it P.

Suppose you have an integer by itself.

We’ll call it A.

Let’s say that the value of A is 5.

And if you wanted to say that P points to A,

you could say P equals the address of A.

I’ll put C++ at the top here.

And so now if I set A to 6

then I print P a dereference of P this is not like a full pointers tutorial

but basically by changing a I’m changing what P thinks it sees as a value

assuming ID reference it I could also let me do a print 6 here I could also

just change the value through P I could say dereference P and I could say equals

would actually print a seven right so you know you can have regular variables global variables

whatever kind of you know memory stuff on the stack and to get a pointer to it you really just

need to get its memory location in c++ it’s kind of easy syntactically you can see what’s happening

in assembly you really just need the memory location stored somewhere you could store that

variable that just simply stored the memory location of some other variable.

You could have a 64-bit register store the value of a variable.

Let’s say we have like a, I don’t know, my whatever, my number let’s say inside of assembly.

I’ll do ASM here and we say it’s a quad word and it starts off as this number or whatever.

So if you haven’t seen my previous videos, go see them for the basics of assembly and

of assembly and linking and make files and all that stuff but you know if you

have an assembly program and you have a data section and you define a global

variable like this what you’re basically saying is I want to take this giant

number and I want to write it into eight bytes that’s the DQ it says data quad

word I want to write that giant number across eight bytes and then I want to

get a pointer to it stored in the my number symbol so my number is not

actually the value it’s a pointer to the value so you know later if you want to

you know later if you want to move you know something into a register if you did this

that would move the pointer into rax but if you did this

with deref symbols after it or around it then you would move

maybe i’ll put that into rex you’d move that actual number that we specified into rex

into Rx. It’s important to understand also that pointers are integers even when we’re pointing to

doubles. So for example sometimes people make this mistake they’ll say you know my double

and they’ll say it’s a quad word meaning this is going to be a 64-bit double precision floating

point number and they’ll do like 44.55 or whatever. So that is a double and it is in memory

you know what is the symbol of my double remember it’s supposed to be just a

pointer right it can’t be an actual double because a memory location is not

a double a memory location is an integer so that means if you wanted to move a

pointer into a register you would only be able to move the pointer into a

regular general purpose register not a floating point register and you should

use the regular movement instructions for just regular general purpose

So keep that in mind if you see a signature like this like let’s say function F and we have

You know, let’s say long a and long B and actually let’s do pointers

Let’s say long pointer a and long pointer

B and double pointer C all three of those arguments are actually 64 bit integers

Because they’re all pointers even if one of the pointers points to adult a double

double why did I say dull pointers aren’t dull they’re exciting okay so I’m gonna open up some

code here real fast so usually I don’t explain my uh my driver I’m gonna explain it to you this time

because it’s kind of doing a little bit more than my other videos um again if you don’t have uh the

knowledge of how to make a make file see my other videos because that’s explained there for now I’m

what we really need to do is write a driver and an assembly module for a

hybrid program again hybrid programs covered in other videos so the driver is

pretty easy I’m just going to copy paste it honestly here and then just kind of

explain it to you the driver is pretty easy we’re going to do I O stream so we

can print stuff we’re going to mark an external function called point as extern

C so that just disables name mangling which means the C++ module will be able

will be able to call on this function called point and it won’t expect that

the point function has its name mangled like C++ does the reason being is that

point is actually going to be in a side it’s going to be inside assembly where

its name will not be mangled this disables the ability to overload but

that’s okay we don’t care it’s going to take two pointers a pointer to a character

and a pointer to a long since both of those are pointers they’re both

64-bit integers even the character pointer and then we have a function that is internal to this

module called hey driver print this remember we’re inside of the driver program right now

so if you look at the bottom it’s just a function that takes in some pointers

and then prints some stuff so it’s going to print like it’s going to print what the string is

it’s going to print what the long is my dog’s growling at me i’m going to ignore him because

i literally just let him pee and poop at this point now he’s harassing me for treats

now he’s harassing me for treats he always does this okay so uh the string the long the double

this function expects to receive three pointers to different data types it’s just going to print

all of them and the point get it the point of this function is we’re going to go inside of

the assembly module and then have the assembly module call on this function so that we can we

can prove that we can have stuff sent from assembly to c plus plus or c using pointers

using pointers we can have data sent over so anyway that’s why both of these

are in here the point needs to be marked as no name mangling because point is

inside of assembly which will not name mangle and then hey driver print this

that needs to have name mangling disabled also so that the assembly

module can call on this other than that we’re just basically inside of a main

saying hey this is the c string we’re making a c string inside of the main function notice how

this is a local variable so that c string is going to show up on the stack it’s going to show up in

the area that is owned by main for main stack area same thing for my long that’s a local variable on

the stack um and but then we can actually send pointers to those pieces of data to another

function in another module you don’t have to only transport globals or stuff on the heap

or stuff on the heap, you can transport pointers to local variables. Just make sure that by the

time this function finishes, then nowhere else is actually using that data because,

well, being on the stack, once main function or once any function finishes, then its portion of

the stack will be cleaned up and removed and it’ll be junk data. You’ll probably get a seg fault.

But for now, we’re not going to use anything on the stack. We’re not going to use these local

just going to use them quickly on this call to point and then we’re going to return to the

operating system and finish the program. So that’s the driver. Now the hard part. Let’s do this in

assembly. So for starters, I’m going to make a data section and just explain it to you very,

very quickly. Again, if you don’t understand the basics of YASM x86-64 assembly, did I mention

that that’s what this language is at the beginning of the video? I guess I should put that in the

put that in the description or record an announcement that I can tack on at the beginning

or something. Anyway, so if you don’t understand how to do this, see my other videos, but basically

we’re going to make a data section. We’re going to define some strings. Here’s like an announcement.

Oh, we’re inside of, you know, the module now, the assembly module. And now we’re going to print

the received string. And then we’re going to make a string that is owned by assembly, which we can

into C++ when we call the function inside of the driver.

So this string is owned by the assembly module.

Notice how these are null terminated strings.

I just have like a comma zero there,

which means I have some extra functions

I’m gonna paste in that we’re not really gonna talk about

because they’ve been discussed in other videos

just so that we can print null terminated strings.

Then I’ve got a new line here,

you know, carriage return line feed.

And then I’ve just got some numbers

that are owned by the assembly module.

Then I’ve got a system write call,

call code one for the system call writes and file descriptor standard output so I

can print just to the terminal again if you don’t understand this see my other

videos so now let’s start the actual text section so this is where our

instructions start so we got the text section here and we’re going to use some

external symbols don’t worry about these I’m just using my own little library to

and input integers if you have access to this library use it if you don’t if you’re watching

at home and you don’t have this library then that’s fine you can use you know printf or

scanf or something like that to get and print floats from and to the user

but yeah I’m just using that and then I’m marking an external function here called hey driver print

this if you recall the driver module has a function called hey driver print this so

just allows my assembly code to call on that external function. Okay now next

piece of code. This is going to be… actually I’m going to paste the print

null terminated string function and related code because it’s just like a

big giant mess and we’re mostly going to ignore it. So just to show you what I’m

doing here I have a function called print null terminated string so that I

can print these strings up here and then I have it rely on a function called

string length that I have implemented up here and all it does is just

implemented up here and all it does just calculates the length of the string and

then a crlf function so I can just call that so that’s all explained in other

videos don’t worry about it for now we’re going to start the actual entry

point remember the driver was just gonna call point right so now we just have to

implement point in the assembly module so that’s gonna be like down here our

our entry point so the signature for this function is going to be character

pointer and then a long pointer and it doesn’t return anything and remember

that if we look back at the driver that should match the signature right it’s a

character pointer and a long pointer and of course this is just a comment that

reminds me of what to do in assembly you don’t really have a signature you just

sort of use registers but I’m reminding myself that RDI is going to be a

character pointer and RSI is going to be a long pointer.

Here’s a note to myself that I’m going to use R12 and R13, which means

the first thing that I should do, well actually before I even do that, I should

return from this function because it is a function. I marked it as global

so that the other module could call it, the driver module could call it. Again,

see my other videos for hybrid programs.

But so now the, you know, if the driver calls this function, then now we’re inside of

and there’s a return statement so it’s a valid function I should preserve the

registers that I’m going to use that are marked as Kali saved for the ABI so I’m

going to go prologue and then an epilogue and I’m going to say push r12 and push

r13 and then I’m going to pop r13 pop r12 they should be in reverse order if

you’ve seen my other videos you’ll know this and the the thing about this

the thing about this particular program is we’re going to run into stack alignment issues

so uh if you don’t know about stack alignment and how it can crash your program without you

realizing what’s wrong see my other videos but for now we’ll assume you know that and uh i i

already know from running this program in advance that it’s going to be out of alignment by eight

bytes so i’m just going to push an extra register onto the stack and that’s going to put it back

I know it looks weird, but this is going to work.

Let me get rid of this here.

Okay, so.

And then maybe if I can remember at the end of the video,

I can just remove that extra push-pop pair,

and you’ll see the program starts crashing.

But at home, you can do it just to double check.

So the first thing I really want to do is,

after I push and pop,

is save our incoming arguments.

Remember, the first integer argument

and the second integer argument,

argument they come in as RDI and RSI in assembly per the ABI if both of these

things are pointers it doesn’t matter what the data type is it could be

pointing to anything including a double and these would still be considered

integer arguments because well RDI and RSI are just going to be loaded up with

memory locations which which are integers so I’m going to save our

arguments to R12 and R13 now justifying our push and pop pair then I’m going to

little welcome message so print a little welcome message again you don’t need to know about this

function but it’s explained in other videos that I’ve already published we’re going to print our

hello beginning message I’m getting nervous he needs to take a second poop sometimes it’s poopoo

number two time for him and he’s not really just lying about a treat but he did go pee and poop

But he did go pee and poop already.

Okay, he just left and walked away.

Okay, if he comes back, I’m letting him out this time.

I’ll pause the video if he does it again.

Okay, I’m pausing the video.

No pee lied.

He went outside, lifted up his little leg, and a couple of drops of pee came out.

Now he’s staring at me like he deserves a treat.

Sorry, buddy.

I wish I could eat constantly all day long, too.

But life isn’t always fair.

isn’t always fair anyway let’s see I might even lined up on the camera

anymore I don’t even know so we’re looking at this code here is going to

print a welcome message let’s see if that actually works so I’m gonna do make

run again make files are whoops what did I do loader dot asm what did I do what

did I do I somehow copy pasted the wrong make file

What’s the name of my source code file?

It’s point.

I guess I’ll just change it, and then it’ll probably work.

It’s still in assembly module.

Hopefully that didn’t mess it up too bad by copy-pasting the wrong source code.

Okay.

What is going on here?

Floater.

Oh, I need to change that.

Hang on.

Let me fix this.

I don’t know if I’m going to edit this out.

out. It’s fun to watch me struggle sometimes. There we go.

Point.

Alright, let’s give it another try.

Oh no, star dot so no such file a directory. Dang it.

Okay, now this seems to work. I may or may not have edited

that out. I copy pasted the wrong source code into my make

file. So I had to manually adjust it. Then I forgot to

copy paste my library file into the build directory. So I had

The driver sees my long as whatever.

What’s going on?

Print an alternate string begin.

Oh, the driver is printing a bunch of stuff.

Okay.

I started to think, why does it look like the program has a lot of stuff going on?

Oh, that’s the driver.

Okay.

So the driver says it sees its long as 100.

And then now we’re inside of the point module.

So that’s the only thing we’ve done in assembly so far.

so far then the driver has regained control maybe I should add a couple of

new lines in there so I don’t get confused again we will do a C out and L

and we’ll do two of those run the program again and then I won’t get

confused about the messages okay so now we’re inside of the point module and

nothing is happening so points let me get rid of the make file here and

and we’re just printing a welcome message nothing else so now let’s print

the received string so what am I talking about so we’re gonna print a prefix

basically saying hey we received the following string right so if you look at

the symbol message received string it’s just gonna say we’re now printing the

received string and then it’ll print it so what are we actually printing we’re

What is R12? R12 is a character pointer to the print me string. And so basically this

function print null terminated string, it takes a character pointer. So we’re giving it a character

pointer that we received. When point was called by the driver, notice how it gave a pointer to

the C string. You know, all arrays are basically pointers. They’re just different syntactically

just different syntactically sometimes so if i declare an array of some length and i give the

symbol somewhere that symbol is really a character pointer so um by calling point with my c string

i’m calling point inside of the assembly module with this character pointer so that means even

though this c string is owned by the driver by the c plus plus module the assembly module has access

So that means we should be able to print it right now already.

