Cmp assembly stack overflow Modified 9 years, 8 months ago. OR and AND both write all FLAGS; they set CF=OF=0, not leave it unmodified, just like cmp. In my notes it says that TST doesn't change the values of its operands. However, you have to use 2 cmp [tipoBandeira],01 je T1 cmp [tipoBandeira],02 je T2 T1: mov ah, 40h mov bx, 1 mov cx, 08 mov dx, quad int 21h T2: mov ah, 40h mov bx, 1 mov cx, 11 mov dx, rect int 21h is In the example they gave, the math is 127 - (-128), which is effectively 127 + 128, which causes an overflow because a signed 8-bit number can only go up to 127. I have a word-array called "lent" filled with some numbers. The difference in signed vs. setb will make AL zero and seta will make DL zero. minuend. C code to assembler. How to work with the cmp parameter in assembly x86. Ask Question Asked 9 years, 8 months ago. Explore Teams. Improve this question. ;----- mov esi, string1 mov edi, string2 L1: mov al, [esi Here's an alternative A1 loop. The point of CMP is that if the two operands are equal then the result is zero, which means cmp a, b is simply a - b. Cmp instruction mismatch. You could also reorganize the code such that only the inc p is jumped over, so both cases would use the same si comparison (similar to your Your basic problem is not signedness (jl is correct for a signed comparison; if you wanted unsigned you'd use jb) but the order of operands to the cmp instruction. CMP and jmp variations in assembly. Namely the flags SF , ZF , AF , PF , and CF are set. What you're got there are addresses of strings, or pointers to (the first characters of) strings. int string_cmp(const char* str1, char* str2) { int res; I'm new to x86 and I'm trying to put the larger integer ( at location 4(%esp) and 8(%esp) ) into register %eax. I'm learning assembly language and encountered a problem that I don't know even how to ask Google. To be exact, here is the meaning of several Sounds like your lecture slides are slightly mixed up. That's because cmp is usually equivalent to sub (subtract) but without actually changing the value. CF is set by the cmp instruction if there is borrow out. cmp %eax, %ebx jg < something > would jump to < something > if eax was greater than ebx. To learn more, see our tips on writing great Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; SPARC assembly has a cmp instruction, which compares two numbers (using subtraction, if I understand correctly). Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Here is a code to sort the given array in assembly language . Hot Network Questions Each instruction set or processor design arm vs x86 vs powerpc, etc has different designers that do different things. 40. I'm having a hard time understanding conditional code in assembly. sub to bring one end of the range to 0, then cmp / ja (unsigned compare). 1. There are plenty of cases where a 3-byte cmp would be better than a 2-byte or, though, on out-of-order exec Ask questions, find answers and collaborate at work with Stack Overflow for Teams. Either use the 8 bit registers or move with sign or zero extension as appropriate. STACK 100h . Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. THIS is the real reason why the compare will fail. On a single line there is the following instruction : cmp d,[eax],0DADADADA I In assembly, the code is doing: cmp $0x5, %eax However, when I examine the contents of %eax, I get: \020\343\377\377\377\177 when examined as a string. For example, if you subtract 10 from 12 (either via cmp or sub), the zero flag is cleared (since the result is not zero) and the carry flag is cleared (there was no need to I'm working on a program in ARM assembly (Raspberry pi), but I have a problem. TITLE pilot DOSSEG . It would then set flags in the status register such as N (negative, if R1 What I don't get is what TST does differently. al is an 8-bit register (it's the lowest byte of eax), so this way we properly copy/compare 1-byte quantities. I'm very new to assembly and all I can figure out is that it might have something to do with the registers I'm using. Since the compiled output shows that there is only 1 byte between the two messages, you will need to change the size of the CMP instruction from word to byte: cmp byte [bx], 0 Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, Assembly - CMP not working as expected. Here is some assembly code in Intel syntax: // Jump to done if rsi >= rax. It also has subcc, addcc, andcc, and others. On overflow, the overflow flag Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly - CMP not working as expected. When I print what is under 0 index it shows ASCII 0 (NULL). Hot Network Questions It seems that Y86 does not have cmp instruction. Im preparing for assembly language exam and im doing examples given by our because they are the same. cmn a, b means a - (-b) - which under two's complement arithmetic is exactly equivalent to a + b. This means that the following setb and seta instructions would produce garbage!. 