Quick but complete guide to writing proper functions in YASM x86-64 assembly on Linux. See how to pass integers, pointers & floats, return values, follow the ABI, disable C++ name mangling with extern “C”, and call back and forth between C++ and assembly in a real working example.

Great for people moving from NASM/GAS or trying to mix assembly with higher-level code.

00:00 Introduction and Video Overview
00:28 What Are Functions in Programming
00:56 Why Functions Are Harder in Assembly
01:24 Topics Covered in This Video
01:59 About the Makefile and Prerequisites
02:28 Hybrid C++ and Assembly Program Plan
03:01 Using extern “C” to Disable Name Mangling
04:03 Main Driver Function in C++
05:00 Adding a Callable C++ Function for Assembly
05:42 Explaining extern “C” Placement
06:16 Assembly File Skeleton and Data Section
06:41 Creating Null-Terminated Strings
07:49 Section .text and External Symbols
08:52 Declaring my_cpp_function as extern
09:20 Defining my_assembly_function
09:40 Labels vs Real Functions
10:31 The call Instruction and Return Address
11:16 Why Jumping Instead of Calling Crashes
11:47 Global Directive for Exporting Functions
12:32 Basic Function Structure
13:20 Implementing my_assembly_function Prologue
14:50 Receiving Arguments in ABI Registers
16:30 Printing Received Integer Arguments
18:10 Handling Pointer Arguments (C Strings)
19:40 Passing Floating-Point Arguments in XMM
21:15 Printing Floats from Assembly
23:00 Calling Back to C++ Function
25:40 Preparing Arguments for my_cpp_function
27:20 Loading XMM0 and XMM1 for Floats
29:10 Making the Call to C++ Function
30:50 Receiving Double Return Value in XMM0
32:30 Saving Returned Float to Memory
34:10 Printing the Returned Value
36:00 Final Messages and Program Flow
38:20 Fixing String Pointer Crash Issue
40:00 Correcting Argument Loading
42:10 Passing String Owned by Assembly
44:00 Observing Successful Output
45:47 Saving and Restoring XMM0 Safely
47:14 Printing Final Returned Float
48:32 Importance of Following the ABI
50:29 Summary of Covered Topics
51:03 Closing Remarks and Call to Subscribe

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

Thanks for watching!

Find us on other social media here:

• https://www.NeuralLantern.com/social
• Twitter / X: https://x.com/NeuralLantern
• Rumble: https://rumble.com/c/c-3696939
• BitChute: https://www.bitchute.com/channel/pg1Pvv5dN4Gt
• Daily Motion: https://www.dailymotion.com/neurallantern
• Minds: https://www.minds.com/neurallantern/
• Odysee: https://odysee.com/@NeuralLantern:5

Please show your support!

• Buy me a coffee: https://ko-fi.com/neurallantern
• Subscribe + Sharing on Social Media
• Leave a comment or suggestion
• Subscribe to the Blog: https://www.NeuralLantern.com
• Watch the main “pinned” video of this channel for offers and extras

Hello there.

In this video I’m going to teach you how to write functions in YASM x86-64 assembly inside

of Ubuntu, although probably any YASM assembler will be fine.

I’ve covered these topics before in other videos, but I thought it would be nice to

put them all here in one single video to make it a little bit easier to understand.

Okay, so first off, what the heck am I talking about?

Well, you know, when you have a program, let’s say we have a higher level language program

and we have like, you know, void or actually probably not void main, let’s say int main

and we’ll forget about the arguments and inside of main, it just sort of calls f and then

we have an f function over here and it, you know, does stuff, right?

So that’s the basic idea of having a function that you can call and calling it.

We sort of take the whole process for granted in higher level languages, but in assembly,

in assembly we have to do a lot more from scratch so I’m going to show you about how to pass

arguments how to pass integer arguments pointer arguments floating point arguments return types

you know double like floating return types integer return types pointer return types

how to use special registers to actually pass the arguments into the functions how to write a

so that’s basically what we’re going to do before before i get any further here i just want to point

out that um this video is not about the basics of assembly nor is it about the basics of um

make files or some of the other related technologies that i’m going to show

i’ve covered everything in this video in a previous video um so if you if you find yourself

getting lost and you don’t understand what i’m saying or what i’m doing you probably want to

look at my previous videos because this is sort of a summary video to kind of help you

of help you help you really lock down on the concepts of writing functions okay so for starters

this right here that you’re looking at is a make file this is not a make file video if you want to

learn how to make make files or why they’re awesome see my other videos but i’m just going to assume

at this point you’re either willing to research my other video or you already know or you don’t care

how to make a make file so you know we’re just going to skip this i’m going to close it

function or sorry a driver module and what I’m going to do is I’m going to write this program

that I’m showing you now as a as a hybrid program a program with modules of different languages so

this module right here is going to be c++ and then we’re going to have an assembly module and

then they’re going to be able to call each other so the first thing that I’m going to do is I’m

going to name some functions that are going to be inside of my assembly module and also name some

module and also name some functions that are going to well name a function that’s

going to be just inside of the C++ module but it should be callable by the

assembly module so for that we use a little block called extern C and long

story short that disables name mangling which C++ does in order to provide

overloading functionality so we’re basically going to disable overload

functionality in order to have simple function names that way assembly can

and also when we attempt to call a function inside of assembly we’ll just call it by its

simple name rather than try to mangle the name based on the the arguments and return type and

such so that that’s something we have to do for compatibility at least for this video and I’m just

going to move on so basically you just take you just take the keyword extern and you put a c in

a quote and then you make a block right a little braces scope and then you just list prototypes of

types of either the functions inside of the current module which you would like to demangle

or other modules functions that you would like to be able to call in a demangled or

I guess a non-mangled way.

Mangled meaning name mangling.

Okay, so then I’m going to copy paste the main function that I’m going to do.

So this is a hybrid program, which means you know there are several different modules of

different languages.

driver which just launches the program and it contains like the entry point

like see how there’s like the the main entry point which if you know how to do

other languages this is usually where you start in C++ or like the GCC

libraries so this is the entry point for our whole program and then once we’re

inside of here we’re just kind of print out a message to the user we’re gonna

make a C string and then we’re gonna call on the assembly function we’re

gonna give it you know some numbers some integers and some floats and we’re

gonna give it our C string and then when we’re inside of the assembly

and then when we’re inside of the assembly function we’re just going to print those things to prove

that we know how to call and we know how to receive data from c or c plus plus into assembly

and then we’re just going to print you know goodbye basically okay so uh i’m going to make

another uh let’s see another function in the driver so that assembly has something to call

because i don’t just want to be able to call assembly from c i also want to be able to call

put in something that we can call on from assembly here.

I’m going to call it my CPP function.

And it’s just going to have like some arguments,

just some nonsense arguments.

And all it’s going to do is just print out that it’s entering,

print out the arguments and then just sort of say goodbye.

And then just return a value to the caller.

We’re going to return a floating point value to the call.

And we’re going to return a floating point value to the caller.

floating point value to the color okay so notice how at the top here i have uh in the extern c my

cpp function it’s the same thing that we see at the top is now at the bottom and that just means

uh this normally would be compiled and linked with name mangling and so because it’s in the

extern c block there’s no name mangling and the reason that works with an extern c block is

because there’s no name mangling in c that’s sort of like a c plus plus thing with overloads and like

whatever okay so we have like the basics for our driver it’s a simple module the hard work is going

to be inside of assembly so uh maybe before we write assembly oh you know what let’s write the

skeleton of the assembly and then i’ll start explaining uh what it means to call back and

forth and to make functions and things like that okay so for starters i’m just going to make a data

section with a bunch of stuff inside of it and i’ll explain it just very briefly again this video is

basics of assembly in Yasm. If you want to learn the basics, go see my other videos.

But for now, I’ll just assume that you understand why we’re making variables in the data section

and kind of how to do it. So I’m just, you can see here, I’m making a bunch of null terminated

strings. They’re null because they have a zero at the end. And I’m just saying, well, here’s a

message when we begin the program. Here’s a message to announce we’re about to print some integers,

to print a C string, to print some floats. Here’s another message for this, another message for

messages right then a crlf that’s just you know taking the cursor down to the

next line and then back to the very beginning so it’s like a basically a new

line and then I’m gonna have some floating point numbers and I’m gonna

have an a and a B number and then I’m gonna have a B and a D that I’ll send

somewhere else and so I’m gonna populate them with numbers from the start the B

and the D that just are not being sent they start at zero because we’re gonna

we’re going to receive those numbers from C and then we’re going to get a return value as a

floating point from the C function that we call so we’re just going to store it there so I’m just

like making a bunch of room to store some from some variables and then here we’re just going

to do system right and we’re going to send to standard output that’s covered in other videos

but basically we’re just going to be printing stuff okay so section text now this is where

actually starts. Well, sort of. This is where the program contains instructions to execute.

So section text. And the first thing I’m going to do is I’m going to name some external symbols

that I have for myself just to help me print integers and floats. So, you know, if you’ve

seen my other videos, you know that you can do a wide variety of things to sort of print characters

and numbers if you were clever. And you can also call on C library functions like printf.

functions like printf if you really wanted to print floating point numbers

and signed integers and stuff so you don’t have access to this library so

don’t worry about putting this part in your code if you’re working at this from

home unless you’re someone that has taken a class that I’ve taught somewhere

and for now just just assume that this just lets me print very easily no

problem you don’t really need that if you’re just kind of experimenting with

function this is the name of the function that we made in the driver my cpp function notice how it’s

over here my cpp function we’re just letting our assembly module know that there’s a function in

another module that we should be able to call on even though all these modules are going to be

linked into the same program we still have to do this just to let the assembly module know

okay so now we need to make a function called my assembly function if you look at the top right here

We had this other D name mingled prototype, my assembly function.

So we have to actually write that somewhere.

It’s not in the driver.

Let’s write it inside of assembly.

So my assembly function is going to start right here.

Boom, right there.

So this is when we kind of start on the basics of writing a function.

So for starters, hopefully you understand labels.

If you don’t understand labels by now, you might want to see my other videos.

just some sort of an alphanumeric symbol that you just kind of write as sort of

sort of like a variable name.

You can’t use every symbol available.

I would just start with using letters and numbers in the underscore.

Of course, you can always experiment if you want to see what’s allowed and what’s

not just type something and see if it compiles.

So the label starts with just, you know, this name and then a colon, right?

So that’s a label.

You can jump into labels.

Like if we were somewhere else in the program right now, we could like, if we

we could like if we wanted to do an infinite loop we could say jump and then the name of the label

and what would happen is execution would go back up to this label and then it would jump again

it would go back up it would be an infinite loop but we wouldn’t technically be calling a function

we’re just sort of jumping so labels are sort of the first step we have to do a little bit more to

actually make a function so we have an assembly function which right now is just a label and in

side we have to mark it as global so remember when we marked my CPP function

as extern that means we can call a function that lives somewhere else now

we’re marking this function as global which means it lives here but we want

other modules to be able to call on our function or our label okay so how do we

actually make a function let’s see the first thing that you should do is try to

understand that if that all functions return even if they don’t return any any

Under the hood, what this is actually going to do is crash the program if you just jumped

into this label and didn’t call it like a function, because the return instruction will

look onto the call stack and try to find a return address for wherever this function

was called from and then go jump, do a jump instruction to return there.

And so if you just jumped into this label and then there’s a return, it’s going to pop

something up the stack that’s not actually a return address and probably crash the program.

So that means now that we have a return statement, we definitely have to call this like a function.

And I guess that’s pretty easy because, well, if we were inside of assembly and we wanted to call another function,

let me just show you real fast.

I’ll say some other function.

If we were inside of this label, for some reason, we could call the above function by just saying call.

And then the label.

So now it’s being treated like a function.

it’s being treated like a function.

What the call instruction really does is it just looks at the address of the next instruction

that would have been executed.

So, you know, whatever, let’s say we have like a nope on the next line, whatever the

memory location or the relative memory location is of that nope instruction, it’s going to

push that onto the call stack.

And then it’s going to just do a jump instruction to this label.

So then later when we hit this return statement, then it’s going to look onto the call stack

and it’s going to find that return address, which is going to correspond with the nope

instruction and the return instruction is going to do a jump back to that address.

So basically you have to use the call instruction to get there and then use a return instruction

to return from there.

And then now we can actually call it a function.

But then there’s other things that functions have to do to behave themselves without crashing

the program.

So for example, notice up here that I have copy pasted the let’s see call on assembly.

Oh, I think I forgot to update that.

I forgot to update that. Let’s see. Call on this. Yeah. It’s supposed to be called my assembly

function now. Hold on. Let me update my, uh, my solution. Whoops. Hold on. Call on.

There we go. My assembly. Okay. So I’ve got my solution up above on another monitor.

So I have the prototype of the function here. Uh, it’s just going to return nothing. And it’s

going to like take in a long and a double and a long and a double and a character pointer.

And then my intention here is to save these arguments with some registers.

So notice how I’m saying we’re going to use R12, 13, and 14 to save the arguments A, C, and E.

And we’ll talk about why we’re not saving the other ones with those registers in a second.

But basically we’re going to use those registers.

So the thing about the registers is we have to do something called respecting the ABI.

Respect the ABI.

The ABI stands for the Application Binary Interface.

interface and it’s just sort of like a standard that governs all of the things that you can do

and you’re supposed to do when you’re working with x86 64 assembly so the abi is pretty cool

hang on a second the abi is pretty cool because it actually standardizes things

for instance if we didn’t have the abi and we didn’t respect it then we couldn’t actually call

plus or vice versa because c plus plus the higher level languages they’re going to use the abi so if

you try to do things your own way then all you’re going to end up doing is wasting time and energy

coming up with two different ways of doing things when you could have just done it the abi way

right so and one of those and one of those ways your way wouldn’t even work uh for cross you know

module calling it would only work internally to your own program so we’re going to respect the abi

does the abi say about these registers and again i’ve talked about this extensively in other videos

but let me pull up a fun book that i love to uh to talk about uh so this book right here i did not

write it uh the book here is written by a really wonderful uh professor dr ed jorgensen phd and um

this book is totally free it’s not i’m not selling you anything uh this is literally you can just go

to this this professor’s website and download his book for free and he’s already given me permission

and he’s already given me permission to just tell everybody to just share it with everybody if you

if you look in the license area it’s a copyleft license so it’s sort of like the spirit of open

source just sort of like sharing knowledge and stuff so it’s awesome so I suggest everybody you

know follow the link that I hopefully put in the video and and grab a copy of this book but

basically what I’m going to do is I’m going to go to a special area and I’m going to search for

here and so that would be I guess section 12.8.2 and it’s called register usage subsection 12.8.2

notice how it lists all of the registers and kind of how they are typically used

this falls under the scope of the abi so this is not all the abi is but this is one of the

things you’re supposed to do when you’re respecting the abi is you’re supposed to sort of like

are supposed to be used. Notice R12, which is one of the registers that we’re going to use,

is designated as callee saved. So that means whoever is being called has to preserve that

register if they intend to mess it up. So for example, if I just had this function here,

move some value into, you know, R12, I have now basically broken the program. If any, you know,

C function or other library that you didn’t write, just any anywhere else calls on this function,

anywhere else calls on this function then their version of r12 is going to get destroyed by what

i just did and so i’m not respecting the abi when this function returns to the caller they’re going

to expect that their original data was intact and if it’s not program is not going to work it’s going

to screw up so respecting the ai means you have to preserve any registered mark as callee saved or

that’s one of the things that it means so notice how r12 through r15 are callee saved so we have

the abi now and sort of preserve 12 13 and 14. we can do that pretty easily with some push and pop

statements so i’m going to do push r12 push r13 push r14 and now what happens is those values

are actually on the stack now and i can retrieve them later at the end of the function even if i

destroy them while i’m inside of the function i’ll just restore them right before i return so

because it happens before the function does anything.

