Mastering System Services in x86-64 Assembly Programming

Neural Lantern

Ready to master x86-64 assembly? This video breaks down system services, showing you how to use syscalls to print messages, handle file descriptors, and exit programs cleanly. We?ll walk through a real assembly program, explain key concepts like standard output and file handles, and share tips from a top book on the subject. Whether you?re new to assembly or sharpening your skills, this tutorial is packed with clear examples and practical advice. Subscribe for more coding deep dives, and check out our upcoming file I/O video! #AssemblyProgramming #SystemCalls #x86_64 #CodingTutorials  Introduction to System Services 00:00:00 System Services in x86-64 Assembly 00:00:04 Recommended Book on Assembly 00:00:35 What is a System Service? 00:01:01 Example Assembly Program 00:01:21 Syscall Instruction Explanation 00:02:52 Standard Output and File Descriptors 00:04:01 Printing a String with Syscall 00:05:16 Exiting a Program with Syscall 00:07:21 Exit Codes and Program Success 00:08:02 Book Reference for System Services 00:09:51 Detailed System Write Service 00:11:01 Checking System Call Return Values 00:12:08 File Operations with System Calls 00:14:35 Closing Files and Best Practices 00:15:50 Other System Call Codes 00:16:43 File Permissions and Modes 00:17:20 Handling System Call Results 00:19:03 Future Video Plans and Wrap-Up 00:20:40 Call to Subscribe and Outro 00:21:28  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

Transcript

00:00hey everybody in this video we're going to talk a little bit about system services

00:04in an x86-64 machine while programming in assembly

00:15this video is going to focus on yasm assembly but this should also work for any other assembly

00:19language that you program as long as you have the right kind of cpu x86-64 is also known as

00:25amd 64 architecture so uh bear that in mind okay so for starters let's see here's a wonderful book

00:33i love pushing this book i did not write it the person who wrote it is a genius it's called x86-64

00:39assembly language programming with ubuntu it's a free and open source book so anybody can go get it

00:44find his website and um here's the version i'm using and uh honestly a really old version of

00:50this book is still really good he just keeps you know making little improvements uh but it was it

00:55was good even a long time ago anyway so what is the system service for starters before i go into this

01:01book a little bit more uh what is the system service suppose for the sake of argument that i'm i've

01:06already written an assembly program a pure assembly program not a hybrid module program or anything

01:12if you want to learn assembly see my other videos but for now i'm just going to assume you kind of

01:17already know how so i'm just going to open up a pre-made program here i'm going to do nano so i

01:22can edit the assembly file just so you can see what's inside of here okay so uh i hope the red

01:30is not too hard to see oh the tabs got ruined on nano hang on let me see if i can just open this up in

01:35genie should have done that to begin with okay yeah okay i wrote it in genie and i guess i don't have

01:41the tabs set up in nano very well okay so you know here's like my assembly program you know i just have

01:49a little string here and um a string length and crlf for like a line feed and some file descriptors and

01:57stuff that i talk about in other videos here's the entry point of the program uh just like a little

02:03start entry point because it's pure assembly and uh all i'm going to do is i'm going to print a message

02:08right i'm just going to print a little hello message and then call on crlf all crlf does is

02:14just it just prints a new line so the program is really really simple um if i run it right now you'll

02:21just see what happens if i say let's see clear and make run then you can see there's some stuff happening

02:29up here in the make file which i forgot to update the title ignore that uh basically the real action

02:36is this line right here hello i'm printing using a system service if you already know assembly then

02:40you have probably already seen this before and it seems a little boring that's good but i just want

02:45you to know that we're using a system service to actually do the printing kind of take it for granted

02:52here this little instruction called syscall what is a syscall well it's just basically you set up some

02:58variables some some codes some data whatever in in some special registers one of the registers lets the

03:06system know what you want it to do one of the registers is like or i guess all the following

03:10registers if they're needed are just input to the system call that you're trying to do so in this case

03:17rax is letting the system know i want you to do this this code i want you to do this certain thing

03:24i have a defined set up here called system right but if you look up in my little defines area

03:30it's just a number one so that means if i've set the number one in rax i'm letting the system know

03:37that i want it to write to some pipe to some file uh then you know depending on what code you're using

03:45rdi rsi rdx and other things may be needed or may not be needed at all in this case when i'm writing

03:51somewhere it wants three arguments so i have to use rdi rsi and rdx because those are the three arguments

03:58the first argument that it wants is where to print see my other videos for a more in-depth explanation

