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

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

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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.

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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

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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

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

Want to write a single program that uses C++ for high-level logic, C for portability, and assembly for speed? This video shows exactly how to do it.

We build a complete hybrid program from scratch:

• driver.cpp – contains main(), prints intro, calls assembly
• c.c – simple printf() function
• cpp.cpp – C++ function (with extern “C” to disable name mangling)
• hello.asm – assembly module that prints messages, calls C and C++ functions

You’ll see:

• Why hybrid programs matter
• How source files → object files → final executable
• The role of the driver and why main() lives once
• Name mangling in C++ and how extern “C” fixes interop
• Global / extern in assembly to expose and call functions
• A clean Makefile that compiles each module separately
• Live build and execution with output explained line-by-line

By the end you’ll know how to call across languages safely, pass basic args via registers, and structure projects for fast incremental builds.

No prior assembly required – just basic C/C++ familiarity. Code is copy-paste ready.

Timestamps in comments. Source files linked below.

Like & subscribe for more systems programming!

What is a Hybrid Program 00:00:00
Why Use Hybrid Programs 00:00:21
Program Modules Overview 00:01:10
Compilation to Object Files 00:02:23
Makefile Introduction 00:04:26
Driver Module Setup 00:06:43
Name Mangling Explained 00:07:18
C++ Module with extern C 00:07:43
Driver Main Function 00:10:12
C Module Implementation 00:11:52
Assembly Module Setup 00:12:41
Assembly Data Section 00:13:03
Assembly Text Section 00:14:18
Global and External Functions 00:14:35
Assembly System Calls 00:16:20
Calling C from Assembly 00:18:58
Calling C++ from Assembly 00:20:32
Program Execution Flow 00:21:05
Running the Program 00:23:59
Passing Arguments Between Modules 00:26:23
Hybrid Program Summary 00:29:11

Thanks for watching!

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Please help support us!

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Hello there. Let’s talk about hybrid programs.

What’s a hybrid program?

A hybrid program is basically a program where you have multiple modules,

many written in different languages, all compiled into one executable program.

Why would you want to make a hybrid program? Well, you know,

maybe you have C++ as one of the modules that you’re creating and C++ is a high

And C++ is a high level language, so it makes it easier to write big, robust programs with GUIs and like big design, big fancy design patterns and things.

But maybe at some point you want to interface with the C program.

So it might make sense to, you know, kind of jump into some C code.

Maybe also you want to write a little module in assembly because maybe one part of your code needs to be really, really fast and efficient in some way.

the code compiles down to machine code.

Assembly is just, you know, a level up from machine code.

So let’s see, I’m just gonna write a quick program

to demonstrate that you can do this

and you can have these modules call each other.

First off, I should probably draw a little bit

about what I’m even talking about with multiple modules.

So I’m gonna do my little annotator here

and yeah, there we go.

Basically imagine that you have a C++ module

and we’ll call it the driver.

and we’ll call it the driver and I’ll say like driver.cpp. Okay I think I’m

drawing this backwards. I’m gonna draw this down here. Driver.cpp.

Excuse my penmanship I don’t have a smoother setup on this thing. So we have

driver.cpp that’s one source code. Maybe you have like another module, another

just some C code. The driver, usually the way I like to refer to it, and one of my favorite,

you know, assembly books refers to it is the driver is where the main function is so that

it’s sort of like the entry point of your program. But then maybe we’ll have another module that has

some other kind of CPP code in there. I’m just going to call this file cpp.cpp. And maybe we

have another module that’s written in assembly. I’m going to call this hello.asm. Okay, so just

Okay, so just several source code files.

We will compile them each to an object file.

Sometimes, I think, with new programmers,

they think that you just compile a source code directly to the executable.

We’re not going to do that in this case.

We’re going to get a little bit fancier.

Doing it this way, by the way, will increase your compiling efficiency

when you start getting used to make files.

I don’t know if you’re already using those, but it’s a good idea.