And then we have to make sure that we pop those values

because you have to be careful with the stack.

If you just start pushing values

and you don’t restore the stack to its original state

by the time you return,

you’ve basically broken the program.

It’s either going to crash right away

or whoever called it is not going to function correctly anymore.

So let’s do some pops.

We’re going to do three pops

and we’ll call this the epilogue,

which means uh you know something we do right before we exit the function and keep in mind

that the order of the pops should be reverse of the order of the pushes notice how we’re popping

in reverse order from what i did before so we’re going to pop uh 14 13 and 12 where as we pushed

12 13 and 14 before if you pop in the wrong order like if you try to do it in in the same order as

the pushes then uh you’re still going to end up destroying data for the caller because you’re

because you’re going to be restoring data to the wrong registers.

So just keep that in mind.

OK, so now the ABI is being respected.

Let me see, by the way, do we have enough to actually even run the driver right now?

We’re just calling it and then it didn’t really do anything.

Yeah, I think we could probably this might compile.

OK, let me let me just check this out.

I want to say clear and make run.

Yeah, it compiles.

OK, so the driver printed its hello message.

again it’s just sort of saying like hello from the driver and then when the driver comes

back it says the driver regained control and nothing really happened because well we didn’t

do anything in our assembly function yet but at least we’re calling multiple modules and again

if you want to know how to do hybrid programs and linking and compiling and all that stuff

see my other videos for now I’m just going to move on so the next thing we have to do is we have to

have to try to understand like how are we going to receive these arguments so this is one of the

other you know building blocks to making functions notice how the assembly function i’m sending in an

integer and then a float and then another integer and then another floats i’m kind of mixing the

arguments then at the very end i’m sending in a pointer if i look back up at the prototype here

which matches what i’ve what how i’ve used it it’s a long a double a long a double and a character

longs and doubles, they actually, they’re called mixed arguments and they don’t actually count

against each other when you’re looking at the order of the registers to stuff them into.

So for example, let me show you here. We have, what did I just do? I clicked on the wrong

computer. Okay. I’m on the wrong computer. Okay. Let me close this real fast. So

When you think about registers for incoming arguments, you basically start to think about this.

RDI and RSI, those registers represent the first and the second integer arguments.

If we look back to the book real fast, we can see in that same section, RDI is the first argument,

and then RSI is the second argument, and then RDX is the third, and then the fourth.

We can do up to six arguments with R9, and then after that, we have to start pushing arguments to the stack.

stack i’m not going to go that far in this video i’ve actually done that already in a previous video

but basically we for for now we can just use six registers to push arguments but if you think

about it these registers these are not floating point registers these are general purpose registers

they’re meant for integers and pointers the reason they’re used for in it for pointers also is because

a pointer is just an integer a pointer is just a 64-bit integer unsigned which represents a memory

can use these registers only for integers and pointers but we can’t use them for floats so that

means the first integer argument is going to be RDI and the second integer argument is going to be

RSI but if there was a float argument in between then RSI would still be the second integer argument

the floats don’t count against the integers and vice versa so for example if we’re talking about

float registers you know the first one available is XMM0 and then we have XMM1 and then we have

we have XMM1 and we have XMM2 and it goes all the way up to I think XMM15 so we have 16 floating

point registers. So we could pass in 16 floating point arguments just using these registers if we

wanted to and then if we want to do even more than that we probably have to get you know funky with

the stack or something or maybe hopefully you just have an array somewhere and you’re just going to

pass in a pointer. But the way you have to think of these arguments is that even though they might

they might be mixed in the prototype of the function that you’re calling from a higher level

language you shouldn’t think of them as being mixed when you’re actually loading up registers so

again if we just kind of like go back here to this prototype

notice how that a variable we’re going to say that the a is rdi because a is along it’s an integer

but then right after that there’s a double b we would not skip rsi or assign b to rsi

b would just be the first float argument and then when we go back to long c long c is actually the

second integer argument because that double doesn’t count against the integer so it’s going to be c

is going to be rsi and then uh for double d uh same thing uh we’re not going to skip xmm1 just

because there was a an integer we’re going to go straight to saying that xmm1 is the second float

so it’s the d then for the last argument that we have i’m just going to erase this stuff down here

it was going to be e and remember pointers are integers they’re just unsigned uh 64-bit integers

we have to go on to the next one which i think i recall is like rdx let me just double check

i don’t want to say this wrong rdx yeah the third argument so i’m going to do rdx and now

that we’ve kind of like mapped this out we we know now what what registers we should be looking for

we should be looking for when the function comes in in order to receive

our data we should look at those registers for those variables another

thing to keep in mind by the way is that usually when we return something in

assembly we will move a value into RAX right like some value if you’ve been

following my assembly videos so far but that only counts if you want to return

an integer or a pointer if you instead wanted to return a floating point number

use XMM zero and I’ll just you’re not allowed to you’re not allowed to hard code a floating

point number in assembly like this or at least in the ASM so we’ll just pretend that there’s

like a float somewhere and I’ll just load it from memory and I’ll say like the float

something like that so notice how we’re using a different instruction we’re not using the regular

move instruction that works with integers we’re instead using the floating point version we’re

saying let’s move a single piece of data and let’s move a double precision floating point number

xmm zero and then we’ll just grab from memory whatever whatever that variable has we’ll just

take that floating point number and stick it into xmm zero so uh if you want to return an integer

or float you use rax if you want to re sorry if you want to return an integer or a pointer

you use rax if you want to return a float you use xmm zero you shouldn’t do that at the same time

if you have like two assembly functions calling each other you might be tempted to do that and i

you know very standard and it wouldn’t work very well with other people’s code or library code or

higher level language code so only one or the other and it just has to match your prototype

so notice how here inside of my cpp function notice how it’s going to return a double

right so when the assembly module is done calling on this function it should expect xmm0 to be

loaded up with that double precision loading point number okay so that’s the basic idea

Okay, so that’s the basic idea.

So now let’s maybe let me pin this.

Let’s kind of fill this out a little bit more.

So the first thing that we should do

is we should save our integer or pointer arguments.

So I’m gonna leave the respect ABI thing there.

And notice how I’m just looking at RDI and RSI and RDX.

And well, I guess, you know,

we have it in a comment up here, A, C and E,

but I’ll just make another comment here.

here I’ll say like a and a or sorry a C and E and maybe we’ll specify the data types for fun

it’s going to be long a and then long C and then the character pointer C even if this was a float

pointer it would still be an integer because all pointers are integers no matter what they’re

pointing to so just keep that in mind anyway so we’re going to save r12 r13 and 14 with the

14 with the incoming arguments and the reason we want to save those right away is because rdi rsi

and rdx those are not callee saved which means the moment we call in another function they’ll

possibly be destroyed because we don’t really know what’s going on in other functions that we might

call so it’s a good idea to just kind of save right away either to a global or the stack or

in this case just registers being faster okay so we have that now we have to save our float arguments

b and d were xmm0 and xmm1 so I’m just going to save both of those

and if you’re wondering what this float b and float d are that’s just

you know up here I just have a global variable so I can save them to memory

easily and not worry about the stack this is not a stack video so much

but yeah okay so I’m just saving all of the incoming arguments that’s all I’ve

done so far and let’s see it should probably still

Let me see if this works.