So just the rest of it is just like giving a pointer.

And notice how I’m not dereferencing R12.

If I did dereferencing around R12, then we would be looking to that address and seeing what’s there,

which wouldn’t work for printing a null terminated string.

So let’s just run it again.

I don’t know if you can hear him.

This dude is growling at me still because he wants another treat.

He just got denied.

He’s trying to do it again.

do it again. I let him outside people. He’s been outside like three times already and he just went

out like two minutes ago. Okay. I love him so much. It hurts my heart and he knows eventually he’s

going to break me because it hurts my heart or I’m like too distracted. It’s like, you know,

pulling the crank on a slot machine in Vegas. You know, eventually something comes out.

That’s what he does to me. I’ve accidentally trained him. So now printing the received

Now printing the received string and notice how it prints.

Hello, this is a C string owned by me.

So our assembly module is able to print a C string that was created locally by a C++ module.

So we’re handing around pointers.

Nice.

Can you hear me?

He’s getting louder.

So now let’s modify the incoming long.

Can you shush your freaking pants, please?

Shush your pants.

shush your pants you know the sad thing also is he’s so old that he’s deaf now

so he used to know what shush your pants meant it meant I’m not listening to you

and you might as well stop because I’m not gonna do anything based on your

harassment but now he can’t hear me say shush your pants so he just harasses me

all day and all night okay um so I’m gonna copy paste a little bit more code

Modify the incoming long.

So remember again that the point function, it received a pointer to a long.

We’re calling the long change me on the inside of this, but it’s coming in as R13.

And if you notice what I’m doing here is I’m just saying let’s increase the long.

So I’m going to dereference R13 because R13 is a pointer.

So I’m saying let’s go to the memory and change the long that is inside of memory.

And we have to specify that it is a keyword.

it as a keyword so that we you know we don’t confuse the system the system might think are

you modifying a keyword or like a double word or like a word like how big is your data all we know

is it’s an integer because it’s the increase instruction so I’m saying we got a keyword you

know a 64-bit integer sitting at that memory location I want you to dereference it and increase

it and going back to the driver we’re providing a pointer to our long so the long starts off is 100

and we’re just giving a pointer to it the next thing that we can do is we can

ask the driver to print our own stuff actually you know what let’s run the program right now

just to show that the driver can see the change in the long so i’m going to run it again notice how

first when the driver says hello it sees its own long as 100 then we’re inside the assembly module

long and then we return to the caller which is the driver notice how at the

very end of the program the driver sees its long as being 101 so we were able to

modify data that was owned by a different module just by passing pointers

and de-referencing them okay cool so now the next thing that we should do is let’s

ask the driver to print our own stuff that we own because remember if you go

to the very top you know we own some stuff we own some we own a long we own

float, right? So we want to be able to do something with that. So I’m going to copy paste this,

ask the driver to print our own stuff. So I’m going to move three items inside of arguments

for a function call. And then I’m going to make a function call calling the function,

Hey driver, print this again, Hey driver, print this is actually owned by the C++ module.

a pointer to a long and a pointer to a double remember even pointers to doubles are actually

integers so they use the general purpose register so that’s the three arguments right there rdi rsi

and rdx m and then we’re giving the first pointer is going to be the c string so message string

inside asm so you can see that’s this right here and then the next pointer is the long

inside ASM and the third is the float where did I just go I’m getting confused my dog is harassing

me right now so bad notice how I’m not dereferencing so like if when we were increasing the incoming

long before R13 was a pointer so we dereferenced while we increased so that we would increase the

actual value and not the pointer and not the pointer’s memory location but here we’re not

C++ module the actual pointers to our data. We don’t want to give it the data itself. We want

to give pointers to the data so we’re not derefing with the brackets. So then we call it and when we

get back in here it should just be able to print everything. So I’m going to run it one more time.

We’re going to make it and run it and so now let’s see. So here we’re inside of our assembly module

And then here the assembly module has just called on hey driver print this.

Remember the C++ module doesn’t actually call this function.

The assembly module calls it.

So we’re like going back and forth.

We’re kind of crisscrossing.

So now the drivers print this function says we got the following string.

Notice how that’s the string that is owned by assembly.

So we define that inside of our data section in the assembly module.

And then it prints the long.

It prints it as hex.

And it just sort of prints the value.

it just sort of prints the value then it prints it as hex again and then prints at the value

i think actually not hex i think this prints the memory location let’s double check real fast

yeah so remember um in c plus plus i know this is not like a c plus plus video but um

if the long is a pointer then if we just print it without dereferencing it we should see a memory

location so it’s telling us uh that the long’s memory location is this and the doubles memory

location is that and if you stare at those two numbers long enough and you understand hex which

And do you understand hex, which you can see my other videos for?

You’ll see that those memory locations are right next to each other because that’s the way we define them inside of assembly.

So we now have the ability to have data that is owned by assembly and give it to C++ or C using pointers.

No problem at all.

And then the printing driver thing exits and then the actual driver regains control.

And it just says that it sees it’s long as 101.

it sees it’s long as 101 so uh yeah that’s that’s pretty much all i wanted to show you for this

now you hopefully are an expert at passing data back and forth between various modules using

pointers we’re not using references because references are like a little bit a little bit

less compatible pointers are just really easy they totally work in assembly no problem

one more thing i just wanted to show you real fast before we go even though there’s another

video you should check out for stack alignment I just want you to see what

happens if I remove this extra push-pop pair so now my stack is about eight

bytes off of its previous alignment because you know we’re not pushing an

extra eight byte value and somewhere inside of the let’s see print null

terminated string and then the hey driver print this oh and then we go into

like a bunch of C stuff the program should probably crash because anytime

you use a GCC function or a GCC library or something like that the stack has to

be aligned to 16 bytes so if it’s off by 8 then it’ll crash and how did I know

that I needed this well I just ran it first and it crashed and then I added

the extra push pop pair and it didn’t crash and I realized it was definitely

one more time we should get a seg fault yeah we get a seg fault stack alignment oh no with no

description of what’s going on if you were in gcc you could i mean sorry if you were in gdb you

could probably figure that out eventually but why not just give it a try add another push pop pair

run the program again with no other modifications now it totally works

okay well uh i think that’s uh that’s all i have for this video thank you so much for watching i

I hope you learned a little bit of stuff and you had a little bit of fun.

I will see you in the next video.

Hey everybody.

Thanks for watching this video again from the bottom of my heart.

I really appreciate it.

I do hope you did learn something and have some fun.

If you could do me a please, a small little favor,

could you please subscribe and follow this channel or these videos

or whatever it is you do on the current social media website

that you’re looking at right now.

It would really mean the world to me

would really mean the world to me and it’ll help make more videos and grow

this community so we’ll be able to do more videos longer videos better videos

or just I’ll be able to keep making videos in general so please do do me a

kindness and and subscribe you know sometimes I’m sleeping in the middle of

the night and I just wake up because I know somebody subscribed or followed it

just wakes me up and I get filled with joy that’s exactly what happens every

single time so you could do it as a nice favor to me or you could you control me

up in the middle of the night just subscribe and then I’ll just wake up I promise that’s what will

happen also if you look at the middle of the screen right now you should see a QR code which

you can scan in order to go to the website which I think is also named somewhere at the bottom of

this video and it’ll take you to my main website where you can just kind of like see all the videos

I published and the services and tutorials and things that I offer and all that good stuff and

for

Clarifications or errata or just future videos that you want to see please leave a comment or if you just want to say hey

What’s up? What’s going on? You know, just send me a comment, whatever

I also wake up for those in the middle of the night. I get I wake up in a cold sweat. I’m like this

It would really it really mean the world to me. I would really appreciate it. So again, thank you so much for watching this video and

darkness, which is coming for us all.

Thank you.

The post x86-64 Assembly Pointers & Dereferencing Explained – Hybrid C++/YASM Example appeared first on NeuralLantern.com.

In this beginner-to-intermediate assembly language tutorial, we dive deep into unconditional jump instructions (JMP) in x86-64 assembly using Yasm syntax.

We cover:

• What unconditional jumps really are (basically a “go to” for assembly)
• How labels work and how to create them
• Why JMP has unlimited range (unlike conditional jumps)
• Practical demo showing how to skip code sections using jumps
• Comparison between jumping over code vs letting it execute
• Quick look at why this matters before learning conditional branching

We also reference the excellent free open-source textbook by Professor Ed Jorgensen (May 2024 version) which is highly recommended for anyone serious about learning x86-64 assembly.

Whether you’re preparing for university courses, reverse engineering, operating systems development, or just love low-level programming, this video will give you a clear understanding of how unconditional control flow works in modern x86-64 assembly.

Next video will cover conditional jumps (je, jne, jg, jl, etc.) and their limitations.

Enjoy the video and happy coding at the machine level!

Introduction to Jump Instructions 00:00:00
Recommended Free Assembly Textbook 00:00:23
What Unconditional Jumps Actually Do 00:01:27
Labels Explained with Examples 00:02:40
Unlimited Jump Range Advantage 00:04:43
Overview of the Demonstration Program 00:06:56
Building and Running the Jump Test 00:09:21
Live Jump Test Demonstration 00:10:53
Effect of Removing the Jump Instruction 00:13:50
Jumping in Different Directions Example 00:14:58
Summary and Next Video Teaser 00:17:28
Closing Remarks and Call to Action 00:17:44

Thanks for watching!

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Please help support us!

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Hello there.

In this video, we’re going to be talking about jump instructions in assembly.

This video is going to be about x86-64 Yasm assembly,

but I think probably anyone who’s interested in jump instructions

will benefit from this video because the concept is usually the same

throughout any system that you might use.

So for starters, I want to direct your attention to a textbook

that I think is wonderful.

This is an open source free textbook that will help you become an expert in assembly.

It’s not mine, I didn’t write it.

The author is Professor Ed Jorgensen, PhD.

He releases this textbook for free.

It’s under a copyleft license so you can literally just go to his website and download it and

send it to your friends and everything and it’s wonderful.

This book will take you from zero to hero when it comes to Yasm Assembly.

It’s wonderful and amazing.

This is the book and I just want to show you the section on jump instructions real fast

and then I’ll talk about them a little bit and then I’ll show you a sample program that

uses jump instructions.

So this version of the book that I’m working with right now is May 2024 version 1.1.56.

I’m going to go down to let’s see section 7 instruction set overview.

Inside of there there’s another subsection called where the heck is it control instructions

instructions 7.7 within that there’s a another subsection called 7.7.2 that’s

why I look this other not one of the many reasons that I love this book it

has so many subsections there’s just so many yummy subsections to organize

everything in a way that you can find it all so fast and okay so we’re looking

at unconditional control instructions in other words a jump instruction if

you’re an old-school programmer especially if you use some sort of like

if you use some sort of like a basic language or a language with go-to’s you might recognize jumps

as just being a go-to meaning we’re not actually going to call a function and then return from it

which is what the modern programs tend to do we’re just going to say let’s write a jump instruction

and we will literally just change execution to to jump to some other location just kind of go there

forever maybe we come back but if we do it’s going to be because there was a different jump instruction

instruction that told us to jump back.

So we’re not calling and returning.

We’re just going somewhere and that’s it.

Obviously it’s a little bit more convenient to be able to call functions,

but that’s sort of like an abstraction that has to be implemented after we

understand how to jump. So anyway, the jump instruction is pretty simple.

You just put JMP in Yasm anyway, and then follow it with a label.

So, you know, just as a quick little recap here, what’s a label?

imagine we have an assembly program here and maybe here’s our text section and we put some

instructions maybe there’s like an entry point right here I’ll say a global entry point and

literally just taking the word entry point and putting a colon after it now makes that a label

so if there are any instructions underneath I’m gonna put a bunch of nopes then if someone somewhere

to say jump entry point they should be able to go right here to instruction 8 and then start

executing downward. I guess maybe I didn’t need to put the global keyword global just means let’s

make this label available to other modules within the same program so if you have a multi-source

program or a hybrid program with multiple different languages then you know you should do this but if

it’s just a pure assembly program and there’s only one source code filed you don’t need to mark a

Just as a quick example here, entry points, I’ll just put hello as a label and I’ll say like do exit stuff.

