The full x86 instruction set is large and complex (Intel's x86 instruction set manuals comprise over 2900 pages), and we do not cover it all in this guide. COBOL does not have a while loop construct, but it is does have a PERFORM UNTIL structure, which means that the normal condition used in a while loop must be negated. Overview. To learn more, see our tips on writing great answers. The while loop loops through a block of code as long as a specified condition is true: Syntax. How to find elements in a list by last two elements. Start with a value at 0. Let's examine a generic while-loop: What does this loop do? Syntax of while loop: Related tasks Loop over multiple arrays simultaneously Loops/Break Loops/Continue Loops/Do-while Loops/Downward for Loops/For Loops/For with a specified step Loops/Foreach Assembly - Loops - The JMP instruction can be used for implementing loops. My primary confusion lies within the end of .L3 and .L2: I believe the end of .L3 is storing the register arithmetic in [ebp-12] and then increasing the value of [ebp-8] (I believe this is a pointer to a copy of the char* arg). There are many sorts of loops, but they can all be boiled down to a few similar formats in assembly code. For example, the following code snippet can be used for executing the loop-body 10 times. I am reversing some x86 from an old CTF from 2014 and am trying to understand the below code (it has been shortened drastically). The do while loop is a bottom tested loop: do {body of loop;} while( condition ); This could be translated into: 1 do: However there is one problem with relative addressing. The loop body is then executed, followed by another check: is x still true? How do you deal with A/B testing for small samples? So why can't the jump label occur before the test? For‐Loop While‐Loop 17 Init; while (Test) {Body Update;} While‐Loop Version: Caveat: C and Java have breakand continue • Conversion works fine for break • Jump to same label as loop exit condition • But not continue: would skip doing Update, which it should do with for‐loops • Introduce new label at Update C# While Loop. Loop while value mod 6 is not equal to 0. In this guide we will learn while loop in C. C – while loop. int i = 0; I am reversing some x86 from an old CTF from 2014 and am trying to understand the below code (it has been shortened drastically). Unlike if statements, Do-While conditions are not reversed. Most compilers will optimize cases like this. For Loops in MIPS However, there is method to our madness, so read on. I believe it is performing some sort of while or for loop through a string x number of times where x is the length of the string. Each time through the loop, add 1 to the value then print it. This chapter will discuss loops, how to identify them, and how to "decompile" them back into high-level representations. The loop iterates while the condition is true. The while loop loops through a block of code as long as a specified condition is True: Syntax while (condition) { // code block to be executed} In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5: ... Do while loops. rev 2021.1.29.38441, The best answers are voted up and rise to the top, Reverse Engineering Stack Exchange works best with JavaScript enabled, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site, Learn more about Stack Overflow the company, Learn more about hiring developers or posting ads with us, The Loop: Our Community & Public Platform strategy & roadmap for Q1 2021, Podcast 308: What are the young developers into? WORKING-STORAGE SECTION. If the execution of the loop needs to be terminated at some point, break statement can be used as terminating statement. While loops. Here, key point of the while loop is that the loop might not ever run. Understanding the x86 assembly code generated from a C program. Creative Commons Attribution-ShareAlike License. while loop checks whether the condition written in ( ) is true or not. Since x86_sse_shift_reg_reg is simply the usage of another macro, there need not be a do while loop monojenkins requested a review from vargaz as a code owner Nov 2, 2020 Dotnet-GitSync-Bot added area-CodeGen-coreclr mono-mirror labels Nov 2, 2020 The loop body simply executes, the condition is tested at the end of the loop, and the loop jumps back to the beginning of the loop if the condition is satisfied. While-loops. I'd change it as follows: this construct imho is a do while loop the telltale jmp at start and the conditional jump at end indicates that construct, this seems to be a strlen kind of function takes one char * input and finds the null at the end of the input, code pasted below generates an equivalent assembly in vc2kten express, the cdb commands are set symbol path to local directory to avoid The while-loop then, performs the following steps: And here then is that same loop translated into assembly: If we were to translate that assembly code back into C, we would get the following code: See why we covered the Do-While loop first? nehme ich eine While-Schleife in Hochsprache, die etwa so aussieht: Während. For Loop While Loop 18 Init; while (Test) {Body Update;} While Version Caveat: C and Java have breakand continue • Conversion works fine for break • Jump to same label as loop exit condition • But not continue: would skip doing Update, which it should do with for‐loops • Introduce new label at Update Each time the LOOP instruction is executed, the count register is decremented, then checked for 0. Also, a good C-Programmer could easily "home brew" a new type of loop using a series of good macros, so they bear some consideration: A common Do-Until Loop will take the following form: which essentially becomes the following Do-While loop: Like the Do-Until loop, the standard Until-Loop looks like the following: which (likewise) gets translated to the following While-Loop: A Do-Forever loop is simply an unqualified loop with a condition that is always true. Because