2. In this context, it will only jump if al was 1. I wanted it to print whether 2 numbers are equal though I think the jumps are not functioning correctly. CMP in x86 with parentheses and address. I've been playing with assembly for a while and looking at some code. To learn more, see our tips on writing great In x86 assembly according to your assembler(e. Twos complement allows us to not need to make subtract logic as that doesnt make any sense, instead you use add logic and invert the second operand and invert the carry in, normally the carry in for an add is 0 on the first bit cell, but for a subtract Stack Overflow Public questions & answers; Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly - CMP not working as expected. when I try to compile below code snippet. 1 = 0001 - 2 = 0010 ----- 1111 For the the first rightmost column there is nothing special: 1 - 0 = 1. How i Thanks for contributing an answer to Stack Overflow! Please be sure to answer Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly CMP result differs depending on used register? 3. @user35443 repe cmpsb The FLAGS results of test reg,reg / and reg,reg / or reg,reg are identical to cmp reg, 0 in all cases (except for AF) because:. But yes, code-size is the disadvantage of cmp reg,imm8 vs. Anyway, FFFF0H is a 20 bit address, you can't compare with that directly if you are limited to 16 bits. From what I read CMP is supposed to subtract the second number from the first then set the flags, in that case it should be 0x84-0x30 Thanks for contributing an answer to Stack Overflow! Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. eq instruction proceeds to the next instruction if the condition is false. TEQ is also useful for testing whether the two values have the same sign. Because fails and passes are mutually exclusive, you can defer from calculating the passes to the very end. cmp not working - Intel x86 (IA32) Assembly. I want to jump to a certain point if my value is not between 65 and 90. How do the two instructions differ? How is . Here is my thought process: cmp rax, rcx // a[idx] <= *b I got your point and agree with you. Carry flag in MSP430 is confused. L1 cmp local0, 3 jge . x86 Assembly instruction. Unsure about cmp/jg behavior in AT&T syntax. Why do I have a segfault when using cmp instruction? 1. I'm assuming Intel syntax on operand ordering for instructions. difference between . subtrahend. or reg,reg. cmp al, 1 will set the processor flags (including the zero flag) based on what would have happened if you'd subtracted 1 from al. You can use for example the Intel documentation as a reference for the x86-64/AMD64 instruction set. I thought my answers on the Q&As linked in the questions made that clear, or at least or al,al / test al,al are both architecturally equivalent to cmp al, 0. AL/AX/EAX (only if subtrahend is immediate) Register Memory. g tasm, masm or nasm) you cannot compare immediates or variables with each other. mcruz. In this case, The inc-instruction just increases the register, but the flags set by the cmp-instructions are tested by the jl instruction. To learn more, see our tips on writing great I've got some assembly of a sorting algorithm and I want to figure out how exactly it functions. Load 7 more related questions Show fewer related questions Sorted by: Reset to Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. mov r1, #3 mov r3, #4 add r2, r1, r3 bl Debug cmp r2, #0x0A ble skip$ mov r0, #10 sub r0, r0, r1 mov r3, r0 bl Debug skip$: Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; I would like to Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly - CMP not working as expected. To learn more, see our tips on writing great The e* registers are 32 bits, you can't load them with bytes directly. JNZ & CMP Assembly Instructions. CMP BYTE PTR [ESP+5],61 ; ESP is 0012F9AC they will interpret ",61" above as decimal, so the compare will fail, because 61 decimal is not equal to 61 hexadecimal. 1,532 2 2 gold badges 11 11 silver badges 14 14 bronze badges. The The cmp (compare) instruction is identical to the sub (subtract without borrow) instruction with one important difference - it does not store the difference back into the The cmp, often integer comparison, compare the values then sets a condition flag which jump instructions rely on to make jump decisions. The assembly on the right is for funA() on the left, but I'm having trouble with lines 3-4 in assembly. Provide details and share your research! C is both carry out and unsigned overflow. MODEL SMALL . change #123, R2 to R2, #123). Provide details and share your research! But avoid Asking for help, clarification, or responding to other answers. Assembly - CMP not working as expected. In order to subtract in binary, you would use "twos compliment", where you take r1, negate it, add 1 to it and then add that to r0. CMP R1, R2 Would perform the action R1-R2, but not store the result. ne returnResult In the above code it will always take the break point. I have following code: cmp eax,DWORD PTR [rbp-0xc] j Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, Assembly - CMP not working as expected. (AF is undefined after test, but set according to the There is borrow out indeed. Here is some code: mov di,offset lent mov cx,0d cmp ds:[di],cx Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Yes -- cmp (at least in most assembly languages) does a subtraction. but the Zero and Carry flags are affected ; exactly as they would be by the CMP instruction. 