In Intel Syntax, your instruction would be written: The immediate can be shifted to the left 0, 16, 32 or 48. movq (assuming youre talking about x86) is a move of a quadword (64-bit value). Interestingly, the answer below was marked as accepted although it doesn't actually give an explanation to question 1. Thanks for contributing an answer to Stack Overflow! What do you do with graduate students who don't want to work, sit around talk all day, and are negative such that others don't want to be there? Necessary cookies are absolutely essential for the website to function properly. Find centralized, trusted content and collaborate around the technologies you use most. The X86 assembly language. Asking for help, clarification, or responding to other answers. Starting at 8 a.m. move the contents of the register bx into the register ax. Functional cookies help to perform certain functionalities like sharing the content of the website on social media platforms, collect feedbacks, and other third-party features. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. instance, consider an instruction located at (starting-point + 0x80) that Teen builds a spaceship and gets stuck on Mars; "Girl Next Door" uses his prototype to rescue him and also gets stuck on Mars. So yes, your belief is correct. The first parameter is the variable %1. Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register? I am completely confused. Assembly generated by a compiler contains instructions as well as labels and directives. Normally the CPU executes instructions in sequence. How can I delete in Vim all text from current cursor position line to end of file without using End key? esp points to the last thing pushed onto the stack. Is it possible to comply with FCC regulations using a mode that takes over ten minutes to send a call sign? movq (assuming youre talking about x86) is a move of a quadword (64-bit value). With over 10 pre-installed distros to choose from, the worry-free installation life is here! A push instruction pushes a value onto the stack. @Victor T.: Intel syntax is what Intel specified originally; AT&T syntax is an adaptation of a Bell Labs assembly syntax intended to be used on multiple platforms. How can one know the correct direction on a cloudy day? In a What is Pushq Assembly? the l in movl stands for "long" which is 32-bits in size, The difference between mov and movl instruction in X86? Subtracting from the stack pointer just puts multiple imaginary members at the start of the struct in one step. Example movl %cr3, %ebp movl %db7, %ebp movl %ebp, %cr3 movl %ebp, %db7 Not the answer you're looking for? What is the size of the MOVL instruction? Is there any particular reason to only include 3 out of the 6 trigonometry functions? Thanks, update: I am talking about x86-64 assembly. Program startup performance matters, so the operating system doesnt recompile Exobiology Using Hydrazine as an Alternative (or Supplementary) Solvent to Water. So when you push to a stack, imagine inserting a new member at the beginning of the structure, while none of the other members moved. Stack frame manipulation is mostly just moving the stack pointer to make more or less room in that imaginary struct we call the stack frame. If the stack looks like this: My question is not about stack, even thought stack is involved. A push instruction pushes a value onto the stack. functions and global variables are the same every time). Why is the syntax so strange? thanks in advance for any help Most documentation uses the "Intel Syntax", which has the reverse ordering. Is my teacher doing this for simplicity or have the 'movb' and 'movw' commands become obsolete in recent years? It specifically supports position-independent executables When should one use MOVS opposed to MOV in x86 Assembly? If not, please explain where I am wrong. How AlphaDev improved sorting algorithms? movl will write into all 4 bytes. How can I handle a daughter who says she doesn't want to stay with me more than one day? Thanks for contributing an answer to Stack Overflow! If you prefer you could use some AT&T assembler, like GAS (for Windows or Linux). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. How is ATP made and used in photosynthesis? What does Movl do to upper 4 bytes? Improve this answer. Novel about a man who moves between timelines. How to professionally decline nightlife drinking with colleagues on international trip to Japan? Yes, this is using AT&T syntax, which is of the form: You are also right about the 8(%ebp) moving 8 bytes up from the frame pointer. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Other than heat. why does music become less harmonic if we transpose it down to the extreme low end of the piano? Why the Modulus and Exponent of the public key and the private key are the same? The compiler is reserving space on the stack for locals and whatever other needs it might have. This probably assigns value 5 to x. To learn more, see our tips on writing great answers. Does a constant Radon-Nikodym derivative imply the measures are multiples of each other? Thanks to lea, the compiler will also prefer the base(offset,index,scale) addresses, thanks to the lea (Load Effective Address) instruction. The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". Find centralized, trusted content and collaborate around the technologies you use most. How Bloombergs engineers built a culture of knowledge sharing, Making computer science more humane at Carnegie Mellon (ep. What is Assembly assembly movs instruction? How does the OS/360 link editor create a tree-structured overlay? Most documentation uses the "Intel Syntax", which has the reverse ordering. rev2023.6.29.43520. The MOVL instruction was generated because you put two int (i and j variables), MOVL will perform a MOV of 32 bits, and integer size is 32 bits. dx is 16bit , eax 32 bits. I feel like what this really means is moving the first parameter into the edx register, but I read on Wikipedia that it is the other way around? Also - %ebx, %esi, %edi are all callee save registers. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. I am reading "Computer Systems: A Programmer Perspective", chapter 3 explains mov instruction, and explanation give in a book confuses me. By clicking Accept All, you consent to the use of ALL the cookies. In this Assembly syntax first operand is destination - this is usual case, for example, in C functions: In AT&T Assembler first operand is source. Making statements based on opinion; back them up with references or personal experience. The source string is pointed by DS Home. rev2023.6.29.43520. I think it is moving the 32 bits in %eax with zero extension to %rdx, which will not be generated as movzql since. thanks in advance for any help. "Immediate" means the value is encoded right into the instruction. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. In 16-bit x86 code, there was no way to use the stack pointer as a pointer (hard to believe, right?). Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. What does 0x4 do in "movl $0x2d, 0x4(%esp)"? Is it legal to bill a company that made contact for a business proposal, then withdrew based on their policies that existed when they made contact? Making statements based on opinion; back them up with references or personal experience. The fields are often not zero-padded Or if you have a big array of, say, megabyte sized structures as elements, padding all bytes to 32-bit words DO have an effect. Unfortunately, ebp makes debugging easier so we ended up continuing to use ebp in 32-bit code (it's trivially easy to make a stack dump if ebp is being used). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. movl $10,24(%esp) means: move a literal decimal-10 long (4-bytes) into a 4-byte memory location that begins at the address pointed to by (the esp register plus decimal 24)--basically it is a local variable. movq moves a quadword (64-bits) from source to destination. Use movzx/movsx to zero/sign-extend into the full-sized register to prevent this. The movb will just change the low order byte. 585), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood. Actually, you cannot scale the base. 8 What is the size of the MOVL instruction? never see movl global_int, %eax. How to describe a scene that a small creature chop a large creature's head off? Temporary policy: Generative AI (e.g., ChatGPT) is banned. dereference memory. Connect and share knowledge within a single location that is structured and easy to search. The first members of the imaginary structure are the most recently "pushed" values. What does leave do? It is about values stored in register and memory. Do I owe my company "fair warning" about issues that won't be solved, before giving notice? Thanks for contributing an answer to Stack Overflow! Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. What is the name of new capital of Andhra Pradesh? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Data representation 2: Object representation, Data representation 4: Pointers and undefined behavior, Data representation 5: Undefined behavior, bitwise operations, arena allocation, Kernel 2: Process isolation and virtual memory, Kernel 3: x86-64 page tables and WeensyOS, Kernel 5: Confused deputy attack, scheduling, and process management, Shell 2: Process creation and interprocess communication, Shell 3: Sieve of Eratosthenes, polling vs. blocking, Synchronization 1: Signals, race condition, threads, Synchronization 2: Mutex, bounded buffers, Synchronization 3: Mutexes, condition variables, and compare-exchange, Synchronization 4: Networking and Synchronization, Synchronization 5: Deadlock and Server Programming, Loading a value into a 32-bit register name sets the, Loading a value into a 16- or 8-bit register name. In Practice Problem 3.3, it says movl %eax,%rdx will generate an error. looks very much like code that will walk up through stack frames. Typically, most variables take up the 4 bytes anyway. You are also right about the 8 (%ebp) moving 8 bytes up from the frame pointer. What does the bracket in `movl (%eax), %eax` mean? Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. Asking for help, clarification, or responding to other answers. Idiom for someone acting extremely out of character. Can one be Catholic while believing in the past Catholic Church, but not the present? Thanks for contributing an answer to Stack Overflow! Also note that the compiler has made two optimizations: The assignment to x is removed because the value is not used. instructions and globals relative to that starting point. In a position-independent executable, the operating system loads the program By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Follow. Why is there a drink called = "hand-made lemon duck-feces fragrance"? CS:APP example uses idivq with two operands? The code you typed as, I've always felt that using the equals sign for, How Bloombergs engineers built a culture of knowledge sharing, Making computer science more humane at Carnegie Mellon (ep. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. rev2023.6.29.43520. It copies whatever is in edx to the first parameter of the function (since an offset of +8 from ebp is a parameter)? instruction source, dest. Asking for help, clarification, or responding to other answers. Not the answer you're looking for? What it actually does is undo the changes to the stack and restore the caller's ebp. I understand this like this. Can renters take advantage of adverse possession under certain situations? Introduction to assembly language. We usually see them used in control flow instructions, as in jmp L3 ("jump to L3"). Why is Bellman-Ford used for distance vector routing? Asking for help, clarification, or responding to other answers. Update crontab rules without overwriting or duplicating. Control flow instructions change the instruction pointer in other ways. In 32-bit mode, each push and pop on the stack is 4 bytes wide. How to style a graph of isotope decay data automatically so that vertices and edges correspond to half-lives and decay probabilities? The compiler will change the double percent sign to a single percent sign so you could just as good write: Thanks for contributing an answer to Stack Overflow! mov instructions & registers - confusion ! These cookies ensure basic functionalities and security features of the website, anonymously. movl will move the first parameter to the second parameter. PS: may be the -M intel objdump's argument can help you to have a better comprehension of the disassembly, a lot of man on the Intel syntax can may be find easily. Under metaphysical naturalism, does everything boil down to Physics? I'm trying to understand some assembly code: The first one means put number 244 to the register eax, but what does second mean? Where in the Andean Road System was this picture taken? How common are historical instances of mercenary armies reversing and attacking their employing country? (PIEs), which are programs that work independently of where their code is The effect you want is achieved by movl %eax, %edx since writes to a 32-bit register always zero-extend into the corresponding 64-bit register. Was the phrase "The world is yours" used as an actual Pan American advertisement? 6 What are the four different types of instructions Assembly? This cookie is set by GDPR Cookie Consent plugin. Temporary policy: Generative AI (e.g., ChatGPT) is banned. Not the answer you're looking for? (e.g. Can I use movl instead of movb or movw in Assembly on modern systems? Find centralized, trusted content and collaborate around the technologies you use most. [duplicate]. How to describe a scene that a small creature chop a large creature's head off? Which fighter jet is seen here at Centennial Airport Colorado? When calling function f(), instead of using a push instruction to put the parameter on the stack, it uses a simple movl to the last location it reserved: A more interesting (in my opinion) thing happening here is how the compiler is handling the call to f(2): To answer your question, "immed by the way?" Mov ax, (to) bx actually means move contents of bx to ax ,is there any historical reason to define it this way? 585), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood. For example, to store 15 at the address in ebx: The parentheses make it use the register as a pointer to memory. What is the reason for having 2 different dilects of assembly? The PIEs 10. Jumping to the next "instruction" using gdb, Help on VGA and putpixel intel x86 asm AT&T syntax, Beep command with letters for notes (IBM AT + DOS circa 1984). Find centralized, trusted content and collaborate around the technologies you use most. Temporary policy: Generative AI (e.g., ChatGPT) is banned. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. What does the assembly code `movl %1, %%ebx` mean? What does the assembly code `movl %1, %%ebx` mean? AT&T has the destination on the right, Intel has the destination on the left. Connect and share knowledge within a single location that is structured and easy to search. You also have the option to opt-out of these cookies. Assembly generated by a compiler contains instructions as well as labels and directives. Interchanging is okay? How does the OS/360 link editor create a tree-structured overlay? What are the white formations? Do I owe my company "fair warning" about issues that won't be solved, before giving notice? Temporary policy: Generative AI (e.g., ChatGPT) is banned. What is Lea Assembly? By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Can you pack these pentacubes to form a rectangular block with at least one odd side length other the side whose length must be a multiple of 5. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Which of the following instruction is same as JNE? 585), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood. How could submarines be put underneath very thick glaciers with (relatively) low technology? How can one know the correct direction on a cloudy day? Making statements based on opinion; back them up with references or personal experience. References. However, in class the teacher has only used the 'movl' operand. Why can C not be lexed without resolving identifiers? rev2023.6.29.43520. Adding to the stack pointer makes the first so many members disappear. Do spelling changes count as translations for citations when using different english dialects? movq moves 8 bytes starting at the current specified memory address whereas movl moves 4 bytes starting at the current specificed memory address. Find centralized, trusted content and collaborate around the technologies you use most. movq and movl are variations of the mov command (moving data to a memory location or a register). How one can establish that the Earth is round? This cookie is set by GDPR Cookie Consent plugin. @Ilya: that would be a store, not a load. 7 What is Assembly assembly movs instruction? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. rev2023.6.29.43520. Here I have read that. This works great, and Can the supreme court decision to abolish affirmative action be reversed at any time? The reason it moves 8 bytes, specifically, is because parameters are pushed onto the stack in reverse order ("right" to "left" when looking at a typical function call). Just to clarify; movzx will extend the data to 32 bits via zero padding so that the data fully occupies the register? What are the benefits of not using private military companies (PMCs) as China did? Labels look like labelname: or labelnumber:; directives look like .directivename arguments. GDPR: Can a city request deletion of all personal data that uses a certain domain for logins? what is the purpose of "movl $0, %ebx" or getting 0 into ebx before an int $0x80 Linux SYS_EXIT? Can the LEA instruction mimic every MOV instruction? Introduction to assembly language. What does the following assembly code really do? Did the ISS modules have Flight Termination Systems when they launched? By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. How can I delete in Vim all text from current cursor position line to end of file without using End key? The %1 is used by gcc inline assembly to specify parameter. In a 585), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood. Wasm enables far faster performance for complex and demanding web applications compared with common web programming languages such as JavaScript. To learn more, see our tips on writing great answers. %eax. Get a virtual cloud desktop with the Linux distro that you want in less than five minutes with Shells! GNU AS uses AT&T syntax, which is the opposite of Intel syntax. What kind of architecture is this question relating to?On modern x86, for example, loading with mov[b/w] kills performance due to partial register updates (prevents register renaming/extends the dependency chain). The instruction moves a value from 32-bit source to 64-bit destination but ensures that signed value of the number in the 64-bit destination is equal to the one from source, that is in case of negative number it fills first 32 bits of destination with ones, otherwise with zeros. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, The information is in there, if you control-f search for, And 2's complement sign extension is something you can look up separately, or the Intel manual documents that, too. Why is there a drink called = "hand-made lemon duck-feces fragrance"? Is efficiency lost when using movl as opposed to movb on a char type for example? Did the ISS modules have Flight Termination Systems when they launched? Was the phrase "The world is yours" used as an actual Pan American advertisement? What does the MOVZBL instruction do in IA-32 AT&T syntax? 585), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood. 585), Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood. Before that code there was an. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy.