I don't care what other viewers say. Keep using paper! Sometimes you have to go the extra mile to make a point. I like your teaching style. Thanks for the videos. Very good info here!

As a web dev, watching this makes me feel like I just swallowed the red pill and saw the real world for the first time.

When I first started programming in C (mid 80s) I wanted to make sure the compiler was doing a good job and would always check the assembly for timing critical code. After doing this for a while I realized I could write the C code in such a way to influence the compiler to output very efficient assembly. Nowadays, the few times I do this, I'm amazed at how good modern compilers have gotten at optimizing for speed.

7:05 you can actually notice how each variable takes 4 bytes of memory from the way they are located always 0x4 apart from each other

Idk why but there's something so satisfying about seeing terminal output on paper. Especially C code and disassembled code. Mmmmmm.....

The instruction at 0x10000f63 is moving the result of the printf function (the number of characters written) to a location in memory (even though it isn't used)

One of my college profs was in the Navy and needed to write assembly for the Navy to optimize COBOL code. He wrote it in FORTRAN and turned in the assembly. They had strict goals on lines of assembly to be written and debugged per day. He always met his goals. His reasoning was that FORTRAN was a pretty efficient language, and so he probably couldn't do much better. The Navy never knew they were converting their COBOL to FORTRAN.

I like how the compiler optimised the while(1) into an unconditional jump instead of actually evaluating the expression "1". I know compilers have been doing that for decades, also it's a very basic optimisation, but I enjoyed seeing it on paper :D

In only 10 minutes, you made me want to learn assembly language. Il looks so simple when it's explained so well. You did a great job, Ben Eater.

0:20 how did you get that infinitely long paper?

The eax register will contain the return value of the printf function. Evidently it is being stored on the stack in the expectation that it will be needed later. Presumably you had the optimiser turned off when you compiled it.

I understood in theory how C went up to other languages. Now I understand how C goes down to bits. Awesome work.

Regarding, moving eax onto the stack. eax contains the return value of the printf call. It's not actually needed by this example. It's probably saved to help a C debugger display what was returned and is likely a nuance of the compiler.

At Uni I made a Snake game in Assembly IA-32 for a course. Never again, thanks.

The type of video that makes you ask "How did people come up with this?"

Scrolling through your videos i can see the depth of your knowledge , its brilliant and inspiring. I just subscribed.

Just a little remark for people wondering why the code generated by the compiler contains strange and unuseful constructs. It is simply because the code was generated with the -O0 parameter which means, no optimization whatsoever. This means that the compiler basically does a nearly 1 to 1 translation of the C code to the assembly, without considering if the operation are redundant, unused or stupid. It is only when optimization is enabled that the compiler will generate better code. In this example, for example, it is stupid to read & write x, y, z continuously from memory. An optimizing compiler will assign register in the inner loop and will never write their values to memory. The spilling of the printf return value 'movq eax, 0x14'bp)' will of course not be emitted/

I taught myself BASIC then Pascal then C++. Learning was actually fun with some of the books they had in the '80s. I got a C64 for my 8th birthday, and I got the C64 Programmer's Reference Guide. It's just amazing the things that were in that book. It went from teaching you BASIC to showing you the memory maps, the pinouts for all the chips, and how to do graphics and sound. But it also had a 100-page chapter teaching assembly! It confused me because it made cryptic references to an assembler called 64MON which I had no idea how to get, but that made it more intriguing. The assembler class I took in college was also one of the only interesting classes I ever took. But I'm pretty weird. I was such a nerdy kid that in middle school I wrote letters to Brian Fargo and John Carmack asking for career advice.

Your video is still helpful in 2020 and I'm sure other people would also understand concepts from it in coming years. Subscribed!

This is an excellent video that was easy to understand. You also managed to not skip anything seemingly trivial, which is rare for these types of videos and should be appreciated. Good job, man.

you didnt mention why y is allocated in 0xC. That is because integers have a sizeof 4 bytes so 0x8 + 4 = 0xC

This is so cool, and I think this would be a way more fun/efficient way to learn Assembly than what's taught in colleges. It's way easier to see where these commands come from and what they mean if they're being directly compared to an actual C program. Much harder if a bunch of Assembly terms you've never heard are tossed at you and all of a sudden you're expected to code a program like this.

This came up on my recommended. I remember using this video the night before my computer architecture exam several years ago back in college. Thank you, Ben Eater, for saving me on that one question.

Many thanks for a wonderful and insightful tutorial. I have been writing code for more years than I care to remember but this is by far the best explanation of how machine code relates to back to it's source code. Fantastic video.

Man, well done to you, you perfectly explained in 10 minutes what a professor in University had 6 months to demonstrate and still wasn't able to. Really interesting.