So imagine on line 16, you add some instructions just to kind of exit.

If I wanted to skip all these nope instructions for some reason, I could just do this.

I could say jump hello.

And what would happen is execution.

Oh, I can use my pen.

Execution would just sort of, you know, it would come into the text section.

you know, it’d come into the text section.

It would go down through the label and it would execute this first jump

instruction and then execution would jump over the nopes into the hello label.

And then, you know, if there was other stuff here, then it would get executed.

So by jumping over the nopes,

I’m essentially saying that the nopes should not actually end up being

executed. They’ll be there in the program, but they won’t actually execute.

So that’s the basics of a jump instruction. Okay.

So what else do I need to tell you real fast?

What else do I need to tell you real fast?

Oh, one thing that’s really good about jump instructions is they have unlimited jump range.

So you can jump from a place at the very, very beginning of your assembly program and

jump to a place that is at the very, very, very end of your assembly program.

There’s not going to be a limitation on how far you can jump.

I mean, in theory, there’s a limit, but practically speaking, there’s not a limit.

Why would you care that there’s not a limit?

not a limit well because in a future video that i’m going to release we’re going to talk about

conditional branching which is sort of a jump that only jumps if a certain condition is true

and those have limited ranges where they can jump so there’s going to be a bunch of different

instructions but one of the conditional branching instructions is jne and another one is jge and

there’s another one that’s je basically you know jump if something is equal jump if something is

can only jump about 128 bytes away.

So after your assembler assembles and compiles

down to object code,

and then after your linker links your final executable,

wherever it is that the instructions happen to end up

inside of your program,

the conditional jumps,

the conditional branching instructions,

they can’t jump more than 128 bytes away

to some other instruction.

So keep that in mind.

Even if later on you graduate

to making decisions in your program,

like I’m going to do in the next video,

in your program like i’m going to do in the next video you can only jump so far and if you have to

jump too far you actually might not be able to jump at all unless you jump a very short jump

to a regular jump instruction and then that jump instruction jumps very very far away that’s kind

of the workaround for it i’m not going to talk about that in this video though this is not a

video for uh conditional branching i just wanted you to be aware of one of the benefits of regular

Okay, so we’re looking at the book here.

There’s not really a whole lot to the jump instruction, just jump and then a label.

We talked about its benefit over conditional branch instructions,

but we also talked about its, I guess, its shortcoming,

meaning it can’t actually make a decision.

It will always jump to a label no matter what.

There’s no condition.

So there’s the book there, and now I’m going to make a sample program

and show you how to run it.

I’m just going to run it.

I’m just gonna run it I’m show you what it does in order to implement conditional branches so for

starters I want you to know that there’s a make file that I’ve generated under the hood and we’re

not going to be talking about that in this video this is also a hybrid program so there’s a C++

entry point a driver module under the hood of this we’re not going to talk about that if you

want to know how to make hybrid programs you want to generate make files you want to learn the basics

videos for now we’re only going to be talking about jump instructions so I’m

going to skip a lot of information okay so for starters I’m going to make a

little data section here and again this is explained in other videos but for now

we’ll just trust that we can make a data section that contains strings C strings

and other values so pretty much I’m just going to make a string called begin jump

test just to announce to the user that we’re we’re going to start doing this

We’re going to start doing this and then I’m going to make a string called this message

should not appear.

So in the code, I’m going to try to print that message, but then I’m going to jump over

the call to print it just to prove to you that there are instructions that would print

that message, but we’re jumping over them with the jump instruction.

And then there’s like an exit message.

And then there’s a CRLF, which is just a carriage return line feed.

Again, all of this stuff is in other videos already.

So we’re going to use system call one to print.

We’re going to print a file descriptor one, which is just standard output for your program.

Then we’re going to start the text section where the actual code lives.

So this text section is here and it’s supposed to be at line 37 already.

I think I missed a bunch of lines.

Oh no, I think I missed some comments.

Anyway, so we have a text section here and an entry point and I’m calling it cool.

calling it cool and I am marking it as global because in this particular program that I’m

building it’s a hybrid program there’s going to be a C++ module that will call on our cool

function so cool has to be global and then I’m just going to call on a method called jump test

I don’t know I have the words load there I’m just going to get rid of that real fast locally and in

my solution up above and so we’re going to call a function called jump test and then when we’re

finished we’re going to return to the caller which is going to be the driver and that’ll

pretty much be it.

So if I comment this out real fast, let’s see,

this might actually work.

Let’s see if I can get it to run in the terminal.

But there’s a bunch more code that we have to add, so I’m not really sure.

So let’s do clear and make run.

And it seems to not have a shared object directory.

Let me pause the video while I copy paste one of my stupid libraries into the

program. You don’t need this library.

It just helps me print things.

okay so now I have copy pasted my shared object which allows me to do extra printing stuffs

just for just to make this demo easier for me but you don’t need to know it or you don’t need to have

it to to learn jump instructions anyway so I’m going to do that again and now it actually prints

something okay so hello from the main CPP driver and then it says the driver has regained control

make a call to jump test here and then let’s start the actual jump test function. So I’m going to do

well I guess this thing is kind of short I could copy paste the whole thing all at once.

So let’s do yeah let’s just jump let’s just call the whole thing. Okay I’m going to copy paste the

whole thing then I’ll explain it a little bit to you. So there is a function that I have in here

It’s just a convenience function that I made so I can print a carriage return line feed.

The real interesting thing here is the jump test function.

So we were just making a call to jump test.

Now we’re making the actual jump test function.

It’s got a signature of just void with no arguments.

So it’s not super interesting from the caller’s perspective, but it does some stuff.

So for starters, it has an intro message.

So this will print, you know, hello, welcome to the jump test.

jump test. In fact, if I do a return call here,

it should actually just print that and do nothing else. Right. Okay.

Notice how it printed, begin the jump test.

And then right after that,

there’s a jump instruction just proving to you that we can jump over other

instructions. So look at this,

this piece of code should never actually be called because we’re going to jump

over it. What it is, is it’s printing that jump shouldn’t happen message.

at the top here jumps shouldn’t happen so it’s trying to print out this message should not appear

but we’re going to jump over that by using this jump instruction here on line 66.

Again note that the jump instruction is just jmp followed by a label the label specified has to be

where you want to jump it’s never going to return from that place unless you specifically jump back

somehow later on like i guess if we wanted to we could put a label on line 67 call it the return

call it the return point and then jump back from it after the jump point in fact maybe that would

be kind of interesting to do at the end of this video but otherwise we’re gonna you know just

let’s see we’re gonna end up jumping over so let me reduce the front size just for a second here

so imagine execution uh comes into this program you know we’re executing uh instructions we’re

calling crlf we’re just executing executing as soon as we hit this jump instruction then execution

then execution jumps over into the label that I specified.

So this whole code section here just never even gets called.

So that’s why we will not see that message.

And then at the very end, all I’m doing is I’m just properly,

you know, I’m printing the exit message.

So I’m just printing another string saying the exit or the jump test is done.

I return to the caller execution goes all the way back up to just you know right here right after

call jump test was executed and then the cool function will return to the caller and that’s

just a c++ main function that does nothing so at this point we should see the whole entire point of

the program and then I’ll start tweaking it so you can kind of see the difference with the jump

instruction uh there and not there so let’s run one more time and notice how it says begin the

says begin the jump test and then end jump test and then it goes back to the driver that is

regain control it never says this message should not be printed so this whole section was just

skipped let’s comment out line 66 so that we don’t actually jump over that code and then now you’ll

see that that message does get printed so notice how it says this message should not appear okay

and then run the program one more time.

Now that message does not appear.

Pretty cool.

Now let’s do that double jumping thing just to show you.

I mean, this is not something that you actually want to do.

You probably want to write functions and function calls,

but if you wanted to, you could do something like this.

Here’s the exiting.

And maybe right after this, let’s make another label.

Let’s do, oh gosh, what am I going to do?

what am I going to do? Because if I jump after the exiting label and I jump back up to some label

up here, it’s just going to be an infinite loop. So maybe, um, I don’t know, let’s make a, I mean,

if I make another label down at the bottom, you’ll kind of think it’s a function just without

a return statement. So let’s actually jump within the same function. Let’s do, um,

over the never area.

So I’m going to say jump test and I’m going to write never.

So now we have a label that tells us where the never printed message actually starts.

So if we jump over it to the exiting, then we’re good.

But then if I up here, if I say jump instruction that subverts

never message so I’m just I’m just leaving a comment not code I could then

say let’s jump to jump test never and what will happen now is we’ll still see

the never message because what will happen is execution comes down you know

through here all these instructions are executing and then we see a jump that

tells us to go to the the never label so we actually jump over this exiting jump

over this exiting jump or this like the skipping jump,

the jump that skips the message.

And then we actually do print the never message

and we just keep going down and down and down

until we’re finished with that.

And we end up just sort of exiting normally.

So that means the only code that doesn’t get executed

in this case is the one right here

that skips over the never message.

Hopefully that makes sense.

Let’s run the program just to prove it real fast.

So I’m going to do this again.

And now you see the message should not appear.

This message should not appear.

You see that message.

So again, if we comment out that jump that subverts the skip, then execution will fall

through and we’ll end up executing line 69, the skipping instruction.

again now that message does not appear.

We could jump back and forth if we wanted to.

I don’t know.

Should I do a back and forth?

I don’t really want to.

I think at this point you understand we can jump anywhere we want, right?

I could take a bunch of time in this video to rewrite this program and have it say,

let’s jump downward and then let’s jump upward again and let’s let it fall through

and then let’s jump over something and whatever.

let’s jump over something and whatever. I mean, just wherever you want to jump,

just make a label and then jump to it. Then you have to figure out what your execution path is

actually going to be. And maybe that’ll be complicated, but I hope I’ve made my point by

now. Anyway, so that’s the basics of just jumping around. It’s not super useful. Conditional

branching is a lot better. So see my next video. And I thank you for watching this and I hope you

learned a little bit and had a little fun. See you soon.

See you soon.

longer videos, better videos, or just I’ll be able to keep making videos in general.

So please do me a kindness and subscribe. You know sometimes I’m sleeping in the

middle of the night and I just wake up because I know somebody subscribed or

followed. It just wakes me up and I get filled with joy. That’s exactly what

happens every single time. So you could do it as a nice favor to me or you could

you could troll me if you want to just wake me up in the middle of the night.

Just subscribe and then I’ll just wake up. I promise that’s what will happen.

Also, if you look at the middle of the screen right now, you should see a QR code which you can scan in order to go to the website

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It’ll take you to my main website where you can just kind of like see all the videos

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Clarifications or errata or just future videos that you want to see

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You know, just send me a comment, whatever. I also wake up for those in the middle of the night.

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enjoy the cool music as, as I fade into the darkness, which is coming for us all.

Thank you.

The post x86-64 Assembly Jump Instructions Explained: Unconditional JMP with Full Example in Yasm appeared first on NeuralLantern.com.

In this in-depth x86-64 assembly tutorial using YASM, we dive deep into implementing complex if-else and if-elseif-else control structures from scratch. Starting with the fundamentals of conditional branching, we build up to full chained if-elseif-else blocks with multiple conditions – exactly how high-level languages handle them under the hood.

You’ll see real working code that:

• Takes user integer input
• Tests against specific values (5, 6, etc.)
• Handles greater-than/less-than comparisons
• Properly branches so only one block executes
• Uses labels, cmp, conditional jumps (je, jl), and unconditional jumps (jmp) correctly

We cover the classic pattern: compare to conditional jump to true block to execute true code to jmp to end to false block falls through or jumps in. Everything is shown step-by-step with live compilation and runtime demos.

Perfect for anyone learning low-level programming, reverse engineering, or wanting to understand how compilers translate if-else chains into machine code. Prerequisites: basic conditional jumps (see my earlier videos).

Code shown works on Linux x86-64 with YASM/NASM syntax. Grab the concepts and apply them anywhere.