39 - CMP Reg, Reg 3A - CMP Regb,Memory 3B - CMP Reg,Memory 3C - CMP AL,Byte 3D - CMP (E)AX,Immediate 80 - CMP RegByte,Immediate Byte = 80 81 - CMP Memory,Reg = 81 83 - CMP Reg/Mem,Immediate (sign extended) byte Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly CMP result differs depending on used register? 16. While looking online for the cmp instruction, I've noticed that it actually would compare the value in r2 with 0 (pseudo-C will be r2==0, right?)So I was wondering if 0h has the same meaning? Recall that labels just mark a spot in your code. C is the overflow flag when the numbers Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. The CPU does not know that labels exist and execution happily proceeds past labels. cmp %rax, %rsi jae done This hurts my brain every time. It sets the flags based on the result of that subtraction, but Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Problem comparing two numbers with the cmp operation in assembly 8086, and need help to fill array backwards. But I have another piece of code that seems to contradict this: cmp $0x2, %eax jg < something> as it jumps to < something > when eax has the value 3. in which AL is first This instruction then triggers the Overflow flag. Then je wins will jump for result 7 at code at wins: and that one will do some useless pointer stuff, reset ecx to 2, display message and jump back to main roll loop. var_1 dd 100 var_2 dd 200 cmp dword[var_1], dword[var_2] Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company I'm writing a program in assembly that is suppose to take the value of a key press, cmp r1, #0x30 beq savekey cmp r1, #0x31 beq savekey cmp r1, Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. There is no need for a cmp-instruction before the jump. Useless Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; I am trying to write the string_cmp function in assembly where you try to compare if two strings are the same. Or even do a single According to my understanding cmpl compares unsigned. e. You can use two 16 bit registers to calculate the physical address, and do a 32 bit comparison using those. cmp al, [var2] respectively. like this: mov ax, 5 cmp ax, 6 or. – Jester Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. , -4095 has the 64-bit 2's complement representation: ffff ffff ffff f001. This is a problem because var and var2 only store 1 byte each. Just correct like this: CMP BYTE PTR [ESP+5],61h ; ESP is 0012F9AC. This assembly sorts an array of three elements. If the result of the addition is all 0s it flags to equal. I think your setup instructions are OK. It may be, in hexadecimals: mnemonics . PS: All your len_* definitions are in the wrong place. 5. using CMP statements in assembly language. But strangely enough if i omit the cmp eax,0 and jz exit lines,the program is seg faulting when it encounters end of file (i. CF = OF = 0 because test/and always do that, and for cmp because subtracting zero can't overflow or carry. i. Example A: A DW 1234,35235,1234,5678 B DW 4532,32735,5678,1234 N=4 If the ECX register is zero then the repe cmpsb is not executed at all. However the code does not take the jump. ok, so i altered the code to: cmp [tipoBandeira],01 je T1 cmp [tipoBandeira],02 jmp T2 T1: mov ah, 40h mov bx, 1 mov cx, 08 mov dx, quad int 21h jmp s1 T2: mov ah, 40h mov bx, 1 mov cx, 11 mov dx, rect int 21h s1: thing is, i would then have to ask and compare another value and, by using both values, draw either a x-color square or rectangle. Now how cmp is subtracting operands? Is it subtracting 1st operand from 2nd or vice versa? CMP subtracts r1 from r0 (r0-r1) and then sets a flag for the BNE as to whether or not the two are equal (the result is 0) or not equal (the result is not 0). For example. model small . Understanding the pop instruction in assembly. Unexpected behaviour of x86_64 assembly code. cmp minuend, subtrahend. For example: CLD LDY #$03 LDA #$00 LDX #$05 CMP Y BEQ Equal STX My initial guess is that you will have more instruction space for the immediate in your branch instruction when you first use a CMP instruction. Thus, 0x5 is greater than 0x1, so it should be taking the jump. cmp rsi, rax jae done This makes sense to my brain: you jump if rsi is "above or equal to" rax, matching the order of arguments in the cmp instruction. The inc-instructions can set or reset flags. I'm a little confused on some of the instructions, particularly the cmp and jle instructions, so I'm looking for help. But in your case next instruction was cmp+jne with read from %rsi, and that instruction can't be finished until data from memory is written to %rsi (CPU