I’m going to go make run.

Okay.

So nothing happens, but it at least worked.

So I’m going to same as other windows for that so that we don’t have to look at it anymore.

And now that we’re done saving our float arguments, let’s, let’s print a welcome message.

So I’m going to do, you know, welcome.

And I’m going to use a special function that I’ve made in previous videos called print

null terminated string, which means I should probably copy paste that into this program

now.

What is print null terminated string?

It’s just another convenience function that I wrote.

I’m not going to explain it too much because it’s in other videos and

I’m already here trying to explain functions to you in general.

So long story short, it takes in a C string and a file handle, you know, to like where

you want to write, like if you want to write to a file or you want to write to standard

output or standard error.

And it just takes those arguments and then it sort of says, all right, how long is the

string?

And it uses another function called string length to figure out how long the string is.

how long the string is and then it just uses a system call to actually print the

string and again system calls are covered in other videos this one looks

really convoluted because it’s like you know customized for this function but

just trust me on this this prints a string next function I got a paste in

real fast again explained in other videos is the string length function so

all this does is it just takes a pointer to a string and it sort of scans the

string is and as soon as it sees a zero like a null terminator then it just says all right that’s

that’s the end of the string and it’ll just return the length to the caller so that’s all you need to

know about this covered in other videos then i’m going to make a convenience function here called

crlf and all that’s going to do is just print the new line that we talked about earlier so just

just a bunch of convenience functions on top of the real part of the program so now that we have

the convenience functions in there we should be able to see the welcome message let me just double

yeah now inside assembly entry point oh what did I do wrong now inside I keep forgetting to change

the strings on this now inside where’s that now inside my assembly function okay let me change

that in my solution to you know I’m just kind of like writing these things and I’m having fun and

I keep changing my mind about what they should be named and then I I get some inconsistencies okay

all right so then uh we printed the welcome message and now let’s print the integer arguments

but first let’s print a little introduction uh to the integers let’s just say hey we’re about

to print the integer so that’s just this other string a message saying we’re about to print the

integers nothing really that complicated so far it says now we’re printing all the integer arguments

so now we can actually print the integer arguments so we have two integer arguments

if you recall let’s see yeah we had like a and c those were integers I’m not going to talk about

e right now because that was a pointer but you know right now we’re just saying a and c so that

was r12 and r13 so I’m just going to paste some code here to actually print those and

talk about this library in other videos but basically I’m moving r12 which is the first

integer argument that functions typically receive and once that’s loaded up i’m just going to call

on my special function to just print the integer and again you could use printf from the c libraries

if you actually wanted to print it and not just experiment and stuff like that so i’m going to use

r12 and r13 and i’m just going to print both of those integer arguments and then after each one

is printed notice i’m calling the crlf function which is just the convenience function of just

which is just the convenience function of just like doing a new line.

So now we should see two numbers. Yeah.

Now printing all integer arguments,

we’ve got an 88 there and then that other giant number there.

Let’s just double check that that’s actually what we’re supposed to be seeing.

So I’m going to do this and I’m going to say,

the driver called the assembly module with these numbers.

It gave it an 88 and then for the next integer,

it gave it the 287 giant number. So great.

so great we’re printing the integers now let’s print the floats so we should see

like a 99 point something in a 32 point something next okay so let’s continue

with printing oh sorry actually let’s print the the C string because that’s a

pointer that’s still more closely related than the floats so the first

thing I’m going to do is I’m going to

print the received c string how about like announce that we will print the c string because

that’s what we’re doing right here let me change my solution to match and then we’ll uh we’ll

actually print the received c string next so same stuff as before first we call print null terminated

string to print out a little welcome message or just like an intro message like we are going to

print the c string and then we’ll use that print function again but we’ll give it the c string so

it the c string so it just prints the whole c string out and this should prove to you that we

are indeed receiving a pointer to some data owned by the c plus plus module so if we run this real

fast it should just tell us two more things it’ll give us the announcement now printing the received

c string and then on that same line it says hello this is the c string owned by main and if we just

that’s exactly what string is inside of that variable so hello this is a c string owned by

main and we gave it to the function by just kind of passing it in and we know that character arrays

are basically character pointers or any array is just a pointer to the first item in the array so

my c string is really a pointer to that h character so if we pass that in then a pointer

based print function should be able to work and that’s what happened okay so we’ve done that

So we’ve done that.

And then the next thing we should do is let’s print the floats.

So first let’s announce that we’re going to print the floats.

Same thing we’d before we’re just printing like an announcement message.

If we run the program again, it’s just like now printing the floats, but it doesn’t actually

do anything.

So then the next step is let’s, let’s grab the first float into XMM zero.

And then let’s call a function to print it.

so right here we have like that 99 number that we expected from before by

the way so why am I doing it this way why am I not just keeping XMM zero

because you remember before we had XMM zero had that had the float that we

received and then we’re using it again down here but remember XMM zero and all

the other float registers they’re not designated as callee saved which means

the moment we call any other function we should expect that that data has been

so I can’t actually count on XMM 0 surviving just this little simple function instead I have to save

it somewhere to the stack to memory you know whatever so I’m just that’s why I put that into

a global variable so it’s sitting in float underscore B right now and then we saved it at

the beginning to float underscore B and if you just kind of look up to the data area well it was

just float underscore B was just a little quad word you know eight bytes of memory that can hold

hold our float. So we have like float allocations for B and D, the first and second float arguments.

So we’re saving it there. And then we’re recalling it here. And remember, the first function argument

is going to be XMM0, regardless of where that data originally came from. So if we look at the next

one here, if we kind of like, let’s see, copy paste this, and we want to grab like the D float,

still going to load it into XMM zero because right now it’s not about what we originally

received as an argument.

It’s what this function expects as an argument.

This function only takes one argument.

It just wants a float so that it can print it and that’s it.

So both times we’re going to load it up into XMM zero and then we’re going to print a new

line.

Okay.

So let me just run the program one more time and we should now see we’ve got two floats

and they should match what the driver tried to send in.

Right?

99, that’s the first one.

And then 32 point something, that’s the second one.

So cool, we have received integers and pointers and floats.

We’ve recalled them and then we’ve printed them.

Pretty slick, what do you think?

Anyway, so we’ve done that.

And the next thing that we should probably do is…

Well, at this point, we just maybe have to mess with return types.

Even though I’ve told you about it, we’ll just mess with it a little bit.

mess with it a little bit. But let’s call the C++ module. So what I’d like to do first is just sort

of announce that we’re going to call on the C++ module. Again, typical design pattern, let’s just

print a message saying what we’re about to do. And if we run this now, it’s going to say assembly

module will now call on the C++ module. So nothing really too complicated. So now we’re going to call

on that function. Let me let me paste the name of it here. Actually, I’m going to put this one right

I want to put this one right here.

It’s the my CPP function function.

So if we look back at the driver

and look at the signature for my CPP function,

whoops, it has this signature.

And if you wanted to look up higher,

you totally can just look into the name mangling section.

Whoops, the my CPP function, it returns a double.

It takes in a long, a double, a long, a double,

and a character pointer.

Basically the same thing as the other one,

as the other one except it returns a double so what’s going to happen is when it takes all of

these in it’s just going to print all of them and then i’m just going to have it return just kind of

like some random double that i decided to type because this is not an arithmetic video okay so

cpp function and then um now how do we interpret that so if we uh let’s see let me maybe just for

help this usually helps me when i’m trying to do this i’m going to take the prototype and just sort

and just sort of like paste it right where I’m about to call the function.

So I’m going to do this just to remind myself of what I’m actually calling.

Let me add that to my solution, by the way.