the While-loop becomes a Do-While when it gets assembled. In essence, it's a While-Loop with an initial state, a condition, and an iterative instruction. Your reconstruction was pretty accurate. This process continues until the condition is false. From Wikibooks, open books for an open world, //remember: in If statements, we reverse the condition from the asm, https://en.wikibooks.org/w/index.php?title=X86_Disassembly/Loops&oldid=3677372. Does the silence spell have an effect on a Rakshasa? One way to do this is to realize that while rsi and rdi change every iteration, rdi - rsi is loop invariant. a register or memory). Using the 16-bit programming model can be quite complex. Otherwise, if the condition evaluates to false, the loop is terminated, and the program control will be passed to the command that follows. Injecting C++ code, while also maximizing compatibility with other injections, Invalid file path error while trying to rebuild with apktool and aapt2. C++ While Loop. This page was last edited on 16 April 2020, at 06:17. Everyone’s getting AWS…, Opt-in alpha test for a new Stacks editor, Visual design changes to the review queues, Loop through letters of a string with index, Segmentation fault while pushing onto the stack. In the previous tutorial we learned for loop. A loop is used for executing a block of statements repeatedly until a given condition returns false. What is a For-Loop? statements inside the while loop are executed. The condition/expression is evaluated, and if the condition/expression is true, the code within all of their following in the block is executed. Note that the LOOP instruction ignores REX.W; but 64-bit address size can be over-ridden using a 67H prefix. In MIPS we can use three types of loops for, while and do-while. will help loop performance. 3.1 Machine-Independent We rst consider optimizations made independent of the x86 architecture. C only has Do-While, While, and For Loops, but some other languages may very well implement their own types. PROGRAM-ID. The syntax of the while loop is: while (condition) { // body of the loop } Here, A while loop evaluates the condition; If the condition evaluates to true, the code inside the while loop is executed. My loose conversion of this (disregarding much of .L3) is as follows in c: Would someone be willing to explain the incrementation of the [ebp-8], first four lines of .L2, and confirm/deny that during the return the value of eax or loc1 as I called it will return based on the end of .L2? This chapter will discuss loops, how to identify them, and how to "decompile" them back into high-level representations. The while statement executes a statement or a block of statements while a specified Boolean expression evaluates to true.Because that expression is evaluated before each execution of the loop, a while loop executes zero or more times. check the condition. In MIPS we can use three types of loops for, while and do-while. How to identify 'main' ubuntu repository apps. ... into the location referred to by its first operand (i.e. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. While register-to-register moves are possible, direct memory-to-memory moves are not. While-loop in C: while(x==1){ //Do something } The same loop in assembler: jmp loop1 ; Jump to condition first cloop1 nop ; Execute the content of the loop loop1 cmp ax,1 ; Check the condition je cloop1 ; Jump to content of the loop if met For the for-loops you should take the cx-register because it is pretty much standard. Is there a way to do multiple replacements with sed, without chaining the replacements? In assembly language, we deal with registers directly, so it makes it a little bit difficult to learn. In this article. While not loop-speci c, optimizations such as moving variables to registers from the stack will help performance, simply because of the gains of the optimization will be realized in each iteration. X86 Assembly, if-else control Structures, loops Comparisons. ; If the test-expression is evaluated to true, . If I buy 1 share of a company's stock, do I get to vote at the next shareholder meeting? i> = 0 und x < 5, wie würde der Assembler-Code in x86-Look ? A word to describe a company which other companies measure themselves by, execute until the start of desired function with. Thanks for contributing an answer to Reverse Engineering Stack Exchange! The loop could be either a for loop, or a while loop. while (condition) { // code block to be executed} In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5: Example. C# while loop. x86 Assembly Guide. The condition is evaluated before executing the commands. When the condition becomes false, program control passes to the line immediately following the loop. Performing arithmetic operations. We see that the loop control variable, b, is initialized to 0 before the loop, and is incremented by 1 each loop iteration. site design / logo © 2021 Stack Exchange Inc; user contributions licensed under cc by-sa. Each time through the loop, add 1 to the value then print it. I use the for loop quite often but I also find the while and until loops useful. Making statements based on opinion; back them up with references or personal experience. Some forms of the loop instruction (LOOPcc) also accept the ZF flag as a condition for terminating the loop … In assembly language, we deal with registers directly, so it makes it a little bit difficult to learn. Loop-While. The key is to update less counters and shift some of the work to the magic x86 addressing modes. So we can compute it once and do this: The loop tests b against 100, after it gets incremented , so we know that b never equals 100 inside the loop body. If you want more flexibility with where you test the condition or what result you test it for, you might prefer the Do...Loop Statement. check the condition, if it is true, jump to 2. if the condition is not true, fall-through the end of the loop.