Introduction to If-Else in Assembly 00:00:00
Explaining the If-Else Design Pattern 00:00:56
Drawing the Basic If-Else Flow 00:01:01
Comparison and Conditional Jumps 00:02:30
Labels for True and False Blocks 00:03:07
Unconditional Jump to End 00:04:50
Diagram of Execution Flow 00:05:51
Alternative Pattern with Inverted Jump 00:07:00
Recapping the If-Else Pattern 00:08:45
Starting the Code Example 00:09:16
Setting Up Input and Strings 00:09:40
Calling External Functions 00:10:57
Entry Point and Prologue 00:11:40
Asking User for Integer Input 00:13:09
Creating the if_test Function 00:14:56
Preserving Callee-Saved Registers 00:15:51
Printing Begin Message 00:17:03
Implementing Simple If Block 00:18:29
Comparison and je Jump 00:19:18
True Block: Equality Message 00:21:08
Testing Simple If Examples 00:23:48
Transition to If-Else Blocks 00:24:21
Creating if_else_test Function 00:24:47
Setting Up Complex If-Else 00:26:25
First If: Equal to 5 00:27:22
True Block for Equal 5 00:28:33
Else If: Equal to 6 00:30:29
Else If: Less Than 10 00:34:17
Final Else Block 00:37:33
Done Label and Goodbye 00:38:23
Recap of Full Flow 00:39:06
Live Demo of All Branches 00:40:54
Signed vs Unsigned Jumps Note 00:43:38
Recommended Assembly Book 00:44:12
Conditional Jump Families 00:45:05
Closing and Practice Advice 00:46:48

Thanks for watching!

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Hello there.

In this video we’re going to talk about implementing simple if-else blocks in YASM x86-64 assembly.

Although if you’re writing in a different assembly language, this video will probably

still be useful to you because I’m going to explain the design pattern or how we can achieve

that at the assembly level.

So x86-64 YASM assembly, also known as AMD64 YASM assembly.

So if you have not seen my other videos about how to do conditional branching in the first

place, you probably want to go check that first because that knowledge is required for

this video.

There’s also a lot of other stuff that I’m just not going to explain in this video, such

as creating a make file, you know, compiling and linking your executable and so forth.

So see my other videos where all those concepts are explained already.

how to implement a simple if-else block.

So for starters, maybe let me draw a little bit.

Suppose we had, let me get rid of this thing

and then I’ll just do a regular notepad.

Suppose we had a higher level language,

just so you know what I’m talking about.

Suppose we had a higher level language

and we wanted to say if, you know, some expression is true.

We wanted to say if expression is true,

then print, you know, it was true.

Anybody remember that old movie, Little Nicky?

Somebody got exploded and then the guy next to him goes it’s not true

Okay, it was not true

So this is the basic idea of what we’re going to implement in assembly

I’m going to write a full program for assembly to show you this but um

You know you’re in C++ you’re in C you’re in I don’t know whatever language you’re in and

You obviously know how to use if-else blocks at this point hopefully

And now we’re going to just try to figure out how to implement them in assembly

So it’s important to understand that really under the hood

really under the hood there’s a bunch of stuff happening surprise right okay so

first off we look at this expression which could be I don’t know let’s say

five is greater than ten or a is equal to B or whatever it is that you put in

there you could make a very complicated expression we’re going to use simple

expressions for this video so if we’re comparing a to B we’ll end up using the

compare instruction remember there are two steps to conditional branching and

in YASM we first use the compare instruction against two operands and then that will end up

filling up the rflags register so that we can later conditionally jump based on the results

of the comparison. So we do a comparison and then you can imagine that the beginning of this

you know the the true body notice how it has a scope I’m going to just put this brace on another

line here to indicate that there is a scope from line four to six indicating all this code in here

in here is executed only if the if statement was true and then all of this other stuff is executed

only if the original comparison was false right so you can imagine now label is something like

my if was true so we can make a label for where that body starts and another label for where the

label is something like if was false and then what will happen is we can use a

conditional jump instruction after we do our comparison maybe I should put to

compare right here oh I’m using I’m using assembly style comments I should

be using a C style comments if I’m actually writing C++ here let me just do

that okay so this is the comparison instruction here and then here’s the

here and then here’s the beginning of the body of stuff to execute if it was true you can imagine

that there could be many statements here um if it was true or if it was false

just to prove to you that we can execute like a full body of stuff so we have a label that

designates when that body starts and then we have another label designating when the else body

starts and then we should have a label that uh that designates when the whole thing is over

label is something like if and or if is done or something like that so basically what we want to

do is to implement an if else a simple if else in assembly we’re going to say let’s do a comparison

and then we’ll do a conditional jump where are we going to conditionally jump well if the comparison

was true label. And then what will happen is execution will fall through.

When it reaches the end, we want to have another jump statement that unconditionally jumps

to the end of the if statement. If we didn’t, then whenever the expression was true,

we would end up executing all of the true statements and then it would fall through

to all of the false statements or the not true statements. So we have to have many jumps in here

let’s get into the body and then let’s finish and jump out of the body on the other hand if the

expression was false then our jump instruction is going to jump you know over the true body so

it’s not even going to do that at all it’s going to execute all of the else stuff and then it’s

you know sometimes a good idea if you have another jump here that just jumps to the end but you can

kind of see by the way i’ve written this out that there’s not going to be anything between the end

you know place where we’re finished with everything which means we don’t really

need an unconditional jump at the end of the else body we can just let the

execution fall through so maybe if I can draw a little diagram here I’ll say I

don’t know I’ll do like if put it in a little bubble and we’ll say that if it

was false we jump to one place and if it was true we jump to another place

we jump to another place.

I hope you’ve already started to understand this by now.

So we’ll say if the expression is true, we jump here.

If the expression is false, we jump over here.

And so true would be saying, let’s jump to the if was true.

So we’re going to jump to if was,

I’ll put a T there because I’m running out of space.

If was true.

was I’ll put an F false and then at the end of the true we jump to the done

label so I’m just gonna put done maybe down here

so at the end of the true we just unconditionally jump to the done area

and then at the end of the false we jump unconditionally to the done area as well

here’s something interesting though when we have a comparison instruction and

conditional branch instruction let’s say uh let’s say a equals b we did that comparison and then we

wanted to jump into the true area if a equals b was true so that means we’ll say jump

equal so if a is equal to b after we compare well let’s say a comma b we’re going to use registers

when we come to the code we’ll say compare a and b and then jump if they’re equal to some label

but the conditional branching instructions they only jump to one

potential place or fall through so if the comparison was false meaning if those

two things were not equal then we’re not going to actually be able to jump to a

different label we’re going to simply fall through to the next instruction so

in that case the very next instruction would get executed let’s just put a jump

was

false.

Meaning

we compare A and B

and if the two things are equal

we’ll jump into the true block.

Otherwise we fall through to the next instruction

which

that’s a very poorly written J

but we’re going to unconditionally jump to false.

So if we did not

jump to the true area we fall through to the next

instruction where we will always

jump to false and that implements the diagram

that implements the diagram that you see up above.

I mean, you know, if we have if statement, we compare,

maybe I’ll do, you know, A equals B.

We jumped to true, if was true.

And otherwise we end up falling through

and then jumping to if was false.

And then at the end of both of those,

we have an unconditional jump instruction

that jumps to the done.

So I’ll put JMP down here just to let you know,

you know, at the end of each of these blocks,

that they are jumping out of themselves at the very end to the done area maybe

I’ll put an arrow here so that we know both of these jumps end up jumping to

the done area that’s the basic idea for how to implement an if-else block a very

basic one we’re gonna do more complicated ones later but for now we

kind of have the idea down I think let’s look at some code dang I blew 10 minutes

already on that okay so I’m gonna copy paste some code from my solution here

code for my solution here this again this is not like a beginner’s assembly

video if you need to learn how to write assembly in the first place how to

compile you know link create a make file and so forth you need to see my other

videos first but for now we’re just going to assume that you know how to

make a data section in Yasm and we’re gonna say let’s make a bunch of strings

so first I’m gonna ask the user for an integer and then I’m gonna make a bunch

of decisions like I’m gonna do you know an if-else block to test what kind of

what kind of number they put you know did they make a number that uh

equal one if it was equal to something oh did I ask twice I can’t remember what I’m doing

but basically we’re going to print something if their number was equal to something else

we’re going to print uh something if their number that they inputted was equal to five we’re going

to print something else if their number was equal to six we’re going to print again something else

a bunch of stuff and then wait isn’t this the complicated example oh no i think i am using code

for my more complicated example else less than 10 i don’t know maybe this is the simple one

let’s double check i guess if i put more complicated code in here you’ll probably be happy

but whatever i thought this was going to be a simple example so i’m just defining strings at

codes stuff that is covered in other videos now i’m going to start my text section so my text

section begins with a declaration that i’m going to use two external functions so this video is

not about this library that takes input and sends output i have other videos for that but basically

i’m just using a library that lets you type an integer into the terminal and then it will print

a different number to the terminal for you so you can imagine if you wanted to follow along

imagine if you wanted to follow along with this code at home and you don’t

have this library you can just hard code your numbers just to prove to yourself

that you can get it to work or you can use a different library if you have a

hybrid program and you’re linking against GCC you can just use scanf and

printf pretty much those take a little bit more work to do but you can do it

anyway so our entry point is going to be called if tester it’s a function called

Again, this is a hybrid program and hybrid programs are not covered in this video, but you can imagine that there is a C++ module elsewhere in my source that is just going to call on a function called if tester.

And so that’s why I’m marking this as global so that other modules in my program can call on if tester.

So it’s just a little label that we can jump into or in this case call into.

Let me go down to the bottom of that.

We’ll put a return statement at the very end.

very end so now this is officially a function and notice how i made a note here that says r12 is

the user’s inputted integer so that means i’m going to be using r12 and since r12 is designated

in the abi the application binary interface as callee saved or callee preserved that means i

have to do a push pop pair to preserve it or i’ll get in lots and lots of trouble my program will

prologue which my favorite book also calls the prologue and then I’m going to

say epilogue and so now we have a function that basically doesn’t do

anything but we can at least jump into it let’s see if this compiles as is I

think it probably will yeah so the driver just says hello that’s the C++

program with the actual main function and it calls my if tester function but

nothing happens the if tester function returns control to the driver and then

the driver just says okay i got control back so nothing really happened so now let’s ask the user

for some input so again this is not a video about this library or how to print with this with system

calls see my other videos if you need help on that but basically we’re going to print a message to

the user hey please input an integer and then we’re going to call on a library function that

lets them type in a number and then returns it to us in rax we’re then going to store it

well just basic stuff right so if I run this again it’s going to ask for an

integer please enter an integer if I can type that and then nothing else happens

okay now we can kind of start making more decisions so I’m going to let’s see

if test if test if test okay so what I need to do now is run another function

called if tests several times and I’m going to compare the number that the

times and I’m going to compare the number that the user inputted so remember r12 is the user’s

input so I’m going to load that as the first argument of a function call and then I’m going

to load the number five as the second argument of a function call probably not a great idea to hard

code numbers in your assembly you should probably define them up in the globals or the the data

section at least as just regular defines and not numbers in memory but I’m going to just keep these

So three times we’re gonna call if test and then also another function called CRLF. So first I’m gonna paste in

CRLF

Where the heck is that? Oh, dude?

Again, this is not a video about the basics

So you’re just gonna have to trust me or go watch my other videos if you don’t understand what I’m doing here

But I have a function called

CRLF and its whole job in life is just to print a new line for me just because I like to do it that way

handles the CRLF call. Now let’s make another function called if test.

So I’m going to start that by designating its label right after this block of

code here, maybe before CRLF.

So we have like a basic if test function and here’s my prototype just to remind

myself what I’m going to be doing.

It’s going to take an input and it’s going to take another input for a test

against me. So the first one is like the user’s input.

The second one is the number that I want to test it against.

number that I want to test it against they’re both longs which means they are both integers

which means the incoming arguments are going to be rdi and rsi if you’re respecting the abi

and then some notes to remind myself I’m going to be using r12 and r13 inside of this function so

I’m going to start by putting a return statement there since that is what it takes to make a label

into a return into into a function then I’m going to preserve the callee saved registers again if

don’t know what i’m talking about see my other videos we’re going to push r12 and push r13 so

that they are not ruined for the caller we call this the prolog then at the very bottom of the

function we have the epilog which just restores uh the registers in reverse order you’ve got to

do it in reverse order see my other videos if you don’t understand why okay so that’s basically a

function that can get called it doesn’t do anything let me double check that the program still actually

works. 66 and nothing happens. We just printed CRLF a bunch of times. Okay, so now we’re ready

to continue. So let’s grab the function arguments. Remember we were going to use R12 and R13 for the

user’s input and the number we will test against. Those came into our function with RDI and RSI.