Okay.

So we’re going to call myCPP function,

which means it’s expecting some registers to be loaded up with arguments.

If we don’t actually load up anything right now,

it’ll probably do some sort of nonsense.

Let’s actually see what happens right now.

If we don’t load up the appropriate registers,

then C++ will still look at those registers expecting to see valid data.

Let’s see what happens.

I don’t know if it’s going to be good or bad.

This would probably be something called undefined behavior,

meaning you did something wrong,

and probably sometimes your program will work,

and sometimes it won’t.

Sometimes you won’t understand what’s going on.

So I’m going to do this right now,

and it’s saying, oh, it’s segfaulted.

Okay.

Why did it segfault?

all right well I guess maybe because I did something naughty I don’t know

if this say faults by the end we’re going to be in trouble I’ll have to debug on camera

so uh we’re going to enter into my cpp function and then it says we got a variable a

which was a long we didn’t give it a one a one was just probably sitting in there

before we even called that function so like one is definitely not it and then b was like some

floating point number and then C was just like another seemingly random value

that kind of looks a little bit more like a pointer I’m not really sure it’s

probably not but it’s just some junk data coming from somewhere and then D

which I think D was supposed to be a character pointer it says not a number

so we just got like a a really bad value for D let me upgrade this real fast

because maybe we should be printing what printing it as a memory location so E is

So E is, let me bring this down real fast.

So E is supposed to be a character pointer.

Let me, instead of printing the C string as just like itself, let’s first print the memory location.

And I’ll just say memory.

And then we’ll do a static cast.

Oh, I wonder if this will actually change anything.

I wonder.

Let’s static cast both of them just to see what happens.

print e and uh c string here and then it’s going to be um

hmm no i don’t think that’s actually going to change anything

because if i cast it as a point as a character that’s definitely wrong

and if i don’t cast it then it’s going to show up as its original data type how do i get the memory

location hmm oh i know what to do i can static cast it maybe as a as an unsigned long okay

and unsigned long. Okay, so a character pointer, we’ll leave that for the C string. Unsigned long,

long, just in case. And I think I should see the memory location first and then the actual C

string later. If not, then whatever. Let’s try one more time. Oh dear. Invalid static cast from

a character pointer to a type long, long unsigned integer. What have I done? How about unsigned long?

long see if that works character pointer long int i guess i forgot how to cast pointers to longs

i’ll look that up and post another video in the future but i guess for now we’ll just

we’ll deal with this humiliation and i’ll just print e by itself

all right so i’m just going to run it one more time it should say fault again

is wrong let’s uh let’s just fix the arguments so before we make that call we should load up some

stuff right so i’m going to do maybe like a semicolon comment here and then i’m going to do

what else do i have i’ve got three move instructions so the first argument is going to get

77118 the second argument is going to get 1111 and the third argument looks like it’s going to

get a pointer that i’ve defined inside of the assembly module so if you look back up at the

look back up at the top let’s see message string inside asm message string inside asm so basically

i’m going to be sending a character pointer to this t right here which is just this string says

this string is owned by the assembly module and it’s a null terminated string which uh c definitely

needs in order to print correctly if we took that zero off we’d probably get a bunch of junk data

we might crash the program i don’t know so anyway basically we’re just going to be saying you know

we’re just going to be saying, you know, here you go. Here’s a pointer. Where the heck am I?

Okay. Here’s a pointer to a C string. And so we’re giving it one, two, three arguments. We’re giving

it the A and the C and the E. Now we just have to load it up with the other two floats. So

I’m going to do this real fast. Remember the first float argument is going to be XMM zero.

And the second one is going to be XMM one. And you can see that send B corresponds to the first

corresponds to the first float argument.

That’s why it’s XMM zero.

And then the float send D corresponds

to the second argument.

That’s why it’s XMM one.

And that’s the D right here.

And there’s no other floats.

And if we just kind of look back up real fast,

the float send B and D,

I just defined those arbitrarily

to just these two random numbers.

So we should see like a two one nine

and then a nine nine eight eight seven seven six

sort of, you know, weird number.

number then when we’re done loading all those things you know all of these registers up now

the function should be able to receive something i bet you the reason it crashes because we

we had like a bad address for the the string previously because we didn’t load it

so let me go up here and now it doesn’t crash so if you just kind of like look we have hard-coded a

and b sorry not not a and b a and also c so that’s the seven seven one and then the bunch of ones

of ones and then the string which was rdx which was the e so this string is owned by the assembly

module nice we’re now able to print a string that’s owned by assembly in another module in

the c plus plus module and then xmm0 and xmm1 those are the two floats so if we look back up

again that’s uh we should expect to see these two numbers right here whoops um bloat send b and d

and then D. So the printing kind of turned it into scientific notation. That’s okay. You know,

there’s other stuff you can do in C++ to just not print in scientific notation, but for now,

I don’t really care. It’s basically the same value. So it’s fine. I just want to show you

how to transport data. This is not a C++ video. Anyway, if we go back down to the end of that,

so we called the function, but then this function, if you notice, it returns a double,

we should expect to see something loaded into the XMM zero register.

So I’m going to move a single piece of data and it’s going to be a double precision floating

point number.

And I’m just going to store it into my float got return value global variable,

which I defined up above.

So let me scroll up real fast.

So where is that?

Where is that?

Float got return value.

I initialized it with a zero just because we’re going to receive something in there.

I could have used the BSS section if I wanted to, probably would have been a little bit

bit smarter and more performant but i don’t know i personally don’t like using the bss section in

yasm unless uh i want to allocate an array if i’m just doing one variable here and there then i’m

just going to put it in the globals or or the data section so float got return value i’m saving it in

there and then uh i want to grab it from xmm zero so that’s where it comes from when uh my cpp

return the double to us inside of XMM0.

So I’m just moving that into my data section into the global variable.

And then now that we have finally returned from the C++ module,

I’m just going to announce that we have indeed returned from the C++ module.

Let’s get that far real fast.

So this string is owned and then my CPP function exiting.

exiting and then now it says the assembly module received this return value from the

C++ module and then it says the driver has regained control because we were supposed

to print the float and do a line feed after that and we didn’t so basically assembly just

printed this string that was the last part we just added and then it returned right away

to the driver so let’s finish this up so next thing we’re going to do is we’re going to

move the float value back into XMM0.

Remember, anytime you call a function, all your float registers could be destroyed.

So it makes sense that we saved XMM0 and then we loaded it only several instructions later

because you don’t know what’s happening under the hood inside of print null terminated string

or any of the system calls that it uses.

Your XMM0 is likely destroyed, but you don’t know for sure, right?

If you count on it not being destroyed, you might be introducing undefined behavior to your program.

So that’s not good.

Anyway, so we’re just loading back up from that global into XMM zero.

And then we’re calling that function again to just sort of print the float.

And it wants the float as argument zero.

So we’re just sticking into XMM zero.

And then we’re printing a new line.