04:05of uh file descriptors and pipes but just long story short every program that gets created ends

04:11up with three pipes uh standard input standard output and standard error standard out is a pipe

04:18number one let's see where's that yeah it's just assigned a number one so every program has its own

04:24number one pipe which is just standard output and um uh i guess i should say that it gets piped to

04:32file descriptor one it's kind of like an ambiguous term sort of but uh the the system write call will

04:38write uh data to a file and if i give it a file handle of just one that lets the system know oh actually

04:45i want to write to this standard output for the one particular process that we're inside of

04:51but you could open any file that you want and get like a file handle to it and the operating system

04:56will realize oh you know i i associate that file handle i gave you with this certain open file

05:02that you created or opened for reading or whatever um and so when you give the handle back it knows

05:08where to write so in this case it's treating the standard output just like a file just a file with a

05:13special handle of just one i'll talk about that more in another video then the next two arguments

05:19it wants is just the uh the a pointer to the first character of the message that you want to print and

05:25then just an integer representing the length of the message that you want to print and if we just look

05:30up here again at my little hello my hello symbol points to an array of bytes just characters there is

05:38no null terminator on there it's just bytes only and so when i give it you know message hello by

05:45itself what i'm really giving it is a pointer to that h letter just a pointer to the memory look you

05:51know i'm pointing to the memory location of the very first character in that string and then for the

05:56length i'm just using a special uh token little shortcut thing where you just put a dollar sign minus

06:03and then the name of another string and then the assembler will compute the length of that string

06:09so this right here is the same thing as me just typing the exact length of the string i don't even

06:14know what it is i don't feel like counting it you can at home if you want one two three four i'm not

06:18going to do it anyway so uh we basically just say you know i would like to write somewhere i would like

06:26to write to the standard output file file handle i would like to you know to that pipe i would like to

06:31write you know this string and then i would like to tell you that this is how long that string is

06:37and then after you set all that stuff up you just say system call and the system will go ahead and do

06:41all of the hard work for you it'll go figure out how to actually write data to the file and you know

06:48figure out how to do everything that it's supposed to do and all you have to do is a little system call

06:52here right after this you can see that i have a call crlf that's just a function call i talked about

06:58functions in other videos but basically you know i'm just calling a function here and it does the

07:02same thing it makes a system call but instead of printing the hello string it's printing my crlf

07:07string and if you just kind of look at what my crlf is it's just these two characters 13 and 10 so

07:12like you know slash r slash n for carriage return line feed that's why i call it crlf

07:19so all this program does is just print a message and then a new line and then at the very end it does

07:24another system call to properly exit the program if this was a hybrid program and i had like

07:29the entry point of main you know that the gcc libraries give you then i would probably just

07:34want to return at the end of this function but since this is pure assembly i can actually just

07:38exit the program and be fine so exit with success is a different system call code notice how i have

07:44the symbol system exit loaded into rax and if you look up here system exit is just the code 60.

07:51so if i send a code of one into rax that means i want to write some data somewhere if i send a code

07:56of 60 somewhere that means i want to exit the program the only argument that takes is rdi which

08:02is just what is the exit code that you want to return to the operating system so um if you recall

08:10from some of my other videos let's see if we do echo hello echo always succeeds at least as far as i know

08:15there's like it's like really hard to make it fail if i echo hello then the program echo

08:21launches and it will succeed by just printing the word hello like see how it just does that

08:26so that means after that command i could echo the special variable dollar sign question mark just to

08:32see what the echo command exited with right it should be a zero because it succeeded so you can

08:38see now a zero under the hello on the other hand if i concatenate um let's say a file that exists

08:47i should also get you know an error code that is a zero see how or an exit code so it's a zero at the

08:52very end but if i try to concatenate a file that doesn't actually exist i'll put uh os release 2

08:59because that doesn't exist notice how the cat command fails because it couldn't find the file

09:04and the exit code is a one so all you're doing when setting rdi here is just controlling what exit

09:09code you want your program to exit with this isn't very convenient for for a human running a program maybe if

09:15you want to look at the exit code for some reason but this is really really convenient for programs

09:19that want to automate other programs just keep in mind if you have a program that is executing another

09:24program and it wants to see if that program succeeded the exit code is one of the easiest ways to find out

09:30if the program succeeded so this is the whole idea of this entire program we're just using two system

09:34calls to print something and then to exit the program hopefully this makes sense and now i want to

09:41go show you this wonderful wonderful book if i didn't already say this before sorry if i already

09:51did this book is written by a genius it's called this here's the guy who wrote the book and here's