Old video, but I still want to remark that you can add the "-S" switch to make GCC output assembly directly into the output file.

great video. When I learnt about programming languages, I always wanted to somewhat understand how computers treat the information we feed them, but looking at assembly on your own is just like *question marks* comparing it side to side to C is really insightful!

this brings me back to my assembly class at university, in 2002. i liked that class a lot, but i've never used it again since i didn't go into a career in embedded

This is a valuable analysis. Knowing what’s going on at this level I think is fundamental to truly understanding computing. And code and architecture. Great.

"Back in my day we had to compile code by hand"

Is it just me or you are feeling excited as well when you see machine language? I was learning python and working on stuff with for like everyday in 8 months. I started learning C and now it just feels a lot of fun language to work with! I even gave a break to python for the time being. Watching assembly feels interesting as well.

My 14 year old self back in 2003 would be extremely excited and thankful if someone would explain machine naguage in such a clear way. Thank you and well done!

to any one doing c++ exams on paper.. do a table with all variables and update their values like the code says.. this way you keep track of everything

I feel calm when people use paper to explain :) very educational and relaxing

Very impressed the way you explained with the amount of knowledge you had with that assembly code.

Just fantastic to see how efficient the code produced by the C compiler is. I spent years writing assembler as a kid and used to have competitions with other on how fast and small we could make our code..

Your teaching is great, informative and esthetic. I loved watching it. Thanks!

I appreciate your effort to make this teaching video to share what you know and honestly say don't know to things you don't know. Well done. I'm not sure either what's the point of moving the contents of eax register on to stack.

This is by far the best series explaning how computers works inside. Amazing work!!

This is a good example on why learning coding without understanding how computer technologies layer on each other seems so daunting. Just learning a coding language is not really that difficult. But coding is complexity built on complexity, and each layer down it become exponentially more complex. From an outside perspective, like when when I first started learning code, it feels like you don't just need to know the top layer of knowledge, be it python or c++, but you need to understand what makes that work and how something else makes that work. At the end of the day Id have the impression I was going to have to learn how electricity works to understand the chipsets or ram to understand the next layer to understand the next layer all the way up to my code. The great thing is that these languages were made so we don't have to do that. OOP and modern tech has almost made everything so independent and modular that you can learn the end result without knowing fuck all about how it works. You don't even need to know to code to write games anymore.

Remembering my first programming. You looked up the op codes and entered them on a keypad in Hexadecimal. This literally was writing the cpu instructions directly. I miss the 6502.

Ohhhhh this helped me for my malware and reverse engineering final. THANK YOU!

I'm studying IT, and coursing a few subjects that include C, C++, Assembler and Pentium processors architecture. And this is one of the best, and more interesting video that I've seen. Great work!

gz on the video, man. i like how you are humble to tell people when you're not sure about.

This brings me good memories from my first semester in college when I worked with MIPS assembler code. Probably one of the most interesting things I have studied so far.

Really enjoy your videos, started my programming journey, if you will, about 5 years ago with the idea of wanting to make video games. i later found assembly programming and electronics engineering FAR more interesting than game design. I have been learning 8086 ASM on DosBox lately hoping i can get enough experience to understand how computers work entirely, i am currently in the process of learning how different IC's work on a breadboard and hope to build my own 8bit computer soon. Thanks for getting me started on such a fun hobby i hope to make my job someday, keep up the excellent videos! Hope to see your channel continue to grow :)

I loved every second of this. Ive been recently learning C++ and viewed the asembly code and was lost this helped so much to see whats actually happenning

I did assembly the first 5 years or so of my 45 years in IT and SW dev. Great foundation. Your explanation is pretty good.

Minor correction, because I used to program in 8080 and Z-80 Assembly: Those instructions from the disassembly are more properly referred to as assembly code instructions. Machine code would be represented by nice hex numbers for the opcodes and operands.

The compiler emits movb $0,%al because printf() takes a variable number of arguments. The ABI specifies that when calling such functions, %al must contain the number of floating point arguments. There are no floating point arguments passed to printf() in your example, so %al is set to zero.

When I was a newbie programmer back there in 2000, playing around with assembler, memory and registers really helped me to get a grasp of what pointers and references are.

8:30 after googling around for a bit im about 60% sure its just an assembly representation of "while" because the %eax(Extended Accumulator" - used for arithmetic and logical operations, as well as for storing return values from functions.) is preceding the cmpl of x to hex of 255. I dont really have much experience in assembly but this is my best amateur educated guess but if theres any assembly experts pls explain what that line would mean

8:15 I believe that line puts tge x value to the aex, where it can set a flag. The next line sets the flag, and the next line uses it to determine wether to jump or not.