So then when we’re done here, run it.

and now it says the assembly module received this value from the C++ module and notice how it’s that

special value that I hard-coded 112222 whatever so that’s this value right here

and again if we had this cpp function returning along then all we would have to do is look at

the RAX register and if we had let’s let’s say for some reason that this assembly function we

function we wanted to get something from it we could say you know double temp

equals that then all we’d have to do is make sure that we returned sorry we

loaded XMM zero with our return value in the assembly module because XMM zero is

the return register for floats or if we wanted to do like a long then just load

RAX with something at the end of my assembly function and that’s how C would

C and C++ they’re following the ABI so if you personally don’t follow the ABI

you’re just going to end up memorizing two different ways of doing things and

your way is not going to be compatible with all the other modules written in

higher level languages so it’s kind of a waste of time and your modules won’t be

compatible with other people’s libraries and functions it’s just a huge waste of

time so just follow the ABI in the first place I’ve literally known people who

said you know what I don’t want to learn the ABI that’s dumb then as they wrote

they wrote a bunch of assembly they just started thinking like oh this is too confusing because i

keep forgetting what registers i was going to pass back and forth between my functions

and like a month goes by i come back to an old program i can’t remember i have to like look

through all my code again so then they started accidentally inventing their own little scheme

like oh i know what to do i’ll put the first argument in this register and i’ll put the

second argument in this all they were doing is just reinventing the abi from scratch and then

got to that point and then later wasting more time when they realized they just needed to learn the

ABI and forget about their way so don’t let that happen to you and you should probably just try to

do it the right way the first time right um a lot of things in coding and computer science in general

are just time savers that feel like wastes of time at first okay what else we have anything

that I wanted to show you that was actually I think the entirety of that program let’s run it

while I check to see if there’s any other stuff that I was supposed to show you.

So let’s do this.

Everything seems to be working.

Let’s see, I wanted to tell you about labels, return, call,

the stack, the return address, prologue and epilog,

pushing and popping, respecting the ABI.

I wanted to show you that textbook function arguments, mixed arguments,

int or pointer return types and float return types.

I guess I got through everything without forgetting something huge, which, hey,

that’s probably a first not that i recall at the moment but it probably is

anyway so uh i guess that’s it i hope you feel like function experts now in yasm x86-64 assembly

also known as a amd64 assembly in ubuntu okay thank you so much for watching this video i

hope you learned a little bit and had a little bit of fun 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. 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.

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 subscribe

and then I’ll just wake up. I promise that’s what will happen. Also, if you look at the middle of

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 if you have

a suggestion for clarifications or errata or just future videos that you

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 um enjoy the cool music as as i fade into the darkness

which is coming for us all

Thank you.

The post YASM x86-64 Assembly Functions Tutorial – Integers, Floats, Pointers, ABI, C++ Interoperability appeared first on NeuralLantern.com.

Join us for a clear, engaging look at x86-64 program segments! We break down the text, data, and BSS sections, explore stack vs. heap memory, and show a real stack overflow in C++. Learn why segmentation faults happen, how virtual memory works, and tips to avoid crashes. Whether you’re new to assembly or leveling up your coding skills, this video is packed with insights to boost your understanding of low-level programming. Subscribe for more coding deep dives!

Introduction to x86-64 program segments 00:00:01
Understanding segmentation faults 00:00:12
Importance of segments in assembly 00:00:32
Overview of assembly program structure 00:01:09
Defining segments in Yasm assembly 00:01:49
Data section: Initialized global variables 00:02:20
BSS section: Uninitialized variables 00:03:23
Text section: Code and functions 00:03:59
Global and extern function declarations 00:04:41
Stack: Local variables and return addresses 00:07:11
Stack overflow example in C++ 00:08:48
Heap: Dynamic memory allocation 00:11:31
Stack vs heap memory growth 00:11:56
Virtual memory and memory allocation 00:14:00
Demonstrating memory overflow with heap allocation 00:16:59
Summary of segments and their purposes 00:17:59
Closing remarks and call to subscribe 00:18:47

Thanks for watching!

Find us on other social media here:

• https://www.NeuralLantern.com/social

Please help support us!

• Subscribing + Sharing on Social Media
• Leaving a comment or suggestion
• Subscribing to our Blog
• Watching the main “pinned” video of this channel for offers and extras

Hi there!

Let’s talk about x86-64 programs and their segments and the purpose of the segments.

Probably when you’ve been programming in the past you’ve seen segmentation fault errors.

If you’ve accidentally crossed the bounds of some appropriate memory location, if you

have an array and you try to index it way outside of bounds or something, you might

have seen a segfault.

SegFaults basically come from the idea that you have crossed a segment boundary.

Segments used to be really, really important.

They’re a little bit less important now that we have virtual memory.

I’ll just talk a little bit about them.

I’m going to be discussing this from the perspective of an assembly program,

just because even in higher level languages,

your languages will typically be compiled down to assembly

first before they actually hit machine code and your actual executable binary.

So if you’re not interested in Yasm Assembly,

Yasm assembly I think this could still be worth it for you to watch because

you’ll be able to see the different segments and just kind of get a better

understanding of what they are okay so I’m gonna actually write a very very

very quick program in assembly it’s not gonna really do anything I just want you

to show I just want you to see the segments that are involved so for

starters let’s just pretend that I’m writing a an assembly program here we’ll

This is a comment just so you know what I’m about to type.

Perhaps actually this is not the best editor.

I don’t know why I pulled this up.

Let me get Gene here.

It’s a little bit better.

Gene.

There we go.

Maybe I’ll save it somewhere.

I’ll say segments assembly.

And then now the comment gets highlighted.

Okay.

So in assembly in the ASM assembly,

we denote various segments by using the section keywords.

keywords so we’ll say section and then dot something indicating the type of segment that we’re

defining so section text and then up here we’ll have another one section dot bss and you know if

you already program assembly you know that somewhere in here is where the code goes right

between these sections so just a quick recap of what the data section is it just is where you

can name and initialize at the same time at the very top.

Globals are not great, but sometimes you want to do that in assembly, depending.

It’s better if you stick all your variables on the stack.

Like if they’re a local variable, it’s a little bit better.

Think of higher level languages.

You typically want to avoid global variables if you can possibly avoid them and make

everything else a local variable or a member variable.

So that means somewhere else other than data, probably the stack, but or the heap.

Anyway, you know, this is like the basic pattern.

We’ll say, you know, my variable and then we’ll decide to say what data size it is.

In this case, it’s DB for one byte of data.

You could put a, I think DD in there or a DW for a word.

Sorry, sorry, DW would be for a word.

And then I think DD would be a double and then DQ would definitely be a quad word.

So here quad word is eight bytes.

And then I can sort of initialize it to some number.

some number and now I have a global variable so not like a huge deal the BSS section is basically

where you have uninitialized variables but there you can make huge amounts of variables you can

make an array of variables so if I say my array something like that I could put resource queue

to say I want the type of one item to be a quad word so you know eight bytes per item and then I

could just say I want you know a hundred thousand items or something and I’ll

double check that after I stop recording this video and correct it if I’m wrong

it’s been a while since I actually typed BSS here and then in the text section

this is where you put code and functions and things you typically want to have at

least one entry point somewhere you know if you want to make like if you want to

make this the entry point of your entire program you’ll probably do global main

main and then this assumes that you use the GCC libraries and just sort of return from the label

sort of makes it a function assuming you didn’t ruin any of the other data.

So those are the three sections that we can start off looking at the data section,

the BSS section, and the tech section. It’s important to note that besides marking functions

as global so that they’re accessible to other segments or other modules, we also mark

We also mark functions and labels that we would like to access that are in other modules.

So if we’re using the GCC libraries, we could say something like global printf,

and then right away we’re able to call the printf function.

Or say if you have another function in another module,

like a different source code file that’s compiled somewhere else,

we could say, you know, my other function.

Something like that.

Just mark it as global.

Or sorry, not global.

Extern.

Xtern. Global is when you have the function in the current module and you want it to be available to other modules.

Xtern is when the function or label is in another module and it’s already been made available to you,

but you want your current module to know that it can access it for the purposes of assembling the program.

So that means inside of here we could do something like call.

