How Linux Kernel Prints Text on Screen
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- Опубліковано 22 сер 2024
- In this video we will dive into a simple call to the printf function and see what happens in the kernel and how it gets eventually printed on screen.
For more information about setting up an environment for kernel dev, checkout the welcome link on my channel.
Docs used in video:
- docs.kernel.or...
- wiki.osdev.org...
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- The display mechanism the kernel chooses depends on the setup, this is what it chose in the case of standard QEMU config & x86_64, but on other machines it may use something else for example a framebuffer. VGA mode 3 is a common x86/x64 choice, especially for older machines, but modern ones may not support it at all.
- The reason it printed 3 "H" letters and not "Hel" is that I recycled the same command that I used to modify the first letter to red, and only changed the color so it brought over the same first letter each time.
@@nirlichtman Pls Make Linux OS But With Debian-Based
Thank you for explaining why H is printed 3 times, because it can be understood as you run over the same char, but in reality you just moved further in “array of letters”.
This video is a tutorial on:
- How printf works in the kernel
- How to the debug the kernel with GDB
- An intro into how to to use GDB
- A guide on how to look up kernel documentation
And several things more. Condensed in 12 minutes. This is perfect!! I've never seen a video as good as this one explaining anything!
I have used GDB for about a year now every week and learned new commands
This is more than an intro on how to use GDB lol, I thought I was decent with it but this is next level.
”we’re gonna have to dive in a little bit to the assembly”. What a madlad ❤
You're making one of the nerdiest linux videos I've seen on windows using MS edge...saying I'm confused is an understatement
What you use doesn't matter. It is how well you use it...
On Windows at least he doesn't have to deal with Wayland issues :D
What I was wondering the most is why didn't he turn off the search box in the taskbar :P
@@marsovac ...as someone who runs wayland without Xwayland or X11 compatibility at all...and as a developer of reasonably widely used wayland software...I take issue to that remark lol =D...that aside the search box is terrible.
He's under three VMs
@@marsovac wayland on newest nvidia drivers works well right now. Fedora btw
A lot of corporate environments demand you use Edge.
I always wanted to know kernel that deep 🙂
we all like it deeper
no unnecessary talking, straight to the point, extremely well explained, this is why I pay for internet, thank you brother
The rabbit hole is much deeper than I was thinking!
The magic behind a standard library function-and the POSIX system calls-is what every CS student would have wondered about for once. Thank you for making this video. It brought back memories of being curious about how a computer works.
This series of videos is awesome. Just explains and explores a low-level topic quickly and without any fluff.
Excellent video as always. Love seeing well-made low level computing content that does deep dives into while keeping it just dense enough to watch in your free time.
I feel like many videos on this topic are always either way too short/simplified or way too long and drawn out. You always hit it on the head.
this feels like i'm watching paid content for free
This is like a whole subject at uni in less than 15 minutes. Subscribed!
I don’t know why but the way you say ‘config’ sounds satisfying
I just discovered this channel today, and with two videos that I have seen I feel that in half an hour I have learned more than what I learned in many classes at the university. Subscribed!
I love the confidence in your voice , i wana be this confidence when coding .
I like the way you explain things. I also noticed that you are very good and very at ease with programming languages. I will look into more of your videos / media when i jump into c programming. thanks again
I didn't know untill now that there's a lot of stuff under the hood just to print something on the screen but with all of this it really makes sense on why is this.
Really cool video btw.
Really fast paced and right to the point. New sub 🎉
kys_write?? how rude...
oh
oh "kms" exist
You should write text to the screen NOW!
I agree, the side effects of this function are wild.
@@jackkendall6420 xD
i have longed for this exact video for years. Thank you.
Just found out about your channel and I'm already hooked with it!
שמח לראות שאתה מישראל 😊.
Keep up the great work!
This was super dope! Loved this technical deep dive! I learned a ton!
What happens when you have a console window, rather than raw VGA as your tty? How does it "know" that stdout is mapped to VGA in this case? is it done during process creation?
When a process writes to stdout (file descriptor = 1) typically it is managed by a parent process. Parent process can catch this data (like for example console window process) or pass through to it's parent. Init is a special process, that starts all other proceses, so it's stdout is rendered by a kernel (also it can pass through data from a process, that is it's child)
@@thebuggerdev So the is full process something like this? (let's say, XTerm) printf -> kernel -> parent process -> x11
@@eitantal726 yes, in this example XTerm is a parent process
@@eitantal726 and to be precise: communication of xterm with x11 is also done by kernel's functions
clicked for the kernel, stayed for the gdb tutorial
Short and descriptive. Great video.
Thanks man, really useful. Keep up the great work!
Have been looking for a channel like that for a long time, thank you!
these videos are so cool and nerdy and the best of it you learn so much
This is such a good reference video for anyone wanting to dive into the kernel
Programmer of 10 years here. This is straight up black magic.
I use gdb a lot but somehow didn't know about the 'advance' command, I would put a breakpoint when I wanted to advance forward (that I had to delete or disable later). So that's nice, thanks :)
It might be worth adding that the vga text mode is a very legacy functionality. Qemu still supports it since its very easy to use from software but I'm not even sure if modern gpus support that. I think all modern gpus support only a normal linear pixel frame buffer where you'd have to blit your font characters manually.
That's a good point, added information about this in the pinned comment.
We are going to need some more of that gdb magic you are doing. Nice video!
Dang breh aside from just screen printing, I learned some gdb and vim. Lots of good stuff.
Finally found this vid. I clicked on a different one when seeing this in the periphery and was searching for it ever since.
That was awesome, I'd love a full series of this
Came here to learn about the kernel, but instead got an intermediate GDB tutorial. I'm thankful, I'm a lot more interested in GDB than the Linux kernel, actually.
As for the video mode, yeah, I remember programming assembly as a child, I know how that mode works. I produced several full screen vomits of blinking colors.
Thanks a lot! I'm trying to grasp system calls and standard library for my own OS. I am always shocked by the complexity of the code in Linux kernel and the amount of knowledge and talent it required to write.
Now THIS is top quality content
Amazing video
But I have a question, all those boot logs before printing the H, how are those printed.
printk(); in kernel space, in (very, very) shorts. I do believe they use different ways to printf() to implement printk().
wow i didnt expect displaying text to be so complicated, that's interesting
beautifully broken down. thanks!
This video is insanely good, not many like these!
You're a freaking wizard, Nic. Great video! 👍😊
essential things, nicely done. But IMHO you missed one point, how vga works (logical level), ie what the kernel write to the vga memory in order to see an 'H' on the screen. Anyway thanks for sharing.
THE HELLO WORLD TYPING SPEED IS CRAZY
Hoping more videos like this. ❤
This man is making some of the most informative and interesting Linux videos on all of youtube... using Windows 10
Very interesting insight! Great content, thank you sir
Nir please I want to know what you way to learn efficiently ? can you give me our method ? some tips ? . Great video thanks
Thanks! Check out my welcome page for more information and recommended resources (link on my channel)
@@nirlichtman Very awesome thank you 🙏
Why did it print 3 "H"? I tought it would continue with the other letters...
Reason it printed 3 "H"s is that when I changed to the other colors, I reused the command in which I modified the "H" to red, and only changed the color, so it kept the same character as before.
@@nirlichtman Ah, right. You did not update just the top byte.
Amazing video!
Linux kernel + gdb + qemu = ❤
I hope you manage to build more videos like this. =D
Hardware and software interaction would be great. 😃
That is deeper than I thought
This is great! Thank you very much!
Damn the knowledge I got through this video is amazing
Excellent job and crazy skills 🤯
Yayyy
this is why I love UA-cam
Excellent information presentation 👌
idk why, but this is fun to watch.
how does kernel printk work? i do not want to use format string but just write
some cool gdb tricks I didn't know. But I think I'll use the tui/disassembly view instead of jumping to addresses
would be cool to show how graphical virtual terminals work in low level too.
this channel is cracked
Driver devs can get a lot of hate sometimes, but if it weren't for them, we wouldn't get to see the hard work of printf()!
learns a lot, thanks!