09:58the version that i'm using but old versions are good you can get this book for free on his website if

10:03you go find it so i'm just going to go to this assembly book which deals with a lot of yasm assembly tips

10:08and tricks and tutorials and explanations and things and i'm going to go to the section uh labeled after

10:15system services so for me this is a chapter 13 system services just to give you a little explanation

10:21of what this is uh you know this is basically what i said it gives you you know a chance for your

10:27application to ask the operating system to do something for you that you don't want to program

10:30from scratch in assembly or that you can't program but here's the real juicy details here if we go to

10:36uh yeah yeah okay okay if we go to uh the appendix which is it appendix c which is section 23.0 system

10:48services there's a whole bunch of system services in here that you can uh that you can read about so

10:53for starters if we go down a little bit notice how it says basic system services if we go down a little

10:58bit you can see right here the system right service that we've been using in the sample assembly program

11:04you know when we're just printing we say code one is to write something and so this is what the table

11:10says it says here's the call code where you which you place inside of rex before syscall

11:15call code one is going to be to write characters and then it tells you what arguments it needs so like

11:20rdi is the file descriptor where do you want to write to just like i said before you could give

11:25it a file handle that you already received from opening a file or creating a file

11:28or you could give it a zero or one or two if you want to try to write to one of the pipes although

11:34actually file descriptor zero is standard input i don't think that would actually work you could

11:39give a file descriptor zero to reading if you wanted to read the user's input or or you know any input

11:45that that program was given through its standard input rsi is just the address of the characters to

11:50write like we talked about before and then rdx is the count of the characters to write notice how there

11:55are no more arguments and that's why i only gave it three arguments besides rex notice how

12:02it says here if unsuccessful it returns a negative value if successful it returns the count of

12:08characters actually written this is a really really good idea if you think about it a lot of new

12:14programmers they don't really look at return codes when they call system services or built-in c functions

12:19or built-in c++ functions they just kind of call it and hope for the best but imagine if you uh if you

12:24try to write a huge long string in uh using system write call code one and maybe some of the characters

12:32did write but the system decided to only write i don't know half for whatever reason maybe it ran

12:37out of buffer maybe you know it interrupted you know you're right or something like that so it could

12:42happen what that would mean is that the return value it would be greater than zero indicating that some

12:47characters were written but it'll tell you exactly how many characters were written so you'll know

12:53that you got to keep calling the sys call until all the characters were successfully written

12:58why would you do this i don't know maybe you have a gigantic string like maybe a full huge file like a

13:04like a several gigabyte file and you wanted to copy it to another file so you'll be calling system right

13:11over and over again and system right it's only going to write so many bytes at the same time

13:16so you use the return value to figure out how far forward in the read buffer you need to advance

13:21or you know whatever so that you can uh well write the entire complete file or the entire complete

13:28string or whatever it is without any gaps without it being truncated and so forth and then of course

13:32if it returns a negative value then it totally failed like you tried to write to a bad file handle

13:36like standard input or a file handle that was closed or a file handle that was open for reading

13:41only something like that and then you can use some if else you know branching logic

13:45i mean not if else in assembly it's just going to be comparison and conditional jumping or branching

13:52and but you can have more control over your program right because you want your program to be able to

13:56respond to errors let me just quickly cruise through all of the other options and let you know that

14:01in another video i'm going to do a full tutorial for how to copy one file to another by opening one file

14:07for reading and then like using system calls to read from it and then open another file for writing

14:13and then you know use system calls to write to it and i'm going to do that looping buffer stuff that

14:17i talked about before anyway so but for now i'm just going to go through the rest of these system

14:21call codes let's see so first off we got uh open um well if you want to open a file then you just uh

14:32you know give it call code 2 and uh you pass in the address here you know rdi you say here's the

14:37address of a null terminated file name so that means somewhere in memory you have to have the file name

14:43that you want to open uh with with a null terminator like a zero at the end of the string

14:48either the full path or relative path to wherever the program is currently running

14:52and then rsi is going to be file status flags and what does that mean we can actually just search

14:58for that i'm going to say ctrl c to copy and then i'm going to do ctrl f to search

15:03for that and uh let's see what page am i on right now 340. if i go down to the search results for that

15:09it just explains what the file modes are so if you put value zero that means read only you're opening

15:16the file in read only mode a value one that means uh write only a value two allows reading and writing

15:23to the file so if you don't know what that means well there it was what was i just on before 40 oh

15:30gosh completely lost my oh yeah 40. okay so opening pretty easy right like if you didn't know this