Simple and interesting explanation, I have experience with assembler, and C ++ is my main language, but I tried to watch this like i'm a beginner. And in my opinion, that was very easy to understanding. Big respect!) Sry for my bad eng)))0

I found your teaching so understandable by non programmers and beginners. I dont know to do programming and wanted to learn. I have interest in c programming and now assebly. Thank you for video you are great teacher

as someone who works on high level programming, your channel really brings me back and piques my interest again

In my last year of college. I now finally understood this! Thank You! What I realised is that the teaching (at my was good but they didn't put enough efforts to link this to higher level of programming we were practising daily. But now it makes sense. Thank you again!

This actually makes sense. As mainly a C# dev, C isn't actually hard, first off. Pointers and such can get a bit complex, but they make sense. This code is certainly simple. The assembly makes sense too. It is beautiful how simple it is and how it uses such sinple functionality in order to create more complex end results. This helped my understanding of Assembly and it might be one of the things that help me finally make a PS2 game one day.

I always wondered how the C program looked in Assembly language. Today I got to understand how it all works. Great video!

Dude this has shed light on so much stuff amazing vid

Sorry for the noob question, but isn't this actually assembly? I thought machine language was basically just ones and zeros?

I don't know C nor assembly but I watched this from start to finish with my mouth hanging open. So interesting.

Gives a true appreciation for compilers! So many layers built upon layers to translate everything into machine code.

That was awesome! This is something I was never ready to dig in, and now this explanations is so good, that it makes it look easy!

Compilers were invented in 1952. People in 1951:

Nice video bro!

you can tell the presenter really has a finite grasp on the information when he says 'i'm not sure what this other thing is' but hey i'm really glad you enjoy this hobby of yours

i didn't understand much but i really liked your approach of going to the roots of the system,hope you gonna keep continue doing this.Thanks a lot.

I miss programming in assembly. The first code I ever wrote was 6502 Assembly on an Atari 600xl. I also programmed in the following assembly languages over the years: 8088, 80286, IBM 360, R10000 and MIPS. After 20+ other languages over the years, assembly is still the one I liked best. It just felt natural. When I first learned C and was using the Turbo C compiler, I often wrote the function headers and variable declarations in C, and just inlined the guts in assembly. Those were the days...

THATS SO COOL, i always programmed in C and was thinking about how it worked inside of the processor

Thank you so much for this tutorial. Thanks to you i understand those concept now. Please make more of those C vs Assembly Tutorials. You are explaining it so good :D

Your channel is awesome. Ta chaîne est géniale. Merci

This is the kind of programming videos I've been looking for, how programming in a language converts to machine instructions, awesome! THAT BEING SAID, your handwriting is abysmal oh my god xD What have you done to that 0x8 at 4:29!?

After ignoring 5 video ..I subscribed this guy . He's worth sharing and learning from.. thanks bro

Oh man! This is awesome. This takes me way back to early software engineering classes at Purdue in the 90s. We used C. Great to understand how it compiles into machine code!

rax is a 64-bit register eax is a 32-bit register which refers to the lower 32-bits of rax ax is a 16-bit registers which refers to the lower 16-bits of eax ah is an 8-bit register which refers to the upper 8-bits of ax al is an 8-bit register which refers to the lower 8-bits of ax gcc -S -masm=intel program.c ATT syntax is ok, but I prefer Intel personally... you’re welcome and thanks for the good video!

This guy is beyond words. Cannot describe how grateful I am; I found the video channel I have been looking for

You know, I come and go from watching this channel, but you are probably my favorite youtuber, period. When I try to learn programming or programming-adjacent skills, I always find myself frustrated by the way it's taught. I learn better by knowing what's going on under the hood and accounting for it, than rote memorizing things and "just accepting that they work that way". If I "just accept it", I basically cannot wrap my mind around how to use it. But people who teach programming tend to not understand that well enough to explain it, so I find myself not actually being able to learn the subject. I had almost given up. I thought it was just something I couldn't do. You almost exclusively do videos explaining the things that have given me road blocks on my tech journey, and it's a cathartic release to have the chance to watch them. I could almost cry. It's like removing a nasty splinter.

Legend!!! Thanks mate...I became one your best followers...sending regards from Colombia

Using paper. I've gotta give you a thumbs up.

I also check the assembly output of my c++ program to check the efficiency of the compiled code. Also sometimes I'm curious to see how will the compiler optimize polymorphed codes (vtables, etc)

Thank you for a nice explanation. It's great that the compiler generates so clean code. Of course it could do better, that is the registers should be used to x,y and z variables, but it probably would with optimisation option. One remark: The addresses of x,y and z differ by 4 (0xc-0x8=4 0x10-0xc=4, since the long type is used, so each variable takes four bytes.

We need more of this maybe for every instruction that you know

Thanks for the video! Glad to find others who think this is super cool. I just finished my assembly course and I'm sad its over. I'm pretty sure I'm the only student who actually did my assignments and didn't just find code to poach on stack exchange. I'm even more sure I was the only one who really enjoyed the class and preffered it over C++ and way more than Visual Basic. My C++ teacher has been giving me a hard time. Assembly is "neat" he says, but VB can make "real world programs" Humph. I figure if I love something that most people dislike, even if I don't do it directly, there's a market for doing that kind of thinking....???????

wtf am I doing here, I can't even code

This brings back the nightmares from writing mainframe assembler in college in the mid 80's. It was frustrating, fun, gave me an excellent appreciation for what's going on behind the scenes, and here I am 40 years later and damned thankful I haven't ever needed to program in assembler since ♥.

Excellent simple explanation. I will refer to your video in a comparison discussion between classical CPUs and Quantum Computers. For those commenting on the efficiency of the assembled code, (correct me if I am wrong) the C compiler options did not seem to specify optimization options. Optimized code can be more difficult to relate to the original C code and would not have helped the simple explanation. What you should expect with using the optimizer is that registers would be used instead of memory where possible and unused code removed (among other things). Before optimizers were standard, we used to deliberately assign registers to key C variables by using the "register" C modifer.

gcc is likely an symlink to clang on 2015 macbooks. otool is specific to OSX's mach-o executable format. On Linux, you'd use binutils' objdump with `-d` to disassemble. The line you drew to separate the assembly separates the "prolog" which backs up the previous frames' base pointer onto the stack, then sets the frame pointer to this stack pointer so that the current stack pointer can be advanced for the call to printf (think of the base pointer as being the base of the current stack frame, while the stack pointer is adjusted for local variables, and will be the future base pointer for child stack frames, like printf in this case). The movb %0x0, %al is to specify that the variadic function printf is not receiving any floating point arguments as per the sys-V ABI. I can't work out what's being stored at -0x4(%rbp) or -0x14(%rbp) though, and why 32 bytes was allocated to the current stack frame (the stack needs to be 16 byte aligned on x86 to invoke calls, but idk why it didn't just allocate 16 bytes since we only need room for 12 bytes, rather than allocating 32 bytes (0x20 == 32)). I wonder if the assembly had stack canaries (I seem to recall OSX adding them) and the author removed some code, but not all of it (missing -0x4(%rbp) or -0x14(%rbp))? %eax would be retaining the return code from printf at that point, but based on the source code shown, it's never used so there's no point to repeatedly only write it to the stack and never check it. Otherwise, I don't think this is actually the disassembly from the source code shown (close, maybe a slide modification where the author removed some of the assembly, but not all of the parts they should have). The epilog is omitted, but the compiler may have noticed that there's no return from main(). The prolog is still needed otherwise printf() and functions it calls cannot allocate things on the stack correctly. Well, just the stack pointer adjustment, the frame pointer on x86 was meant for debugging, but is not used with optimizations turned on (and you'd probably just want to use DWARF tables to unwind the stack anyways, not as fast at unwinding, but faster happy path, smaller code, less instruction cache pressure, external to program). Great video though!

Almost a year late... On x86-based computers, eax is usually for return values. Don't forget that printf is not void, it returns a length. The compiler is a macro-assembler so it stores it on the stack anyway. What you can do is ignore the stack & use only the registers ebx, ecx & edx to store x, y & z, so in theory, it should execute faster. If i remember well, if you only want 8 bits, you can use even bx, cx & dx, or even b, c ,d

finally ! i can understand some assembly code now. Thank you for this

Thank you so much and very beautiful way of making things clear

Wonder if he tried deleting that "eax" line or replacing it with a no-op or something to see if it mattered, or if it was eronious compiler overhead.

I've always regarded C as a sorts of macro generator. You can almost see the result in asm when you write C. Although with any level above O1, things get totally too much for a human to read, unless you wrote the compiler.

thank you, I don't know a single programming language but I wanted to see how compilers translate code because it blows my mind, this was very useful! what's happening inside a modern PC is 1000000 times more complex, but at least I can vaguely picture it now :)

Incredible video, the most useful channel on UKposts for computer science students

this also nicely demonstrates how intelligent the c compiler is written. the last line that contains the "jmp" order to go back to the infinite while(1) loop doesn't check any conditions...the compiler knows that this is an infite loop and thus just places a jump command. Also, its nice to see how all the programming textbooks tell you to "AVOID GOTOs! OMG GOTO IS THE WORST!" ... when under the hood of the compiled machine code, all the nice and orderly loops, and ifelse and break and continue statements...are just GOTOs :D

Must be a professor or something. Your expression is great Ben. Thanks for the good work.

For a person very interested in how computers manage to function through tasks, you really pleased me.

This is so cool. This makes me want to learn the basics of assembly and be able to make simple programs like this in assembly.

When I was a C developer I always used the compiler's option to output the assembler code it was creating to check it was creating good code. There are lots of ways of coding and hints that you can give the compiler to help it understand what you want and to help it create good code.