So we’ll write a script to compile or make file to compile

or some kind of a build system to compile.

We’ll compile the CPP driver to just a file called

driver.o, O for object file.

So O for object file depends on driver.cpp.

We’ll have another file called c.o,

so the object file c.o depends on c.c and so forth.

So I’m just gonna go cpp.o, circle it,

and then down here we’ll do hello.o.

Notice how all of these modules, regardless of the language they are written in, compile

down to an object file.

That’s really what it is.

The higher level languages are just an illusion for us humans.

So then at the very top, we want our actual executable file.

We’ll call it main because that’s kind of a standard thing sometimes.

So it’s like our main, we’ll have a file named main that we can execute.

is going to be the main function.

Spoiler, it’s going to be inside the driver.

Remember, the main function, if you’re using GCC libraries,

has to appear in exactly one place, one module in your program.

It should appear one time and one time alone, not zero or more than one.

So the main executable depends on the driver object file,

the C object file, and the C++ object file,

and the assembly hello object file.

Looks complicated at first until you stare at it a while,

complicated at first until you stare at it a while and then you just realize,

oh, every source code is going to compile to its own object file.

And then after we have all the object files,

we’re going to have our executable just sort of linked together at the final

linking stage. So with that in mind,

I’m just going to copy paste a make file that I already wrote.

It’s like my pre solution because this is not a make file video.

And I just want you to know just real fast what it looks like.

All I’m doing is I’m going to make some variables, uh,

for my C++ compiler flags and my C compiler flags and my Yasm assembler flags.

I’m really not doing anything fancy.

I’m just sort of setting up some variables so I don’t have to type.

The newer Ubuntu stuffs need a no executable stack flag and a no pi flag.

So here we’re just like making variables for linking.

My executable’s name is going to be main over here.

I personally like to make make files that default to a cute help menu.

help menu. I know most people just default to building, but, uh, you know, for educational

purposes, I think this is a little more fun, a little easier. So this way later I can just type

make menu or make build or make run or make clean or whatever I want to do. And, um,

I’m going to make a target called run, which is what we’re going to be using. And then a target

called build, which just only builds everything. The run target obviously depends on build so that

to run it and and if you don’t know make files at this point it’s okay this is not a make file video

i’m going to make some make file videos in the future

then my binary the main program is just going to depend on all these object files notice how it

depends on the driver the hello the c plus plus and the c object files so really uh at the linking

stage we’re just going to use g plus plus to just sort of link all the object files together

Then we have a little section to compile each source code file.

So in order to make driver.o, I’m going to compile,

I’m going to depend on the driver.cpp.

And then when it comes to actually compiling, compiling it,

I’m just going to type the commands to compile it here.

Don’t worry about these crazy variables.

I’m going to make more videos in the future.

And then each module gets compiled in its own way.

Then I just have a little target at the bottom called clean because I like to be

able to just clean my build area of object files.

I don’t know about you, but I like a clean build area.

I’m a clean build area guy.

I don’t like a dirty build area.

So that was the make file.

Again, this is not a make file video,

so don’t worry too much about it right now.

The next thing we should do is make a driver.

Again, I’m gonna copy paste from my solution here

because the driver should be easy.

This is really, this video is mostly gonna be

for people who have never done assembly before.

So I’m just gonna assume you know a little bit

about C++ and C already.

So I’m going to say touch, let’s see, driver.cpp so that I can get the driver file in there.

And I’m going to go nano driver, probably put it down in the first place.

Then I’m just going to paste my solution.

Oops, you can tell I copy pasted this from my IDE because I have the extra tabs in there, which I love.

One thing to keep in mind is that, I guess like one of the first rules that I want to tell you for hybrid programs

is that assembly doesn’t understand something called name mangling.

mangling. So what is name mangling? Name mangling is basically when you have a,

oh, this is not going to get called. So this is maybe not a great idea to show you name

mangling. Let me, let me start on one of the other ones real fast too. Let’s do

the C++ module. Okay. So I’m going to go nano CPP dot CPP. I know it’s a silly name. I know it.