We could say, let’s call my other function.

my other function that will work if the other function is actually a function

with a return statement and it respects the ABI you know it pushes registers that

it’s going to modify if they’re callie saved if it’s not another function maybe

we’ll say extern my other jump point so like not a proper function but just

something that you might want to jump to then instead of using call we would use

jump, you know, and so forth. So those are the basic sections. Let’s see, for the BSS,

I definitely talked about token size and count. And then there’s resource B, resource W, resource

D, resource Q for the different sizes. So basically just take all the different sizes you could put

in the regular data section and assume that that could be popped on to the end of RES inside of

the BSS section. If you’re interested, this is a segments video, not an assembly video.

assembly video um another segment uh another segment that we’re not showing here in this

source code is the actual stack probably i guess the best way that i could show it is

i don’t know by messing around with the stack pointer increase uh

oh rsp why did i think it was the instruction pointer we could increase the stack pointer and

stack pointer and then decrease the stack pointer.

It doesn’t really help a whole lot.

But in another video, I’m gonna talk more in depth

about what the stack is and how to use it and such.

But just keep in mind that the stack

is where local variables are stored

and you can use it in assembly and C++

and other higher level languages.

You can even make virtual stacks

in any type of program almost

to just sort of have a stack like structure.

But in C++, if you make any local variable,

any local variable, then it’s usually going on the stack.

And the stack tends to have a fixed size.

And so if you sort of put too much data on the stack,

you risk a stack overflow, which will crash your program.

Besides local variables, the stack will hold other information that the current scope needs,

like its return address.

So when we make a function and then let’s say we call another function inside of it,

actually just up here even when we call another function on line 22 the stack will receive the

return address of where we currently are so that later when we return from the other function you

know the program knows where to jump back and forth it’s actually under the hood just jump

instructions when you do call it’s it’s not it’s not like a special thing it’s just jumping to

another location and then pushing the return address on the call stack so that’s other stuff

other stuff that the stack holds, which means if you have like an endless or like a infinite

recursion happening in your function where just a function calls itself over and over

and over again, or you have some complicated call graph and you end up calling too many

functions without ever returning, you could overflow the stack.

So that’s not good.

Let me show you a quick example that I’m going to make for you real fast.

Oh shoot.

Did I actually make this?

Where is the, it’s on the desktop.

Okay, I guess I’m going to make more on the desktop.

Let’s do a stack overflow.

Let me do desktop here.

Let’s do stack overflow.

That’s CBP.

I’m just going to make a quick program.

In C++, that doesn’t really do anything except overflow the stack.

So I just want you to know the stack can overflow.

I’ll make a function called F.

And F just calls itself.

calls itself. So that’s going to be an infinite recursion.

And that means, you know, every single time F calls itself,

the return address of like line five is going to get pushed onto the stack again.

So it’s just going to get pushed and pushed and pushed and pushed.

The stack will overflow faster if I had local variables because every scope that

gets called, even if it’s the same function name, it’ll have its own allocation on

the stack. So if I just sort of like call this from main and then I’ll just need

and then I’ll just need to make a little uh a little make file that just kind of compiles

I don’t really even need to make a make file I could do a build script but I’m going to do it

anyway we’ll say g plus plus standard equals c plus plus 23 show all warnings show pedantic

the input file will be stack overflow cpp and then the output file will be main

main and then assuming that succeeds I’ll just run main so this is like not a make

file video I have other videos for make files let’s see let me get to the

desktop if I run the make file with just the word make you should see a stack

overflow we should also see a warning about the fact that there’s infinite

That’s funny, I must have done something wrong or different because on a different computer

not too long ago I did the same thing and I saw a warning.

Okay, well thanks for not warning me.

But anyway, you can see that we have a segmentation fault here because the stack overflowed.

We just made a stack that was too huge.

This is a reminder that the stack is not necessarily as dynamic as the heap.

You can definitely overflow it.

It’s not really meant to store gigantic amounts of data.

What am I talking about when I say the heap though?

So just keep in mind that whenever you allocate dynamic data,

well, maybe, well, what do you think?

Maybe, maybe inside of, instead of calling F, let’s call G.

The heap is where you allocate dynamic data.

So anytime you use the malloc operator in C or the new allocator in C++,

you’re creating like a dynamic allocation of data.

I’ll do a new integer and then I’ll do like a thousand integers so this is

basically going to allocate a thousand new integers in the heap which is a

segment that is right up against the stack I should mention also that the

stack it’s important the stack its memory locations go down as the stack

grows keep that in mind because typically when you when you imagine a

stack I’ll make more videos in the future so you can visualize the stack better but most of you who

know what a stack is already you imagine that the stack grows upward visually right but in your

computer the memory locations of the stack grow downward so if I add something to the stack then

the new stack head pointer is actually going to be a lesser address and the reason for that is the

stack and the heap they grow in the same direction towards the same unallocated space we’ll call that

We’ll call that space the unallocated memory space or the unmapped memory space.

And it’s just, you know, a segment stack in the heap.

And then there’s the unallocated space that they grow towards each other.

If they ever meet, then your program is out of memory and it’ll crash.

But the heap actually can be dynamically allocated to store tons of RAM.

I’m probably about to crash this computer.

So maybe this video won’t finish recording.

recording so maybe I’ll try to crash this at the very end I just wanted you to

see let me let me do this I’ll say while true we’ll just allocate a new integer

this is kind of bad form because when you allocate you know a new variable

somewhere you want to have access to it after the allocation you would usually

send that into a pointer you’d say like you know integer pointer P equals new

in something like that right but in this case I don’t really care I’m not gonna

that memory i just want you to see how fast the memory can grow when we did the stack overflow

seemed like the program died pretty quickly but here we’re going to be able to see the memory

boom pretty pretty big before the whole computer just starts to you know fall to its knees

so i’m going to do a call to g here and then i won’t actually run this until

i’m ready to crash this whole video let’s see what else can i say

Oh, the reason that the heap can grow so much larger than the stack is our programs now in the modern era, they use virtual memory.

So it’s not like when we allocate, let’s say we’re accessing a memory location via a pointer inside of our program.

It’s not that the pointer has the absolute memory location through the whole system.

Like it doesn’t, it’s not able to name locations outside of the current program.

Every program sees its own start memory location as just zero.

And then under the hood, there are some extra tricks so that when you try to access a memory location,

let’s say memory location 100, because you’ve made, I don’t know, this many integers or whatever.

Then under the hood, the computer will know what is the start offset of the program’s memory.

So maybe the start offset is 1000.

It’s like totally unrealistic, but just pretend.

The start offset is 1000.

1000 then that means if it’s trying to access a pointer with memory location 100 it just gets added to the offset the virtual offset

So it’ll be a thousand and 100 is the real you know physical memory location or at least at that point

It’ll probably go to the paging system, but that’s that’s another video

So just know that it’s virtual which means the program thinks it has the full range of of the 64 bit memory address space

Even if you don’t even have that much memory on your computer, which there’s no way there’s no way you have so much memory

there’s no way you have so much memory that it’s going to overflow or even meet

64 bits at all but the program thinks it can so it’s just going to grow and grow

and grow until you actually run out of physical memory and then eventually

probably your program is going to start going to the page file or to the swap

file and then when you run out of that then your whole computer probably is

going to start crashing to be honest it’ll probably start crashing as soon as

it hits the swap file because at that point none of the other programs really

None of the other programs really have enough leftover memory to to breathe to do extra

allocations and things so everything’s going to start slowing down it’s going to be pretty bad

and well i think that’s all i really need to say this is not supposed to be a really long video

let me uh let me show you this crasher i’m going to say goodbye now thank you so much for watching

the video i hope you learned a little bit and had some fun too and and that way i’ve already said

if it actually crashes the computer. So I’m going to open up this and then I’m going to do maybe

HTOP so that I can see memory. So if you kind of like look at HTOP real fast,

this is just a Linux command line tool that you can install. It’s pretty cool.

What the heck’s going on here? Oh, that’s my recorder. You can see here’s the memory allocation.