15:37before you didn't necessarily read me you could just look at this table and go what do i want to do i

15:40want to open so here's the code here's the address that it needs when you're done opening a file when

15:48you're done working with a file you want to close that file unless the whole program is going to

15:53terminate at that point you i mean maybe you have a function that gets called somewhat often and you

15:59kind of always have to open a file and sort of look at something or write something and maybe you want

16:03to close it when the function is over right you don't want to have a file handle just open forever

16:08that's a waste of memory and it might introduce bad behavior to your program if you just have a

16:12bunch of file handles floating around that you forgot about so it's proper to close a file when

16:17you're finished with it that's just going to be code three and it only wants one argument it just wants

16:22the file descriptor of the file that you wanted to close so if you open a file for a reading or to

16:27create it you get a file descriptor back you should probably hang on to it somewhere on the stack or like

16:32in a global variable or like you know in a register or whatever then when you're done doing something to

16:36that file and you're sure that it's time to close it just give that handle right back when you call

16:42code three to close it then you can sort of like seek you know forward and backwards to the file if

16:46you want to you can fork the current process that's kind of advanced for this video you can exit remember

16:52before we exited from that program with code 60 so that's it call code 60 if you didn't watch this

16:59video you could have gone through this table and just said oh you know if i want to terminate the

17:02executing process i do a system call with code 60 and i'll give the exit status to rdi which is

17:08typically zero for success a bunch of other stuff's create uh get the time of day and then the file

17:15modes that we talked about before and then file permissions let's see where's the permission i think

17:20it's when you want to create a file yeah right here if we wanted to create a file that's code 85 rdi is

17:26the name of the file you know like a pointer to the string rsi is the file mode flags and then you're

17:32thinking like what are the file mode flags just go down here and then here you go uh all the file

17:39modes that you could ever want you just look at it and go well i guess i can just copy paste this number

17:46right here and it'll end up being you know whatever is described on the left and the right like uh the

17:51group has read write and execute permissions and so forth i mean really if you look at this long

17:57enough you'll kind of realize that this is a quad word because it has a q on the end of it and the

18:02rest is just an octal file permissions notation where this first number here notice how the first two

18:08are zeros but this first actual number here is something that applies to the user and then the

18:13next one applies to the group the next one applies to others who are not part of the user or group

18:17um and this video is not about file permissions i'll probably make one later if i think uh people

18:24will actually watch it but um yeah you can just kind of like look it up here if you want to and just

18:29copy paste that into a define somewhere let's see what else um i just want to show you a little code

18:36snip just to illustrate a little bit more what i was talking about before um people usually make this

18:43mistake they'll do let's see this is what people will usually do like new programmers suppose this

18:50is a function pretend we're in c or c plus plus for the time being you call some sort of a system

18:55function or an api of some other library and you just you just hope that it works you just call it

19:00and then your program continues underneath right it's a bad idea because maybe that failed maybe there's

19:05some action you need to take so instead it's a really good idea to check the return result

19:10or i should say an even better idea is check the documentation usually these types of you know

19:17c system calls or function calls or api calls or whatever they'll return something indicating success

19:23or failure sometimes the documentation will tell you oh you've got to put like a pointer to some

19:27other data structure in the arguments and then the result comes there of course just read the

19:32documentation but usually it's just it returns some sort of a result so you want to grab the results

19:39and then check the result in this case i'm assuming this is sort of a standard function that will

19:45return zero or greater on success like the system write call or the system read call and return less

19:52than zero if it fails so here i'm just responding you know if it's like if the result was greater than

19:58or zero then it's like yay we can proceed maybe i'll continue with my program in some way and if it's not

20:03then i'll print a complaint to the user and then i'll take some sort of an action like i'll write

20:08to a log file send an email somewhere you know do something to try and recover from the error and

20:14alert admins to the error and in this case i'm just throwing an exception this video is not about

20:19exceptions but so i just want you to know this is a good design pattern to not just discard the result

20:25of a call to something you should check the result and see what you're supposed to do

20:28um let's see what else did i want to tip system services file handles um yeah okay i think that's

20:37everything that i wanted to tell you in future videos i'm going to talk about uh you know some

20:44of this stuff much more in depth uh like with my file io video that i'm going to publish pretty soon

20:50which is uh you know it lets you read and write uh to files and it uses a loop to read a little bit

20:56at a time okay thank you so much for watching this video i hope you learned a little bit of stuff and

21:01had a little bit of fun i'm outie or something goodbye hey everybody thanks for watching this video

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