So CPP dot CPP, this is just a simple C++ program, which is meant to be called by

by the assembly module just to show you that we can.

And so we just have a little function here,

void, you know, the CPP function.

So it’s just a function that doesn’t really return anything.

Inside of it, I’m just going to print,

hello, this is the C++ function, no problem.

Notice at the top,

this is the first important thing that you should understand.

In C++, we have overloads, right?

You can have the same function name,

but with different argument lists.

And so that gets resolved under the hood

by the name actually kind of changing

based on what the argument signature is,

what the prototype signature is.

So that’s great and all,

but in assembly, at least at this level,

there’s no way to really differentiate between overloads.

In assembly, we’ll just call a name of some function.

But if we try to call a name,

let’s say of just the CPP function,

if we say call the CPP function in assembly,

it’s not going to work

because the actual function has not been compiled to have that name.

It’s been compiled to have the CPP function

and a bunch of extra symbols indicating what the argument list is.

This is great for C++ programs, but it’s poor.

It’s bad for other modules to be able to call the C++ function

because they won’t really know what the actual name is

after the mangling has been done.

So what you want to do is make a little block called Xtern C,

just like this.

You say extern and then C in quotes and then do a scope with braces.

And inside of that, just put the prototypes of any function that you want to be available to other modules.

So, for example, we want the CPP function to be available to assembly modules and C modules, not just other C++ modules.

So, I’m just going to put the prototype of it right there.

You can put as many prototypes as you want.

I think they should come from the same source file.

different source file where you want something to be available elsewhere,

you would put an extern C block for its prototype.

This disables name mangling,

which means the compiled name of the CPP function will actually be the CPP

function. So it’s pretty good. This will make it available to assembly.

So now that I’ve kind of explained that I can maybe close this and go back to

what I was doing before. So this is the main driver program.

We don’t need to engage in a dis,

we don’t need to disable name mangling for the main function because no one is going to call main

except the GCC libraries or like the OS. But when the C++ module calls other modules, it needs to

be able to call the right name. And remember in C++, there’s name mangling enabled by default.

So if I tell it, let’s say on this line right here, or you can do alt N to get line numbers on nano.

we want to call on the hello module will C++ by default will assume that hello has name

mangling applied to it, which means it will actually try to call some name that is totally

different based on the prototype signature, the incoming arguments and things like that.

But so we want to disable that because we wanted to just call the actual name because

hello is going to be a function that we make inside of assembly.

So it’s not going to be name mangled.

So that’s why up here on line seven, we enter its signature.

Enter its signature. Oh, actually this is from a different version of something that I was doing. I’m not going to return anything from hello

We’ll do name a demangling with our extern C block just void hello

And now when we call hello, it’s actually going to call a function called hello without any other

mangling same thing for the C S E A

Oh

That was from a different version. I wrote we’re not going to be calling C in this module. Don’t worry

We’re going to keep it simple

So pretty much we’re going to print hello and then we’re going to print,

then we’re going to call the hello and then we’re going to say goodbye.

And then that’s going to be the end of the program.

So let me close out of that and just double check what I have written right now.

So we’ve got CPP and okay, we need to do C, nano C dot C.

Okay, so let me get my solution real fast here.

It’s a simple program.

Literally, it’s just a C program that prints something.

Remember in C++, we’ve got the IO stream, but we don’t have that in C.

that in C. So I’m just going to use printf. Oh, by the way, this is a C function. Hi.

So done with that. Pretty easy. Shoot. You know what? I need to start learning how to do

clear and LSA. So I can just go up, up, up. So C++, C and the driver

and the make file, which is huge. I just realized that. And now I guess we can start

or hello function.

Okay.

So maybe I want to write this in the Genie program.

So I’m going to, well, maybe I’ll first, I’ll say touch,

and I’ll say hello.assembly.

And then now that it’s there, I can try to open it up.