may block in the middle of cmp+jne execution or do many restarts of Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Which does push old rbp value into stack (to preserve it), then sets rbp to current stack pointer value (rsp). mov $0x0, %ecx lea 0x3(%rdi), %eax cmp $0xffffffffffffffff, %edi ; ??? cmovle %eax, %rdi I have marked line by ???. Understanding cmp instruction. C is the overflow flag when the numbers I got your point and agree with you. ldr x19, [x0, x_offset] ldr x21, [x1, x_offset] cmp x19,x21 b. intellivision. JE (Jump on Equal) instructions use a result of a previous operation, this previous operation is usually CMP (compare) but it can be any other of a multitude of similar operations (eg ADD, jz means jump if zero. Thus the programmer chose to make sure the flags have a defined state. 1 ;prints the value of DX as hex. From the usage notes in ARM DDI 0100E: "TEQ is used to test if two values are equal, without affecting the V flag (as CMP does). What does the MSP430 cmp. (Also, pow is a C standard library function name; usually best to pick a different [EBP + local_c] is an address, in an indirect addressing mode. 3. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; I am working on a project to organize students mark in 3 exam using assembly language. Here is my code (initial value of dx register is 0xdada):. So all flags that are set by a SUB are also set by CMP . ARM loop cmp issue. To my understanding, my cmp command will set the carry flag to 1 or zero, then adc will transfer that flag to the eax register. To learn more, see our tips on writing great Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly x86 call after cmp. V is signed overflow some architectures just call it overflow but it is specific to when the programmer considers the numbers to be signed. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; using CMP statements in assembly language. For the second rightmost column there is a borrow (i. EDIT: I haven't any comparison in my code. Decoding Assembly Language with GDB. stack 100h . Unfortunately I cannot find a proper translation for lt (less than), gt (greater than) or eq (equal), but I would expect that there is something like that in the instruction set. To learn more, see our tips on writing great Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. cmp eax, win: You use MASM syntax, so this is cmp eax,[mem32] in Intel syntax, will compare eax against value 7 from win "array". Follow edited Feb 24, 2016 at 23:27. My conditions seems to work fine cause if i write cmp [esi+counter] and [esi+counter] it works and outputs number 3, but for cmp [esi+counter] and [edi+counter] it says they are Thanks for contributing an answer to Stack Overflow! Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, Do any assembly languages really use CMP as an alias for SUB when it's only available as a destructive operation? The assembly as follows, I am interested in the testl line: 000319df 8b4508 movl 0x08 (%ebp), %eax this generates a smaller instruction than cmp eax, 0 which is the reason that compilers will generally do it this way. but I thought CMP didn't either. CMP i. Related. About the je in assembly. It does both, in a way. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company When encoding an assembly instruction, immediate data is typically encoded in the instruction itself. I am a bit confused. Compare this to the GNU syntax: // Jump to done if rsi >= rax. : a 1 is "borrowed" from one of the next columns). MIPS Conditional Statement BNE is always Just to make sure you understand the problem: cmp [edi-10],byte 41 will test byte at address edi-10, if it is equal to 'A'. ,e. e eax=0). data ;roll number 2435 Stack Overflow. While I was studying assembly code, I saw below code segment. I don't understand why that line is used in assembly code? assembly : x86 64 bit. Here is what's written in wiki: cmp arg2, arg1 Performs a comparison operation between arg1 and arg2. At that point in time, eax holds the value 0x1. @fuz: x86_64 - Assembly - loop conditions and out of order shows cmp can't macro-fuse with js, jp, or jo on SnB, but test can. Hot Network Questions Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. b instruction do? 0. Assembly - cmp instruction always passes. Note: Need to do that without CMP. gcc inline - operand type mismatch for 'cmp' 1. e. I have just started learning assembly from this tutorial, which has been great so far, This sounds like an XY problem, you should have specified what you wanted to achieve ultimately. assembly; x86; Share. Pop/Push Instruction. CODE BEGIN: mov ax, @data mov ds, ax mov es, ax mov cl, 05h mov dl, 10h cmp cl, dl JE where JNE somewhere somewhere: lea dx, NEQUAL where: lea dx, EQUAL mov ah, 09h int 21h mov ah, 4ch int 21h Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; I have some decompiled assembly: cmp byte ptr [esi+esi*4-7D7A6FF5h], 0Bh sal byte ptr [ecx+ecx*4], 0Bh sal byte ptr [ecx+ecx*4], 0Bh loopne loc_BC58F56 xchg eax, edi or As for the cmp instruction, it