So I’m just going to copy those two incoming arguments into R12 and R13. And you’re supposed

keep the user’s input in RDI then the moment I call any other function or system call I’m just

going to lose that data so I’m going to keep it inside of R12 and R13 so I grab the function

arguments and then I print a begin message just to let the user know that we’re going to start

you know making tests against our number so this is just basically a message saying hey begin the

if test and then print what we’re going to check against so the next thing is

I’m going to let the user know what the second incoming argument was.

If you look at R13 here, that was RSI, which was the second argument.

So the test against me number.

So we’re going to check the user’s input against whatever we called for the second argument.

So I just wanted to print it out.

You know, like we’re testing your number against whatever.

So that means I need to make another call to my little printing library here.

To RDI which is the first argument notice how we already we already destroyed RDI. That’s why I’m keeping the input in our 12 and our 13

And then we’re gonna make that call and we can just assume everything will go according to plan at that point and then

Then we’re gonna print a special message only if something actually happens

So we’re gonna implement the if else block in a moment. Let me just run this real fast

and then it says we’re calling the function three times and we’re saying the basic test has begun

we’ll test against this number five so we’re testing your input against five and then six and

then seven and then we didn’t actually do anything we’re about to okay so then the next thing is we’re

going to print a special message only if the user entered the right number so first off remember we

see if else block and converting it into assembly so i’m sort of placing that in comments for you

so the comparison instruction i i didn’t want to put the r12 equals r13 inside of the same comment

that lines up with the the compare instruction because the compare instruction as i’ve said in

other videos already it doesn’t actually check to see if something’s equal it just makes a bunch of

called r flags with information that we can later use to decide if the two things were equal or not

equal or greater than or less than or whatever so that’s why i chose to put that on on the next line

so at this point we’re saying if those two things were equal and that’s how i implement the expression

in the middle then let’s jump to a special label called if test if was equal so you can come up

with any scheme you want for your labels but for me when i have when i have sub labels inside of a

when i have sub labels inside of a function i like to just suffix the function’s name with an

underscore and then start thinking of sub labels after that so everything’s like kind of clean

there’s less chance of overlap in labels if you have like a giant module with tons of functions

so i’m going to say this is like my main if that i’m checking and uh i’m going to jump to a label

called was equal meaning you know this evaluated to true that means that should be the true part

block let’s see do i still have that code here yeah right so here i’m going to jump to

the true block so you know if was true in the first example that we talked about so i’m going

to say if it’s if those things were equal jump to the code for the true block okay that means i

actually need the true block uh but i guess we’re going to set that up in a second otherwise if that

did not jump away it means that those two things are not equal so i said i should jump to the

I should jump to the else block or the false block.

Wait, do I have else in this example?

Oh, okay, okay.

This first thing that we’re looking at is only if.

So we don’t even have an else block yet.

We’re going to do that as the second example.

So we’re basically going to jump to the done area

if we didn’t jump into the true area.

And then at the end of the true area,

we can either jump to the done area

or we can let the execution fall through.

fall through okay so now that means i need if was equal and if i just copy paste a big giant

block of code and try to explain it to you real fast let’s do this

okay so we were going to jump to if was equal if r12 was equal to r13 and then um

we uh have this you know label here and notice how i’ve kind of like put a brace here indicating

like put a brace here indicating hey this is the beginning of the true block body you should

consider doing this too when you’re first learning and even after you’ve learned because let’s face

it assembly is tough and so in the true area i’m just going to print the equality message i’m going

to say hey your number was equal to you know whatever and then uh i’m going to actually print

the let’s see r13 number so that was uh i think the number to compare against the test against

against me number so that means here in this message we’re going to say hey your number was

equal to the number that we tested against so then otherwise let’s see or sorry after that

we’ll print the suffix of the message um and so you know i just i just like to make pretty

pretty uh printed messages so let’s see where’s the suffix here i can’t even find it um how about

oh i should have put suffix instead of the number two that would have been better so what i wanted

to do is say your input was equal to and then print the number and then after that print an

exclamation just to prove to you how easy it is to to make a pretty message that’s formatted nicely

for the user um or your professor or whoever so uh you know you basically just print a number and

then you or sorry you print the prefix and then you print the number and then you print the suffix

suffix and then that’s the end of the true body and then since we’re done with the true body we

can basically just say all right now we’re done with the if so the next instruction that follows

is the done area again looking back at the example here that would be sort of like here after the

whole entire block was finished we’re ignoring the else in this code but you can imagine we’ll do

that soon so if the user’s number matched something we execute a true body if not we jump to the done

to the done area and if the user’s uh number did not match then we just immediately jump to the

done area so that we don’t do the true area and then we write comments to ourselves to help us

remember oh look here’s like the comparison and then here’s the body of true and then uh the done

area is like we could put another comment if we want we could say like this is done but i kind of

think like i kind of think the label is self-explanatory so let’s see if this worked it’s

was talking too fast we’ll run it and we’ll say 44. okay so nothing matched any of the numbers so

let’s type the number six so that we get a little message on the second one so i’m going to type the

number six and you can see it did not match the number five and it did match the number six so

we got that true block executing when we called on that function where the number to compare to

nothing there. So now we know simple if blocks.

The next thing we’re going to do is if else blocks.

All right.

So let’s see.

Not sure if I’m going to cut the video and split this up into multiple parts.

Probably would have been a smart idea.

Let me know if I ended up doing it.

We’re at about 25 minutes now.

But anyway,

so now we’re going to look at if else blocks.

if else blocks.

So I’m going to start off with another function.

I’m going to call it if else test.

So here’s the tester.

It called on if test.

And then I’m going to, you know, I had the if test function that I did previously.

And so now we’re just going to make an if else test function.

The if else test, it just has one input, one argument for input.

And we’re going to just sort of compare it against different values.

of compare it against different values we’re not going to call this multiple times with different

values to compare against so that means let’s see we only need to use one register so

i’ve designated r12 as the user’s input so that’s where we’re going to store it which means we

should preserve it in the prologue so i’m going to push r12 and then again if you don’t understand

some of the basic stuff that i’m that i’m skipping over see my other videos where i explained

to know to actually have this kind of a program. So I’m going to have a function that enters and

it returns at the end. It uses R12 so we will preserve it with a push pop pair.

And then the first thing that I should do is grab the user’s input from the first argument.

And if you’re respecting the ABI that means it should come from RDI. So I’m going to move RDI

somebody remind me that the first part of the program has to actually call this function or

nothing will happen. So we’re going to say hello. That’s just a simple message that we talked about

before. And then now we’re going to actually implement the if else block. So it’s like a

little bit more complicated than just the simple if block. It’s going to be this whole example that

we talked about before. Let me see. Maybe I should add the calls to this block real fast. So we have

to this block real fast so we have the if tester and then run the complex if else tests

right before r12 okay let me just double check that i’m making a call at the right spot here

so we have r12 and 7 and then epilog okay so now finally in our program we’re going to have a call

to um the if else test function that we’re just making right now

Okay, so we have that.

And let me find that source again real fast.

If else test, we got the prologue.

We take their input and then we say hello.

Okay, so now we need another label that begins the if block.

So what we’re doing is we’re checking to see if the user’s input was equal to five.

And then we’re going to say something if it was.

And otherwise, we’re going to do an else block on that.

block on that so again i like to write my comparison instructions with a blank expression

in terms of the c equivalent comment so notice how the we’re checking to see if r12 is equal to

five i put that on the next block because we’re going to jump to the to the true part of the if

block if it equals five so that’s why i put that there the comparison instruction pretty much just

compares r12 with five sets a bunch of values into the r flags registers so that we can later

so that we can later conditionally jump if we want to.

So basically we’ll jump to the true place if that was true.

And then if not, execution falls through to the next statement,

which will just jump to the else place.

So we need more labels is what I’m saying.

So now we need a body for if the statement was true

or the expression was true,

we want to be able to execute the true portion of the if block.

the if block so that’s this right here so we’re going to jump to if it did equal to five you

could imagine you know making a better label instead of equal five you could say first if

first else if complicated block true scope or true block or something but i just put equal to five

basically saying we’ll execute this code if it was equal to five so again we’re just kind of like

into this label equal five if r12 was indeed equal to five so that means if it was we execute

all this code right here we know we’re finished when we have the very last line saying let’s jump

to finish the the if else block again just just to clarify

we do the comparison first and if the comparison was true then we’ll jump into the true area so

you know we’ll jump into the true area and then all of these uh instructions get executed

instructions get executed but if we don’t have a way to jump out of that block then whoops

all of the else statements are going to get executed too right so we don’t want that

we don’t want to execute both the true and the false statements we want to have a jump instruction

at the very end i’ll put jmp just so that we jump to the to the end of the if else block

you know jump to the place where it’s just all over and finished

Okay, so now I’m going to look at, whoops, turn that off.

So we have equal five.

So we’re going to jump to if else test if done.

Let me just double check to make sure I’m not forgetting anything.

Oh, we need to jump here.

Basically meaning we’re going to jump into the else portion.

So here we covered jumping into the true portion, you know, the regular top block.

Now here we’re going to jump into the else portion.

We need a label and some code for that.

code for that so this is the else six begin i copy that oh man it’s starting to get kind of

hectic in my brain here the copy pasting is worse than actually writing the program

so uh if the comparison was not equal like if r12 was not equal then execution falls through

to line 216 and then we unconditionally jump to the l6 begin area which is like down here

like down here and then we’ll print something else.

We’ll, oh, we’ll, we’ll check again.

So we’re doing like, um, if else, if else.

Okay. So originally when I was talking about the, uh,

if else block, I didn’t do a,

I didn’t do like a very complex if else statement.

We’ll say a is greater than B something like that.

So I’ll just put a more code here just so you know that we can do if else,

if else blocks but just you know again keep in mind that every scope here that you’re going to

try to run based on some condition you just make it its own label and make sure that at the very

end of the scope you jump away so that you reach the very end of all this stuff because just as a

quick review if we are just writing in c or c plus plus only one of these blocks is going to execute

right like if a is equal to b then only that first block will execute the second and third blocks

will not execute it’ll jump after that all the way down to line 21 only if a does not equal b

do we even have the chance of checking to see if a is more than b if it’s false then we’ll have the

chance to check the else and if it’s true we will only execute the code in the line 12 block you

know imagine there are more statements there and when that block is done then we will jump to line

to b and a is not greater than b so just a quick uh you know c plus plus uh you know design pattern

review if else block review so we have like an else if here we’re going to say check if the input

was equal to six so we just do the same thing that we did before we compare r12 the user’s input with

six we jump if they were equal to the else if equals true block and if not execution falls

Less than 10 begin block. So this is going to be like really complicated

How many lines did I actually I think I got excited when I wrote this

Okay, else if equal six true

My challenge to you is to come up with labels that are like way easier than the labels that I came up with

All right, so we’re gonna do this I

Can almost guarantee that when I’m done copy pasting everything something is not going to compile because I forgot a label somewhere

Else if equals six true, okay

equals six true okay so what’s happening here again um uh so at this point you know the first

if uh expression was not true r12 was definitely not equal to five so we jumped down to else if

equal six begin which was here and so then we just make another comparison to see well okay it was

those two things weren’t equal it wasn’t equal to five so let’s check to see if it was equal to six

to yet another scope.

If it was false, we go down to line 237

and jump to yet another scope.

So here is what will get executed

if R12 was indeed equal to six,

and then we’re just basically gonna say it to the user

and then jump to the done label,

meaning like we’re totally done with our if else block.

Notice how the first if here,

when it was totally finished, it jumped to the done area.

this else if block is also jumping down to the if done area.

So eventually we’re going to need that label.

Okay.

So then next we’re going to check to see if the user’s input was equal to a 10.

So like kind of the same thing here.

We’re going to do another copy paste and we’re going to say, all right.

So if the user’s input was not equal to, let’s see,

if it was not equal to five, then we jump down here for our next comparison.

comparison we check to see if it’s equal to six if it was not equal to six then we jump down to

the less 10 begin line which is like all the way down here we do another comparison to see all right

well is it less than 10 you know if it was not equal to five or not equal to six then we check

is it less than 10 if it uh if it is then we uh jump to the else if equal less 10 true block which

I made this way too complicated. I realize that now, but it’s too late. I’m going for it, man.

Anyway, so if that statement is true, if R12 was indeed less than 10, then we jump to this block

and we basically just say that to the user and then we jump to the done. So finally, notice how

this part right here on line 259, it’s basically saying if R12 was not less than 10, then we’ll

then we’ll jump somewhere else notice how it’s just else right so this is like the very the very

bottom so i think the way that i wrote this uh code is uh we have two else ifs right we have like

an if five else if six else if less than 10 so maybe i could do something like this um if r12

r12 I think I said six just now right hopefully I actually did say six

otherwise if r12 is less than 10 then do some stuff otherwise if nothing else

matched then we’ll execute you know this block right here so again remember

every single scope has to have its own label so that you know where to jump and

it’s also a really really smart idea for every single scope to have you know

to have a little jump instruction that jumps past all of the if else if else if else stuff

so we’ll say like you know label you know done so that we can make sure that only one of these

blocks actually executes which is how you’re supposed to imagine c and c plus plus work

um and so it’s just complicated because there’s a lot of stuff to copy paste but you can just

still see you know only one of these blocks ever is supposed to execute so we give the first one

we jump into it if it’s true if it’s not true then we give the second one a chance

we jump into it if it’s true if not we jump to compare the third one if it’s true then we jump

to its scope if not we jump to the else and the else always executes if nothing else above

was true so that’s the basic idea here oh dear i’ve probably lost my place

looks like i just copy pasted less 10 true which was this right here so

so um less than true or less than 10 true okay so we told the user your stuff is less than 10

and then we jumped to the done area so that means we are probably working on the else area okay let

me grab that so now finally we’re going to have the else area which is going to you know finish

this all up that’s going to be after the if done so if nothing else matched then we will end up

nothing else matched then we will end up jumping to the else area and then we’ll basically

just tell the user none of the conditions seemed to have applied and then even at the

end of the else block even though you could probably get away with just letting execution

fall through just to save yourself one instruction you know you could comment that out assuming

you were sure that the very next instruction was the beginning of the done area but otherwise

I’m just going to play it safe and jump directly there then we have to make the actual done

So, just so you know, a label doesn’t have to have any instructions.

We could have something like this, if else test say goodbye.

We could have two labels right next to each other and one doesn’t actually have instructions.

That’s totally fine.

If you jumped to the done area, then execution would just fall through to the next valid

instruction which could go through another label.

So you know, we could have a say goodbye label and an if done label.

if done label, I’m just going to have the goodbye stuff happening inside of the done.

But for clarity’s sake, you might want to keep that label in there that I just deleted.

And all that we need to do at the very end when we’re done with everything is just say goodbye.

So I’m just printing a message.

So just to do a quick recap here, let’s see.

We come in with the user’s input as R12.

We ask, does R12 equal five?

If that’s true, we’ll say that it was equal to 5, and then we’ll go to the done area,

meaning we’ll skip past all the other blocks for the if-else block, or all the other scopes.

But if that did not equal 5, then we’ll fall through to that jump instruction,

which takes us to the next test, which is going to be our else-if equals 6.

So we are checking now to see if R12 is equal to 6.

If that’s true, we jump to this next block.

Why do we need to jump to a block that’s so close?

Well, if we don’t, then we’re going to end up definitely jumping to the next comparison

and you know, that wouldn’t work.

So we’re just kind of jumping over this unconditional jump statement and we’re saying, all right,

your input was definitely equal to six.

And then we’re jumping to the done area just past everything.

But if that wasn’t true, then we hit this unconditional jump that takes us to the next

comparison to see if the user’s input was less than 10.

You know, if that’s true, then we go to the true area to print another message and then

go to the done area.

If it was false, we fall through to the else jump, which will take us to here.

And notice how the else scope doesn’t actually make any comparisons because, you know, when

you have like an if else, sorry, when you have an if else if else if else if, you know,

any number of else if blocks, the else will always be executed if nothing above it actually

did execute.

execute so if you have an else block that means something will execute so we’re not doing any

comparisons we’re just saying you know when that scope is done we’re just going to jump to the done

area so all of these different scopes they’re jumping to the done area when they finish and

the done area is just this label right here where we say goodbye and that’s it let’s see did i copy

paste everything that i was supposed to i think so probably so let’s run the program and see if it

All right, it at least compiled.

So the basic if test, that was the first part of either this video or the previous video,

depending on whether I chose to split this up.

So let me comment out those calls real fast.

So I’m going to just comment out these calls real quick so we can only deal with the complicated

if else block.

So I’m going to enter like a three.

Whoops, let me do it again.

Three.

And so begin the if else test.

test your input was definitely less than 10 and then we end the if-else test so notice how only

one scope executed three was definitely less than 10 what other numbers did we have let me write

them down somewhere so I don’t forget oh they’re written down right here so we entered a three

which was definitely not a five and not a six so that’s why the less than 10 block executed maybe

only the first scope should execute so it says your input was equal to five notice how it didn’t

mention that it was a six or less than ten and then if we do a six it should just tell us that

we have a six it will not mention the five it will not mention the ten so we did we did five

already and then we did six and we did like a two which was less than ten we could also do a one

which was less than ten we can do anything that was less than ten and then the else would be if

And then the else would be if our input is probably like 11 or greater,

meaning it’s not less than 10 and it’s also not five or six.

Just we can see the else block.

So I’m going to do 11, which is the first number that should trigger the else block.

And it says no conditions were satisfied,

which was the message inside of our else block.

Let me just show you that again real fast.

This was the else block.

So message if else else or labeling on my port, I realize that.

But it basically says no conditions were satisfied.

The if else else.

I think that’s basically everything that I wanted to show you.

At this point, you should feel like you’re starting to become an expert at complicated

if else if else blocks, converting those from a higher level language into assembly.

assembly and I honestly recommend that you practice this like crazy while you’re trying

to get more used to it but you know hopefully you have everything you need at this point

keep in mind I’ve said this at the beginning I’ve actually I think I said this in a different video

when we use these branching instructions notice how I have jump equal to where’s that less than

yeah notice how I have a instruction jump less than the the family of conditional branching

conditional branching instructions that compares less than greater than less than or equal to

greater than or equal to the ones that I’m using here apply to signed integers and they won’t

necessarily work with unsigned integers and they won’t work with floats if you compare floats later

so just keep that in mind let me pull up my favorite book again real fast

where what the heck is that book oh dude where’s my document viewer document viewer can I get

document viewer can I get there there we go so I guess I didn’t introduce this

book at the beginning of this video but I probably should have this is my

favorite assembly book you can turn yourself into an expert with this book

by just on its own it’s open source and free the author gives this away he’s a

genius the person who wrote this book is a dr. Ed Jorgensen he’s a professor he

wrote this for his own classes and again it’s like a free open source book you

download a copy from his website if you look this up and and convert yourself

into an expert so let me collapse everything here and then I’ll go to

instruction set overview control instructions conditional control

instructions and I just want you to see real fast just as a recap I talked about

this in a previous video but when it comes to comparing things and checking

operands because if you want to see if two operands are equal you just check to see if

all of their bits are equal you don’t even really care whether they’re both integers or not

on the other hand notice how there is like a family here jump less than jump less than equal

to jump greater than jump greater than equal to that those instructions apply only if your

if they are unsigned or if you’re using floats for your comparisons you have to use this other family

of jump conditional branching instructions called jump below jump below equal jump above jump above

equal and i’m sure you can infer that jump below is the same thing as jump less than right jump

less than or equal to is the same thing as jump below or equal to it’s just that you need to use

a different instruction uh if you’re using signed integers or floats versus sorry sorry

a versus sorry sorry you need to use different instructions if you’re using signed integers

versus unsigned integers or floats so keep that in mind the code demo that i just showed you it’s

for signed integers if you were going to use unsigned integers or floats you would this

program wouldn’t work for you you’d need to replace my my branching instructions with the

the unsigned versions which i mean they work the same thing you’re still going to do a comparison

it still works. Anyway, so after having shown you that and making sure that everything actually

did work, I think we’ve talked about everything that we need to for this video. So yeah, I hope

you feel like an expert practice this on your own, you know, write your own programs just to make

sure you know how to convert a higher level language block into an assembly block. And you

should be on your way. Thank you for watching this video. I hope you learned a little bit of

bit of stuff and had a little bit of fun tell your friends and i’ll see you in the next video

hey everybody thanks for watching this video again from the bottom of my heart i really

appreciate it i do hope you did learn something and have some fun uh if you could do me a please

a small little favor could you please subscribe and follow this channel or these videos or whatever

it is you do on the current social media website that you’re looking at right now

It would really mean the world to me and it’ll help make more videos and grow this community.

So we’ll be able to do more videos, longer videos, better videos, or just I’ll be able to keep making videos in general.

So please do me a kindness and subscribe.

You know, sometimes I’m sleeping in the middle of the night and I just wake up because I know somebody subscribed or followed.

It just wakes me up and I get filled with joy.

That’s exactly what happens every single time.

So you could do it as a nice favor to me or you could troll me if you want to just wake me up in the middle of the night.

just wake me up in the middle of the night just subscribe and then I’ll just wake up I promise

that’s what will happen also uh if you look at the middle of the screen right now you should see a

QR code which you can scan in order to go to the website which I think is also named somewhere at

the bottom of this video and it’ll take you to my main website where you can just kind of like see

all the videos I published and the services and tutorials and things that I offer and all that

for clarifications or errata or just future videos that you want to see please leave a comment or if

you just want to say hey what’s up what’s going on you know just send me a comment whatever I

also wake up for those in the middle of the night I get I wake up in a cold sweat and I’m like

it would really it really mean the world to me I would really appreciate it so again thank you so

much for watching this video and enjoy the cool music as as I fade into the darkness which is

the darkness which is coming for us all.

Thank you.

The post Complex If-ElseIf-Else in x86-64 YASM Assembly – Full Guide with Code Examples appeared first on NeuralLantern.com.

In this hands-on x86-64 assembly tutorial, we dive deep into moving integer data between registers and memory, working with pointers, dereferencing, and pointer arithmetic using YASM on Ubuntu.

You’ll see practical examples of:

• Moving immediates and data between general-purpose registers
• Loading different sized integers (byte, word, dword, qword) from memory
• Why data size specifiers matter and what happens with overflow/underflow
• The difference between moving a pointer vs dereferencing it
• Pointer arithmetic to access array elements
• Using LEA for address calculation without dereferencing
• Debugging with GDB to inspect registers step-by-step

Perfect for anyone learning low-level programming, systems programming, or wanting to truly understand how pointers work at the assembly level. No prior expert knowledge required – we build it up with clear examples.

Code and examples are shown live, compiled with YASM, linked with LD, and debugged with GDB.

If you’re into assembly, reverse engineering, operating systems, or just love understanding how computers really work, this one is for you.

Introduction to Integer Data Movement and Pointers 00:00:00
Program Setup and Makefile 00:01:16
Data Section Definitions 00:01:38
System Calls for Output and Exit 00:01:52
Text Section and Entry Point 00:04:21
Printing Hello String 00:05:05
Moving Immediates to Registers 00:06:04
Copying Between Registers 00:06:27
Dereferencing Pointers from Memory 00:07:16
Specifying Data Sizes 00:08:01
Register Size Variants 00:09:45
Reference Book Explanation 00:10:50
Loading Words and Bytes 00:13:25
Debugger Breakpoint and Registers 00:14:28
Overflow Demonstration 00:16:25
Makefile Adjustments for Warnings 00:17:44
Proving Data Size Importance 00:20:32
Underflow and Overflow Examples 00:21:34
Symbols as Pointers 00:24:16
Array Definitions and Access 00:25:20
Pointer to Array Items 00:26:08
LEA Instruction for Addresses 00:27:32
Dereferencing Manipulated Pointers 00:29:01
Final Register Inspection 00:29:50
Conclusion and Subscribe Request 00:33:26

Thanks for watching!

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Hey everybody! In this video I’m going to talk to you about integer data movement and pointers

in an x86-64 YASM assembly program within Ubuntu but this should work for all of you x86 YASM people.

Okay so what am I talking about for starters? I’m just talking about moving data pretty much. If you

from one integer register to another or from one general purpose register to another.

And if you know how to manipulate pointers in assembly and all that stuff,

you probably don’t need this video.

But if you are not an expert, then this will probably help you.

Let me start off by saying, of course, that there are going to be lots of concepts in this video

that I don’t actually explain because I’ve explained them in other videos.