Let’s see.

Hello.assembly.

Okay, so now let’s write an assembly program.

I’m not going to spend too much time talking about this,

but I’ll say like data section here,

and we’ll say section.data.

We’ll say section.data and then we’ll say system calls.

This is not a system calls video.

I’m just gonna do this very quickly without explaining.

System write is gonna be code one, I guess.

And then we’re gonna do file descriptors, one for stdout.

Whoops, stdout, it’s gonna be equal to one.

This is not a file descriptors video.

I’ll put FDs, I guess.

And then we’ll make some C strings.

C strings, you know, just character arrays, basically.

I’ll say, well, you know what?

I don’t want to type this all out

because this is not a lesson on variables.

I’m just going to copy paste my existing solution here.

And, you know, long story short,

I’m going to try to make this a pretty program

that prints hello and goodbye and things.

And so we just have like some variables

that sort of just you know old strings and hold the strings length and we’re

gonna do one that calls a new line and so forth actually that’ll be a different

video but I’ll leave it in there for now so now let’s make the text section

remember the text section in Yasm is just for the actual code of your program

and then we need to set up some external symbols. So right now, at least for the purposes of this

source code, this source code is inside of the hello module. So you can imagine we’re inside of

hello.o. All the o’s get linked together in the executable, but between themselves, there’s kind

of a little bit of like a border situation happening. There’s like some separation. So

are going to be available to the other modules,

even if they’re part of the same executable.

It’s kind of convenient because you would like sometimes

to be able to write tons and tons of functions

that are only available inside of your assembly module,

and they might not be available to other modules,

and therefore you don’t want to pollute the namespace,

I guess, of the global namespace of the executable.

So by default, if I make a function called hello,

how you make a function, just a quick recap here.

This is not a functions video.

make a symbol with the name of the function you want and then you just put a ret after it and then

you hope that whoever got you there is going to use the call instruction but anyway for now

we’re just going to say this is our hello function okay i cannot call the hello function now from

another module i have to make it visible to the other modules with a special keyword called global

our program the hello function will be available to the other modules but not anything else that

I type without marking it as global so let’s go ahead and do let’s do a welcome message and this

is not a system call a video maybe I should just copy paste this so just looking at it this is a

going to quickly you know breeze through what we’re doing in case you’re interested but you

don’t have to care too much about this right now we’re going to tell rax what system call we want

to make the code for that is what i put up in system right which is just code one means let’s

write to a file we’ll give it the file handle uh that’s going to be the variable we made called fd

std out which means we’re going to be giving it the file handle of one which means we’re telling

to print to standard output because file handle one is always standard output.

And then we just give it the a pointer to the first character of the message

string that we made and then tell it how long that message is. Then we do system call this should

actually um this this should actually print a string. Okay so then uh let’s see down here I’m

going to make a function called crlf because I’m lazy and I don’t like constantly adding extra stuff

constantly adding extra stuff to the string so that there will be like a new line line feed

situation so i usually like to make a little function somewhere called crlf carriage return

line feed is what it stands for and it’s the same thing it’s just like load up the system call

registers so that it knows i want to print the string which is just a line feed

carriage return line feed and then actually call system call and then return to the caller so this

back up here let’s see crlf i guess i am going to be doing it it’s just a string with the the

slash r slash n or the carriage return line feed characters in ascii those are 13 comma 10 so i can

put that there instead of a literal quoted string okay so we got that and then that means after the

welcome message i can just like call crlf in order to have uh the cursor jump down to the next uh

after that string is notice how I don’t have any line feeds after my strings that’s kind of why I

like to do crlf and it lets me just call crlf a bunch of times if I want to just like space things

out temporarily okay so we’ve got the hello function and now we’re going to print another

message here I’ll just explain this real fast so I’m going to print a message about calling a c

So same thing, we’re just going to tell the system call service that we want to print a string and it’s going to be our string here that says, you know, we’re about to print this.

We’re about to, what do we say?