So I’m using two gigs of eight gigs on this virtual machine and then I’m not using any swap

space which means I have not exhausted all my memory and and I have not been forced to go to

the disk for like additional memory but in a second we’ll see that will probably change

so I’m going to do make just to run that it looks like it’s stuck but if you keep looking at the

memory allocation oh it already hit eight gigs now it’s into the swap already probably

The out of memory killer, the OOM, successfully killed the program.

Thank goodness.

Or the whole computer would have been down.

I always forget that that’s going to happen lately.

But you can see it spiked, right?

It went right up to the full physical memory allocation of 7.75 gigs.

And then the swap file went all the way up to 2 gigs.

You can see that the swap file still has about 700 megs inside of it.

That just means that there’s memory that isn’t really being accessed.

the operating system doesn’t feel like it’s necessary to pull it back out of swap and put

it into RAM but I wonder if the video skipped when I did that or stuttered or anything I’m

interested to find out now okay so that’s the end of this video I hope you have a decent basic

understanding of segments and then you know like the segments and their purpose you know the heap

and the stack they grow together the heap can dynamically allocate to grow bigger notice how it

Whereas the stack will crash a lot faster.

If the stack and the heap meet, then you’ve ran out of memory.

At that point, you know, you probably want to like resize the heap or the stack just crashes.

And then we got the text section of the program and the data section and the BSS section of the program.

And these sections are a little bit less relevant in the modern era because we have, you know, virtual memory and stuff.

That definitely means you have tried to access outside of your segment and it’s a no go.

Okay.

Thank you for watching this video.

I will see you at a later date and time.

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

as 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 you could troll me if you want to just wake me up in the middle

And I 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

and uh if you have a suggestion for uh uh 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 um enjoy the cool

Enjoy the cool music as I fade into the darkness which is coming for us all.

Thank you.

The post Demystifying x86-64 Program Segments: Stack, Heap, and More appeared first on NeuralLantern.com.

Get a clear, fun breakdown of data sizes in x86-64 CPUs! This video covers bytes (8 bits), words (16 bits), D words (32 bits), and quad words (64 bits), plus how they fit into assembly and CPU registers. Perfect for programmers, students, or anyone curious about low-level computing. I’ll walk you through the basics with relatable examples, no jargon overload. Subscribe for more coding & tech videos, and scan the QR code to visit my site for extra tutorials! Leave a comment with your thoughts!

Introduction 00:00:00
Data Sizes Overview 00:00:01
Byte Definition 00:00:44
Bits to Bytes Conversion 00:01:27
Word Size Explanation 00:02:20
Double Word (D Word) 00:04:03
Quad Word Definition 00:04:44
CPU Registers and Memory 00:05:21
Conclusion and Outro 00:06:07
Call to Action 00:06:32

Thanks for watching!

Find us on other social media here:

• https://www.NeuralLantern.com/social

Please help support us!

• Subscribing + Sharing on Social Media
• Leaving a comment or suggestion
• Subscribing to our Blog
• Watching the main “pinned” video of this channel for offers and extras

Hello there.

I’d like to talk to you about data sizes within the context of x86,

64 CPUs,

just like 64 bit CPUs for assembly and just whatever.

So what’s going on inside of the machine and how do we describe the sizes of

various clumps of data? So for starters, you know,

of your computer, there’s just a bunch of ones and zeros represented in all the wires,

in all the circuitry.

The human beings with feelings decide, oh, you know, I’m going to take a group of this

many bits, this many ones and zeros, and I’m going to say that it is something else.

I’m going to call it an abstract unit.

So the first thing that we do is we have something called a byte.

That just means eight bits.

So we can say one byte equals eight bits.

equals eight bits. I don’t know why I hit the shift key there. That’s probably going to mess

up my annotator. Nope. Okay. So what is eight bits? Just as a little reminder, you know, we’ll

just say one, two, three, four, five, six, seven, eight, or it could be, you know, something more

random looking one, two, three, four, five, six, seven, eight, you know, so like inside of the

machine, we have eight ones and zeros for eight bits. And when we group them together, we’ll just

Okay, no problem.

So that means by the way, if somebody says,

oh, my internet connection is this many megabits a second.

If you wanna know how many bytes a second,

try dividing by eight.

Or if somebody says I have this many megabytes a second

of transfer speed, try dividing that by eight.

You know, so multiply or divide by eight

to get the right number.

Okay, so if we had two bytes together,

that’s just gonna be 16 bits.

No problem there.

I’m just going to copy paste these bits and you don’t have to do this.

But for me personally, I like to put a little space in between bite groupings just to help

my human brain kind of see, oh, I’m looking at two bites right here when I’m referring

to different data sizes.

So then eventually we kind of need an idea of, you know, let’s come up with a different

word that means more than one bite.

ironically based on what I just said the the term we’re going to use is word a word

the size of a word just kind of depends on the system that you’re on on my system

the size of a word is two bytes so I’m going to say one word equals two bytes

and you can imagine that’s 16 bits maybe I should start doing the double equal sign here just to be

Okay, so equals two bytes.

It also equals 16 bits.

And you can imagine it’s basically the same thing

that we just wrote up above.

So I’m gonna erase this actually, 16.

Oh, I put an E there, embarrassing.

Okay, so one word is two bytes.

Again, the size of a word depends on your system.

If you’re not sure, you should probably look it up.

What’s the word size on my system?

But again, the typical system here in 2024

6664 machines running Ubuntu, um, or just like Linux is going to be like, you know,

two bytes. Okay. So now, uh, we have like words that we can do. So if I had, uh,

let’s say I had two words, uh, two words is going to be double of what we just wrote. Right. So

that means we’re going to have this many, whoops, I blew it. We’re going to have this many bits.

So two words, two words is going to be equal to, um, 32 bits.

And it’s also going to be equal to four bytes because, you know, we multiplied everything

by two.

So this is like four bytes.

So now we need another term to represent this two words, which is four bytes, which

is 32 bits.

We can also call that a D word.

Actually, I’ll say D word like that.

And what does D word mean?

is twice the size of the word length on your system.

So like if we decided our word length is two bytes,

then a D word is four bytes and that’s it.

Four bytes is 32 bits.

You might notice this is similar to like old school CPUs

that ran in 32 bits and maxed out their RAM

at about four gigabytes.

So let’s come up with one more term here.

Let’s come up with the term quad word.

Actually, we’re not coming up with the term.

We’re just using it.

What is a quad word?

A quad word is two D words.

So we’re just kind of doubling it again.

I forgot that extra equal sign there.

Quad word is two D words, which means it’s also four words.

And it also means it’s eight bytes.

64 bits.

And then I guess if I want to represent this to you,

I can just kind of like copy paste that twice.

And this is the size of the memory address space that we have on modern CPUs.

This is also the size of your general purpose registers in modern 64-bit CPUs.

So if somebody says, hey, I got a quad word.

I want you to store it somewhere.

You can instantly think to yourself, well, obviously I could put it in memory.

I could put it in memory.

You could also just put it directly into a CPU register.

And that’s it.

This also means that you could put two D words into one CPU register if you were very clever.

You could also do the same thing.

You could put like four words inside of one CPU register.

Because the CPU registers, they’re always the same size no matter how you use them.

They’re kind of hardwired.

So I hope this has been, you know, kind of helpful.

These are the basics of different data type sizes that we have in CPUs and

assembly language. 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.

If you could do me a please, a small little favor,

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 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

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, 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

and tutorials and things that I offer and all that good stuff and if you have a suggestion 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 this

it would really it really mean the world to me I would really appreciate it so again thank you so

thank you so much for watching this video and enjoy the cool music as as i fade into the

darkness which is coming for us all

Thank you.

The post Master Data Sizes in x86-64 CPUs: Bytes, Words, D Words, and Quad Words Explained appeared first on NeuralLantern.com.