For example, this is not a Hello World tutorial where I show you how to actually,

you know, build a basic Yasm assembly program.

a basic yasm assembly program you can probably infer that just from this video but i’m not going

to explain it also this is not a debugging video so i’m not really going to explain too much about

gdb which is something i’m going to use to look at the registers but it might be nice for you to

get a little preview i will record a full gdb tutorial in another video anyway so let’s see

here uh for starters i’ve just kind of like created a little sample program here um well

a make file which will just compile and execute my program again this is not a

make file video tutorial check my other videos for that I’ve already made one

for now I’m just gonna start making a program and I’m gonna give it a data

section so first thing I’m gonna give it is like well let’s say I’ll give it a

bunch of data then I’ll kind of explain what I gave it again this is not a

basics Yasum tutorial so see my other videos but you know just simply put I’m

You know just simply put I’m gonna write to standard output so I do I remember the system call code for for writing

And then I’m gonna exit because this is a pure assembly program that I’m compiling. There’s this is not a hybrid program

We’re not doing GCC. We’re just doing LD

So I’m just gonna say a call code 60 to exit a file descriptor for stdin actually I don’t really need that

Let me just double check that I don’t need that in my solution

Yeah, I like to reuse these things so we don’t really need standard input

standard input. We’re just going to do standard output, you know, printing to the terminal.

We’re going to exit with a success code of zero. And then I made a couple of, well, just one C

string where I’m just basically saying, hello, my name is so-and-so. So you can know that the

program actually started in case it crashes later. That’s not my name. I just love those kinds of

names. And then I’m just going to make a bunch of integers of different data sizes. So this is kind

sizes but just real fast because these are going to be directly used in our video we uh we have a

quad word here so i’m defining an integer called my long int and i’m saying that it’s a quad word

with dq q for quad word and then i’m just giving a pretty big integer same thing for a regular d

word which is just a double word so that’s half of a quad word and then another thing with a whoops

w not a q let me just fix that in my solution oh dear then i’m going to do a word which is a six two

one one one a word is two bytes of data we’re starting to get dangerously into overflow territory

when i was originally writing this solution i had like a bunch of nines up here and the number that

i ended up seeing on the screen was totally different and i forgot oh a word is just two bytes

so even if it’s unsigned the highest thing that it can actually represent is um

65535. So that’s not good. Then I’m going to just store a byte here with DB and put some twos there.

And then I’m going to make an array of integers. Putting an array, making an array in the regular

data section, that’s not as good as putting it in the BSS section if you want a lot of integers.

But I’m just going to specify a small number of integers so I can just put it directly into the

BSS is for like giant arrays of uninitialized data and the data section is for whatever you can handle typing pretty much of initialized data.

So now I’m going to start my text section and my program’s entry point.

Again, this is not a Yasm basics tutorial. See my other videos.

And so we’re going to start by just saying here’s my entry point where the operating system can sort of jump into my program.

to just you know exit the program let’s actually see if that works so far it

should do nothing but but at least exit with success I’m gonna go clear and make

run and notice how it just kind of like ran it and then nothing happened okay so

that’s great how come it didn’t print though what did I do wrong oh I forgot

to copy paste the code that prints something okay let me get my solution up

here so the first thing we’re really going to do is just use a system call to

call to print the word hello again this is not a system call video see my other videos

and um so now we’re just we’re just going to print our hello string

with a system call let me run it one more time we should see like a quick hello

yeah okay my name is uh chaplain mondover it’s not maybe i should change that to it’s not my name is

not no i don’t want to do that sometimes i like to pretend that my name is chaplain mondover

name is check lane Mondover you should too it’s fun anyway so the first thing that I’m going to

do here is I’m going to move an immediate well let me copy paste a whole chunk of code and then

I’ll explain it one by one I’m basically going to use data movement instructions to move a bunch of

data into a bunch of registers and then I’m going to hit a break point or I’m going to I’m going to

sort of do like a non-operation piece of code so that I can easily break on it in my debugger so

the results. Okay so first off I’m going to move an immediate into R12. The

instruction for moving data into general purpose registers or the integer

registers is just MOV so that’s fine probably you should know that by now but

I’m going to move an immediate into R10 and I’m going to move 12876 into R10.

So that’s how you move an immediate no problem. Then I’m going to first clear

out R11 and then I’m going to move R10 into it. The reason I’m clearing out R11

reason i’m clearing out r11 first is just to prove to you that we are actually moving data from one

register into another because if i just i don’t know if i didn’t do that you might be tempted to

to think well um maybe r11 already had that data in it so first it’s going to be just a zero and

then it’s going to actually be whatever r10 had so we should see r10 and 11 both have one two eight

register or a pointer register from one to another. And by the way, integers, those are

pointers, or rather, should I say, pointers are integers. So if you’re moving an integer or you’re

moving a pointer, the system just kind of sees that as a number either way. So you can use the

move instruction for it. So now let’s use the pointer that was assigned to our long integer

to move that number into R12. So let me just go up real fast. My long int up here,

my long int up here it’s like a gigantic number and remember I said before that my long int is

actually just a pointer to the memory location that begins to hold the quad word so my long

int is really a pointer to one byte and there’s an eight byte allocation because it’s a quad word

so when you sort of put this into your code and you let the system know that you want to move a

then the system will scan eight bytes and interpret that as an integer so

putting the the keyword quad word here I don’t think that’s necessary I usually

do it anyway but you’ll see in the next few instructions that it’s a really

really good idea to specify the data size because if you don’t you’re gonna

actually get something wrong in other words if for some reason I moved I don’t

into R12 I could do it if I if I wrote it correctly but then I would be taking

part of a quad word and trying to interpret that as the entire number and

it’s probably going to be totally wrong we’ll have an example for that in a

moment but anyway so that’s how we move a quad word we just specify keyword right

before the memory location and we use the brackets to dereference if we

didn’t dereference then we would actually be moving a pointer to the long

integer into R12 we’ll do that soon too I’m going to try to do everything in this

to do everything in this video then we’ll move a d word from uh from memory into r13 this is a

little bit different notice how uh here on line 60 i kind of have to specify d word because that’s

the data size that i’m trying to move into the r13 register so this is telling the system that

there’s about four bytes that i need to scan not eight bytes and interpret those four bytes as an

Moving it into R13, you know, I could have sworn a long time ago, you could just move that sort of thing directly into R13.

But when I tried my solution before recording this video, I kept getting assembler errors until I added the D at the end.

And so I just want you to know, oh, you know what, let’s look at a book real fast.

Let’s look at my favorite book.

I just want you to know that there are other forms of every register that can specify the register’s data size.

What do I mean by that?

itself then the system thinks you’re talking about all 64 bits of the 13 register the r13 register

but if you type whoops if you type r13d then the system thinks you’re only talking about the lowest

32 bits of that register so you’re also able to specify the data size of a register again i thought

you should should a long time ago i thought i remembered moving a d word directly into r13 and

it would just clear out the extra bits but i got warnings this time maybe that’s for the best

that’s for the best forcing us to be more precise, but this will solve it. R13 just says,

let’s only talk about a D word. So let me go, let me go to my favorite book here.

Where the heck is my favorite book? Okay. I guess I have to like, there we go.

Okay. There’s my favorite book. So, um, I’m going to go to, uh, let’s, let me just search for,

just search for I can’t remember where it is our 13 D yeah there it is okay so

what we’re looking for is 2.3.1.1 let me show you real fast the top of the book

just to give a shout out to the person who wrote this book I did not write this

book this is a free and open source book that you can download for free

everybody should get a copy to turn yourselves into an expert it’s called

x86 64 assembly language programming with Ubuntu it’s written by a brilliant

professor. This is like a semi recent version. There’s probably a newer one now, but yeah,

it’s a great book. I love it. So I’m going to go back to what the heck was I just doing?

Was it 2.3.1.1? Yeah. Okay. So in this book section 2.3.1.1, the section entitled general

purpose registers, notice how there are a bunch of different versions of each register. So if you

So if you look in this column right here, we have, oh, I get to use my annotator.

We have a, what were we just looking at?

R12.

Notice how R12 here is a designated as a 64 bit register.

But if you wanted to use the R12 register and only use 32 bits of it, the lowest 32

bits, then this is the form you would use for the R12 register.

So 32 bits.

And if you wanted to use only a word’s worth, you know, 16 bits.

Well there you go.

two bytes on these systems or 16 bits and then if you just wanted to use one

byte of the register well you can you just put a B at the end of it it’s

pretty sweet and convenient it’s not too hard to remember that D is for D word and

W is for word and B is for bytes it just gets a little confusing when you’re

using the other registers like the the RAX register it’s got EAX as its 32-bit

version and then AX as its 16-bit version and AL as its 8-bit version if

it’s a 8-bit version if you can remember that there’s an L I guess that’s not too bad but you

know remembering the X and then it changes to an I over here and a P over here I’m personally not

a fan so I always come back to this book to try and remember what I’m supposed to be putting for

these registers because I can’t remember all the time or rather I can almost never remember but we

So, I want to take a D word from memory and load it up into the R13 register.

So I got to specify the R13D.

And, you know, maybe I’ll take that out and show you that it doesn’t compile later, if I can remember.

But I’m going to now take a D word, do the same thing into R13.

Oh, sorry, no.

I’m going to take a regular word and do the same thing basically into R14.

But it’s just going to be a word instead of a D word.

And then I’m going to take a single byte and load it up into R15.

single byte and load it up into R15. Then I’m going to call Nope, which is an instruction that

just does nothing. And it’s useful sometimes if you want to have a place where you can stop your

program and sort of inspect the results of your program in your debugger. So there’s a lot more

to this program. I’m going to go ahead and just run it right now, just to make sure that it compiles.

And then maybe I’ll add a breakpoint here. Let’s go clear and make run. Okay, so it ran,

Okay, so it ran, it compiled, it was fine.

And now I’m going to open up a new little window and I’m going to launch the program.

Notice how the program that has been compiled is called main.

So I’m going to launch the program into the GDB debugger.

This is not a GDB video.

So I’m going to be skimming over this, check future videos for a full GDB tutorial.

So I’m going to do a break point right there at line 72.

So that I can just kind of inspect the state of the register.

break at main.asm line 72 and then just to be sure I did that correctly I’m going to say info

breakpoints or just a b is fine and it’s like looks like I set it up correctly then I can type

run to see what’s up notice how it starts to run and immediately stops at that breakpoint

you can tell it stops there because it’s at line 72 then I’m just going to do info registers or

info r just to print out all my registers and we should see a confirmation of what we were hoping

so if I kind of like maybe I’ll pin this to the top for a second always on top how about that

so first we moved an immediate into r10 so notice how 12876 is in r10 right there and then on line

53 we basically copied r10 into r11 so that means in r11 we see the same value

and then we’ll move a quad word from the long integer we’ll dereference that pointer

at R12 it’s like this gigantic number if I scroll all the way up it’s the same number here as in our

source code so that seemed to work and then we’re going to move the D word next I guess into R13

so that’s this number right here so that seemed to have worked it’s this number right here the

D word and I’m going to get lost so fast scrolling up and down like this now we’re going to move a

one one and there it is right there and then r15 is just gonna take a bite and

it’s gonna take a bite oh now I’m hungry now I’m hungry that activated it it’s

only been like think four or five hours since I ate I guess that is usually when

I need to eat again even though I gorge and fall asleep on the floor because I

home r15 anyway holds the correct value it’s 222 i just want to show you overflow real fast

we look at r14 uh we’re using the word version so it’s only going to read you know uh two bytes

from that memory location and it’s only going to consider uh you know 16 bits as being valid in uh

in that register so if i use a number that’s too big it’ll overflow two bytes the maximum number

three, five or from zero to that number or six, five, five, three, six combinations.

So if I just stick a nine in front of that right now, that should overflow the system.

So let me try it one more time.

I want to do Q because that was the last thing we needed to look at.

And then I’m going to do make.

Oh gosh, what’s happening here?

Okay.

How about I do clear and make build, which is in my make file and GDB main.

So I don’t have to type all this stuff out again every single time.

have to type all this stuff out again every single time what did i just do oh nice okay you know what

i swear it didn’t used to do this before it’s giving me a nice warning saying that value does

not fit in a 16-bit field i guess if i turned warnings off which i don’t want to do then the

program would run and it would overflow eh do you want to turn it off let’s turn off warnings okay

much attention to it. Um, where the heck are the warnings? Where’s the assembler running? There it

is. Yasum flags. And that’s, so that’s my variable up there. There we go. Okay. So I’m just going to

make a copy of this line, comment out the original, and then I’m going to take out, uh, the command

that says, or the flag that says convert warnings into errors. Your compiler is your friend. It was

So I am now going to do a breakpoint at 72.