We’re about to call the C function and then later we’re going to call the C++ function.

So we’re just going to print a message announcing and do CRLF.

So that’s really nothing new at all.

And then down here we actually call the C function, which is not going to be in this module.

It’s going to be in the C module, the other source code.

Somewhere in this program, we need to let our assembly module know

that the C function is available because it kind of goes both ways.

In assembly, I have to mark a function as global if I want other modules to be able

to call that function, but I also have to mark a function as external

if I want to be able to call some other modules function.

So we have to go back up to the top.

I mean, somewhere in the text section, I think would be fine.

But for me, I just like to put it at the top.

put it at the top at the top so I’ll say externals and then I’ll go extern

keyword and then just name the function that I intend to call this way when the

linker is trying to link all the objects together it doesn’t get upset because

you tried to call a C function which is not in the current module it’ll

understand that it’s gonna happen you know like it’s gonna call other modules

for that it usually gets upset right away and then while we’re at it let’s do

Let’s turn the CPP function because that’s going to come from the C++ module.

So now we have both of our external functions set up.

We’ve marked our global hello function so other modules can call us.

And then let’s see, oh, what did I do wrong?

I put the C message in the wrong spot.

I was inside of the CRLF function.

So I’m just going to copy paste that back up here.

Okay.

Then we’ll do the same thing for the C++ message,

or for the C++ stuff.

I will first print a C++ message

with these lines up here.

That’s just like a regular system call,

and then I’ll do a CRLF, so nothing new,

just printing something.

And then I will actually call the C++ function

called the CPP function.

Then I’ll say goodbye with another block of text.

Again, not super important or complicated,

so I’m just gonna paste it.

Just another string.

string. And then when we’re done, I’ll just say, you know, return, we’re done. So that means if

you notice how we don’t have like any symbols in here called underscore start, that means this is

not a pure assembly program. And we don’t even have a symbol inside of here that is called main.

So that means this hello ASM module is not going to be the entry point of our program. It’s just

going to expose a function called hello, which does some stuff. The entry point of our program

really, let’s see, where is it? Oh, gosh, I think I already closed it for some reason. It’s the

driver program, which we talked about. So remember the driver program, it’s got a main here. So that

means this is going to be what the GCC libraries or the operating system calls, however you want

to think of it. So this is the entry point of our program, it’ll just print something quickly,

then it’ll call the hello function, which means execution will jump in to the hello module. So

module and and also it would be really easy to apply this logic to have a c module call on an

assembly module and in fact you can use this logic to have every type of module call on any other i

mean just if you have a c++ function either being defined in the current module or being referenced

in another module just make sure that you have an extern c block and then if you’re inside of a c

and you’re going to call a non-name mingled module like C or assembly,

then just do this block here.

No problem.

What else can I really tell you?

The C module, we looked at that.

The C++, we looked at that.

Pretty easy.

Hello module.

I mean, I guess we’re kind of done already.

So what we should see now is a little intro here.

Hello, my name is whatever.

And then when we jump into the hello module,

we should see a little welcome message about being in the assembly module.

And then we should see a message about calling the C function.

And then the C function should print.

Let me open that up for you.

Let’s see.

The C function should print.

Just, oh, by the way, this is the C function.

And then when it returns at the very end, remember, even in a void function, there is

a return statement implicit at the end of scope.

So when it returns, it’ll come back down here.

And then the C++ message will get printed.

printed and then the C++ function will get called which is going to end up being

just it just prints you know another message this is the C++ function

then when it returns implicitly at the end of the function

then we get down to here where we just basically say goodbye and then return to the caller

the caller in this case is going to be the driver program because this whole time we were just inside

called. So then the driver will say goodbye and then it will return control to the GCC libraries

or the operating system ending the program. Let’s see if that works. Okay, clear. And so,

you know, the way I wreck my make files, I’d like to do a little menu. I type make and it’s like,

oh, the menu, make menu, make build, just in case, just to see what’s up. Okay, clear.

say, let’s do clear and make run because that’s really what we’re after.