This is so cool, thank you
0:56 For the record: the `-j` flag stands for "jobs", and tells `make` how many actions it may run in parallel. It can generally make a complex build faster, up to the limit of the number of CPU cores you have. At least for me, the numeric argument isn't necessary and by just passing `-j`, "make will not limit the number of jobs that can run simultaneously".
this is an insanely good video.... thanks
Could you make a video about how to make a simple wayland-based window manager?
Thank you!
Good stuff
Great video!
Thanks for such an amazing video.
Could you please share the tips to become good in low level things just like you?
Yes, there are a bunch of tips on my welcome page, link in the channel description
@@nirlichtman Thank you Nir :)
You back!
thanks for another great video!
I still don't know why the random modified memory appears on the terminal, but I've learned the basics of GDB, so that's something
On VGA text mode 3 (the display mode the kernel decided to use in this case), the text on screen is manipulated through memory, so when writing to a specific memory address we can change the contents on the screen. (more info on the OSDev wiki article link in description)
That memory was about to be printed anyway. It was H to start with because that was the beginning of Hello World. He just found where it was about to happen and changed the memory just before it did. Then because he kept using the same second byte and only changing the colour byte, it printed more H characters instead of the e and l it would have otherwise printed.
WTF, bro just said. "I don't really like AT&T *assembly* syntax, so I'm going to intel *syntax*"
This man is crazy
"it's more fun to read" omg
what's crazy about that? You pretty much have your preference if you ever worked with x86 assembly.
wonderful explaination ,
Awesome, thank you
Thats a knowledge packed video. I understood it in parts. Is there any past video i need to see to better inderstand this one?
Depends on which parts you were and weren't able to follow. Any GDB tutorial will be a good place to start
I would recommend checking out my videos about making a simple distro, Linux system calls explained, and generally my Linux playlists. Also, welcome to check out my welcome page for recommended resources, link on my channel
Very cool. How do you get so comfortable with CLI tools? It's like you don't even need a mouse.
Check the welcome link on my channel for recommended learning resources and additional information, I indeed prefer using the keyboard over mouse control in a lot of cases, but I wouldn't say a mouse is completely unneeded, for example for graphical stuff it is very useful.
i feel like i am linus torvalds after watching this video
Noob arch user : nooo you can't just run both windows and linux
Chad Kernel dev : anyway I just opened Qemu on Linux from ssh windows
*WSL :)
@@nirlichtman WSL? EVEN BETTER
Oh damn, I'm coming in for another "Neat!"
Love your videos!
Can you start a series on deeo dive into Linux kernal ? (A real detailed one)
More kernel videos are planned :)
Great job, Thanks bro
You are an absolute legend 🤯
How did I end up on this side of youtube again? You know.. that side of youtube where we change ANSI escape codes in the register live from the debugger.
So we can have a prettyier color in our terminal emulator...
i'm not judging you but, why are you using windows when you're very interested in linux?
I am also interested in Windows :)
based
This is such a good video
How does this change in a terminal app in a desktop environment?
Well done.
It’s so interesting👍
Why does it print "H" 3 times? At 12:15 you hit continue execution, so it should go to the next letter, "e" right? I would expect red "H" and green "e".
It would have been an e, but in modifiying the byte for the color, the byte for the character was overwritten as well.
At 12:32, The original value of 0x765 (Light Gray 'e') was changed to be 0x248 (Green 'H').
For Green 'e', the value would of have had to have been set to 0x265. 0x2 being the color Green, and 0x65 being the ASCII code for 'e'.
This is fantastic
I guess if you wanna keep the character value but only change the color, you do bitwise AND to the value, for example $r10w=$r10w & 0x2FF (for green color) right? Did GDB support this expression?
pls do videos about v8 engine debug or idk things like that
Can yolu create 64 bit bootloader with c and asm
great!