So break at main.asm972 and then run and then it breaks.

And then I go input registers.

Then if I look at, what was I just talking about?

R14, was that it?

Notice how 44607 for the word, that’s a totally different value, right?

So this is what happens when you overflow or underflow.

You got to be careful about your data sizes.

I’m just going to do this again one more time just to make sure.

one more time just to make sure that it’s fixed now that I moved it and I’m

gonna I’m gonna read that part about warnings because I love converting

warnings to errors as just a way to help myself write better code okay so let’s

do that and you know what I’m gonna do too I added another target here again

this is not a Yasm or sorry this is not this is kind of a Yasm video this is not

extra file called gdb.txt where I can stick gdb commands and so I don’t want

to continue to type my breakpoints over and over over again every single time so

I know for sure I’m not going to change the program anymore at least I’m not

going to shift the lines up and down before 72 so I’m going to do breakpoint

you can type like break or I think you can type either break or the full word

breakpoint but I just type B name of the source code file a.asm at line 72 and so

And so this file right here, again, this is not a GDB tutorial, but in my make file, I have it set up so that GDB will execute whatever commands I put into this text file.

So it just kind of saves me typing.

So let’s put a breakpoint there and then we’ll say info breakpoints and then we’ll just run the program in that way.

I can have a little bit more information.

So I’m going to do make build instead of make build and then running GDB.

I’m going to say make debug.

That’s another target in my make file.

make file so then you can see it added the breakpoint for me and then it

immediately broke on that and you know what yeah I guess I’ll just leave it in

where I have to type info registers okay so we’re just making sure I think that

R14 is still a valid value and that it was fixed okay so we’ve gotten this far

the next thing we need to do is prove that data size matters by specifying a

specifying a bad data size for our long and then for our byte. So I’m going to copy paste a little

proof here that is followed by another non-operation. So I’m going to do this. And so

I’m going to try to prove to you that the data size matters by specifying a bad data size. I

just proved to you that it kind of mattered or it definitely mattered in terms of overflow and

underflow, but I’m trying to prove to you now that it matters when you’re reading. So I’m going to

I’m going to clear out r12 and then I’m going to move a byte from the long integer into r12b

and you know this would be a warning or I think it this wouldn’t compile or wouldn’t assemble if

I didn’t have the b there so I’m basically telling the computer all right I want you to use only a

byte of r12 and I want you to take one byte from memory and just put it in there but I’m taking it

awful. And then right after that, I’m going to do another naughty thing. I’m going to move

something into R15 and I’m going to word, I’m going to move a keyword of memory into R15,

but I’m going to be taking it from the bytes memory. So remember a byte is just one byte.

And so the next seven bytes that come after it definitely have nothing to do with that byte,

but I’m going to interpret them all as a quad word. So we should get nonsense data

also R15. I don’t really need to zero out R15 because the fact that I’m specifying R15 means

I want to use all 64 bits. So it is going to definitely zero it out. And whatever we see in

R15 is going to be the real thing. But in terms of R12B, we’re only going to be loading up one

byte’s worth of that register. So that’s why I’m zeroing out the rest of it just to prove to you

that we are truly getting nonsense. So now there’s going to be another break point at 82.

I’m not going to break anymore at 72. I’m going to break at 82. So let’s do that.

What’s going on here? We got that.

Oh gosh. Okay. Quit. I guess that means I need to comment this out. Let me do that.

We’ll do break at main.asm at 82. And you could, you could do both of these two. Actually,

let me do it first. We could, we could do multiple break points. Yeah, I’ll just do it first,

So notice how it first breaks at the 72 and then I do C to continue and then suddenly

it breaks at the 82.

But I don’t want to have to hit continue a bunch of times so I’m going to comment that

out for the future.

Let me quit real fast to make sure I remember what GDB expects as a comment.

Yeah, okay.

82.

Okay, so then I’m going to print the registers info registers.

And we’re looking at R12 and R15.

You can see R12 has the number 144, which definitely doesn’t make sense because there’s

no 144 anywhere in our original data.

in our original data so hopefully that’s proof to you that we’ve grabbed some junk

sometimes maybe when you’re doing this at home if you’re just going to grab one bite

maybe accidentally um no no that would definitely still look bad but for the r15

if you’re going to grab a q words worth of data i don’t know maybe the junk data is like

accidentally correct in a way where it would it would still make it look like the intended value

the intended value no it wouldn’t there’s no way that would even happen no i guess the junk data

could could work that way anyway so we’ll look at r15 two eight and a bunch of fours and then a two

three eight um that value does not exist up here so again junk data mixed in with real data is is

bad news okay so now let’s do another proof i want to prove to you that symbols are pointers

you know like I’ve been saying and they need to be dereferenced so let’s first start off by

moving the raw pointer for my long int into r10 and then into r11 we will dereference that

pointer so in r10 we should see something that looks like a memory location or at least a relative

memory location and then in r11 we should see the actual long integer like we saw before

And then I’m going to mess around with some arrays.

So remember up here we made an array.

Let me minimize that real fast.

We made an array, a very small array.

I just specified a bunch of ones and then specified a bunch of twos.

So it should be contiguous memory since it’s been allocated as an array with that comma

there.

We should see if we looked inside of the system inside of memory, we should see eight bytes

representing the ones.

And then right next to that, we should see eight bytes representing the twos.

easy for us to sort of move our pointer around a little bit. So what else do I need to copy paste

here? We got that long int. Okay. So I’m going to grab the first item and the second item in those

arrays just using some pointer manipulation. So you know how to do this already. You just take

like the pointer to some number and you just dereference it and you end up getting a number.

I understand your feeling because you might be thinking, wait a minute, wait a minute.

I understood it when it was dereferencing just one number, but didn’t you say this is an array?

Why is it we can dereference the array and just get one number still?

Well, remember, a pointer to an array is really a pointer to the first item in the array.

That’s kind of how it works.

So either way, whether you consider that to be a pointer to one number or a pointer to array,

when we dereference it, we’ll get one number.

If you want to get the other numbers, then you have to start manipulating the pointer

or derefing in different ways.

So here’s how we can get the second item.

We deref the original pointer plus 8.

8 because it’s a quad word.

So, you know, it probably would be a little smarter to put that 8 up into a define somewhere

so you’re not hard coding numbers, but I’m not going to do it.

So basically in R12, we should see the 1s.

In R13, we should see the 2s.

And then a couple more things.

Next, we will grab a pointer to the first item in the small array.

And we’ll just stick that into R14.

So, you know, we kind of did that with R10 already.

But now I just want a pointer to the small array.

And I’m going to keep it in R14 just so you can see the memory location.

And then after that, we’re going to grab an actual pointer to the second item.

hey how do you get a pointer to the second item well it’s that but then what

if you didn’t want to dereference the pointer like what if R13 you know here

clearly because we have these brackets for dereferencing you’re allowed to put

the formulas inside of the brackets what if I wanted to get the address to the

second item and not actually the second item we would not be allowed to remove

the brackets the assembler wouldn’t like that so without another instruction

way to actually get a pointer with a complicated mathematical formula. You know, you can do like

multiplication inside of here and other things in parentheses. So we wouldn’t be able to do that

without the other instruction that I’m introducing now called LEA. LEA just means, yeah, sure, we’re

still going to use the brackets to figure out where the pointer is, like figure out what memory

location we want with some sort of a formula. But then what will be stored in the first operand is

This LEA won’t dereference just because there are brackets.

LEA will keep the pointer.

And then finally, once we actually have a pointer to the second item in R15,

then we can move the second item into another register.

And this is like an LOL because I ran out of registers already.

So I’m just going to use RDI, which is usually the first argument.

It doesn’t matter. It’s fine.

So if we dereference R15, it’s the same thing as dereferencing this.

We could have also just dereferenced R14,

and it would have been the same thing as just getting the value of the first item.

But I hope you can see now that, you know,

that expression is going to be a pointer to the second item in the array,

and that’s going to be stuck into R15.

So if we dereference that, we can treat R15 like a pointer.

It’s just, you know, it’s a general purpose register.

it doesn’t only need to hold integer values it can also hold pointers because pointers are integers

they’re just unsigned so we deref the pointer that is stored inside of r15 now put it inside

of rdi and then we should see that rdi actually holds the second item which should be a bunch of

twos so the last thing that i’m going to copy paste in here is another nope so i’m going to

break at 108 just so we can see the rest of it so i’m going to open up this right here and i’m

open up this right here and I’m going to comment that out and I’m gonna say break at 108.

Whoops.

Okay.

There we go.

But, but, but quitsies.

Now I’m just going to run make debug again so that I can enter GDB.

And we’ve, we’ve, we’ve hit our break points.

And now I’m going to go info registers and just talk about the stuff that we’re seeing.

Okay.

So the first thing I think this is all on one page, right?

I don’t need to scroll.

Yeah.

one page right I don’t need to scroll yeah okay this is all on one page first thing we see is r10

holds this value right here that’s a pointer to the my long int that kind of looks like a pointer

if you consider that it could be relative and then r11 holds the actual integer and if you recall

that is the gigantic long that we put in there great so now that’s the difference between a

pointer and its value or in its dereference value so then we’re going to grab the first item in the

So then we’re going to grab the first item in the small array.

So R12 should have the ones.

If we look at R12 here, it’s got the ones.

Again, just trying to prove to you that if you dereference a pointer to an array,

you’re actually dereferencing a pointer to the first item.

And then on R13, we’re going to make a pointer to the second item.

Then with the brackets and the move instruction, we dereference that and it should become the twos.

So if we look at R13, notice how it’s the twos.

Then we’re going to grab a pointer to the first item in the small array.

Hang on a second.

What did I want to say about that?

In the small array to R14.

Okay.

So again, we’re just going to grab an actual pointer to the small array.

And so the difference between R12 and R14 is the dereft value or the pointer.

So if we look at R12, that’s the actual value that we’re pointing at.

And then R14 is the memory location that we have in there.

And then notice how it’s kind of similar.

Notice how it’s kind of similar.

Maybe this proves a little bit more that these are pointers that we’re looking at.

R10 was supposed to be a pointer and now R14 is also a pointer.

Notice how those numbers are pretty close together.

I’m sure if we looked back up in the data section and we calculated an offset based

on how big each data item was, we could probably predict what the memory location of R14 should

have been based on R10 because you can see it’s like only increased by a little bit,

by a little bit right it was like 534 to 49 what is that 10 plus 5 so that’s like 15 is that correct

oh I guess I had a byte in there so maybe it’s that’s why it’s kind of odd but yeah we can we

can just compute the offset and then we’ll grab a pointer to the second item using the LEA

instruction remember we have to use brackets if we’re going to do an expression but then the

dereferencing in the move instruction so we just use LEA to not dereference.

So we should see that R15 is about 8 bytes further along in RAM than R14 because both of those items

are contiguous in memory and they should only be about you know a quad word apart 8 bytes.

So if we look at R14 it’s got this number so it’s a pointer and then if we look at R15

you know what’s 49 plus 8 it’s 57 so you can see that the pointer to the second item is only

pointer to the second item is only eight bytes further in memory yay contiguousness and then

sorry i can’t resist so then finally we will uh we’ll look at the dereference to r15 because it’s

now a pointer what was it it was the it was a pointer to the second item that should be a bunch

of twos right because remember in our array we had a bunch of twos here as our second item

have a bunch of twos inside of it so if i look at rdi where the heck is that right there has a

bunch of twos inside of it rdi baby oh my gosh why did i say that anyway so that’s our non-operation

instruction and i think that’s everything that i wanted to talk to you about in this video so now

i hope you feel like you’re an expert in moving integers and moving pointers and

dereferencing pointers and manipulating pointers so you can get a pointer to something else and

pointer to something else and de-referencing those pointers or not whenever you choose.

Thank you so much for watching this video. I hope you learned a little bit of stuff

and you had a little bit of fun. I’ll see you in the next video. Limit out. I’ll never say that

again. Hey everybody, thanks for watching this video again from the bottom of my heart. I really

appreciate it. I do hope you did learn something and have some fun. If you could do me a please,

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