Again, run will implicitly call build.

So do that.

And it seems to have succeeded.

It wasn’t actually very exciting.

Let me do a make clean so you can see it compile real fast.

We run it.

It’s compiling.

That was still kind of fast.

So up here is basically the make file, all these lines.

They’re just calling your normal compiler and assembler tools and linker tools.

linker tools and then here’s the driver like we said it prints the hello message

you know my name is whatever I love that name I have a strange name generator

that I found online that’s not my name you can call me that if you want to and

then we have the message that we’re going to call the hello module so now

we’ve called into the hello function inside of the assembly module assembly

module prints hello you know I’m inside of here now and then the assembly module

Now we’re going to call the C function and then the C function actually

executes and it is now printing with its print F function call that it did,

you know, by the way, this is a C function. Hi.

And then it returns to the caller.

Maybe I could have added another print statement if I felt like I wanted to

spend more time, but it returns to the caller, which is the hello function.

Then we get to the next part of the hello function where it’s like now calling

the C function as a print statement.

Then it actually does call the C function. The C function prints itself out.

prints itself out hello this is the C function and then the assembly module

says goodbye it says it’s finished and then it returns to the caller so finally

at the end of this print statement the there’s like that little return statement

at the bottom of the hello module that returns to the caller which was the

driver dot CPP program so then the driver prints the driver is now back in

the driver is now back in control. You can see that message behind the terminal here.

The driver is now back in control. Then it says goodbye. Then it returns zero to the operating

system and the whole program is done. So it’s important to understand that if you want to

pass arguments, right now I’m just going to say, you know, I’m going to make another video that

is more robust in the future about passing arguments back and forth between these. It’s

important to understand for now that you can pass arguments between all these different

between all these different module functions if you want.

Like if I wanted to give the.

If I wanted to give the hello function.

An incoming argument, let’s say like an integer or a character value or a pointer

or something like that, I could just load up appropriate registers.

Remember, we have argument registers in assembly.

I think I can never remember if RSI or RDI are the first one.

I’m going to say RDI is the first argument just off the top of my head.

I always look that up.

So if I wanted to call, let’s say give an integer argument to hello when I originally

called it, then from the driver, I literally would just have to type an integer argument

in there and then under the hood, the RDI register, assuming that I’m right about RDI

being the first argument, the RDI register would get loaded up with that value.

your assembly program all you’d have to do is do something with the RDI register

you know load it up save it do whatever it is that you think you’re gonna do so

that’s how you pass an integer or like a character or a pointer from C++ to

assembly it works the same exact way if you want to pass arguments when calling

from C to assembly and when if you want to pass integers from assembly to C or

or C++, it’s the same thing just backwards.

First, you load up RDI as the first argument register with whatever value you want to pass

to C or C++.

And then in the signature of the C++ function, you just expect, you know, a long or, you

know, whatever you were going to get and the register will already be loaded up.

Remember, the C++ is not really happening on the machine.

It gets compiled down to assembly.

So this source code that we’re looking at, it’s another illusion for our benefit.

benefit. It’s not happening. It’s getting assembled and then compiled down to machine code. And then

that is what’s actually happening on the machine. So when you write that you have an argument in a

C++ function signature prototype, you’re not really writing that at all. You’re just saying

that the A variable should get loaded from whatever the RDI register had.

Or if you previously loaded the RDI register and you want to call a C++ function, that’s

a C++ function that’s what it would expect the RDI register so it can work pretty seamlessly

just calling things back and forth um so yeah I guess that’s all I really had to say

this is the basics of writing a hybrid program thank you for watching I hope you learned a

little bit of stuff and had a little bit of fun see you in the next video

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

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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’ll happen.

Also, if you look at the middle of the screen right now, you should see a QR code, which you

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

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you know just send me a comment whatever i also wake up for those in the middle of the night i get

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as as i fade into the darkness which is coming for us all

Thank you.

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