is simply a subtraction. To learn more, see our tips on writing great Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; cmp userInput, 18200 jge _tb0 ; userInput being less than, equal, and great than 18200 but it still jumps to the same label. Regarding cmp / jg, jle, etc in AT&T syntax assembly. There is no partial-flag problem, or any interaction with P4's false FLAGS dependency for inc/dec; P4's inc will have a false dep on or al,al just like test al,al or cmp al,al. g. Making statements based on opinion; back them up with references or personal experience. About; Products OverflowAI; Stack Overflow for Teams Where developers & technologists share private knowledge with ,al mov [bx+data1], dl mov swap, 1 noswap: add bx,1 cmp bx,9 jne loop1 Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. As per the document operands could be either one of below. The reason the CMP instruction is not working for single word comparison is because a word must contain 2 bytes. In this case cmp ecx,ecx results in a CarryFlag=0 and a ZeroFlag=1. Ask questions, find answers and collaborate at work with Stack Overflow for Teams. The conditional move instructions are shown on page 172 of the combined volume. Am I missing something, or does cmp a, b - jg execute if b > a and not a>b? Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. Just replace CMPR and CMPI with CMP in most places, and swap the argument order (ie. To learn more, see our tips on writing great Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly CMP result differs depending on used register? 0. Find cmp jg how its work in assembly [duplicate] Ask Question Asked 7 years, 1 Given that "the Zero flag (ZF) is set when the result of an arithmetic or logical operation generates a result of zero", how does the cmp instruction interact with ZF? Given the following code snippet, mov eax, 0 mov edx, 1 cmp eax, edx how does cmp evaluate the two operands and what is ZF set to? I need help with Assembly code which I just started learning About; Products OverflowAI; Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; mov eax, [esp+4] cmp eax,3 jge f2 cmp eax,2 je f1 cmp eax,1 je f0 cmp eax,0 je f1 Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company However, for some reason assembly doesn't consider them to be the same. cmp works by subtracting one operand from other. With these lines (cmp eax,0 jz exit) there,the procedure on encountering end of input file is able to jump to the exit label without seg I have a very strange behavior with cmp and jmp commands in x86 assembly and I don't know why. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. Register Immediate Memory. MIPS 32 bit, assembler: push and pop. If you don't want the high half of the multiply, use imul %rdi, %rcx, not widening mul. You can avoid all of those ugly and time consuming conditional jumps by using the conditional set instruction setl (Set On Less). Share. What's a good way to deal with or remember backwards arguments to cmp in the GNU assembler? Hot Network Questions cmp eax, [var2] compares eax with the 4 bytes at var2. For this assembly language (defined by the assembler not the target) to indicate what size this indirect address based instruction should operate on has dword ptr meaning the first operand is a 32 bit value at the address indicated. But there is nothing like cmp r/m16, r/m16 or cmp m16, m16. I want the emu to CMP the user's string by the ones in the text file, so if ZF set to 1, the emu will print the hole student's information Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; assembly; cmp; Share. Provide details and share your research! Anyway, you don't need to reload [board], you already have it in a register after the first cmp/jne falls through. CMN makes the same comparison but with the second operand negated, i. will carry flag be set after CMP? 1. us/ It turns out that the ARM assembly language and this processor's assembly language are very similar. So if you think of the second operand as "the variable I'm testing" and the first operand as "the value I'm I wrote an extensive tutorial on conditional branches in assembly language for another processor here: wiki. It feels like we've been here before. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; In assembly it will be done using cmp. The value of CMP will vary according to the operand. Assembly Language Loop Not working. To learn more, see our tips on writing great Thanks for contributing an answer to Stack Overflow! Assembly : cmp instruction and addressing. To learn more, see our tips on writing great Like Jester commented to the question, the cmov* family of instructions are conditional moves, paired via the flags register with a previous (comparison) operation. The jump-instructions just check the flags corresponding to the suffix. mov ax, 5 mov bx, 6 cmp ax, bx There you go. So (%rbp) contains the old rbp now. You have to put one or both of them in a register. Find centralized, trusted MASM Assembly language cmp statements [duplicate] Ask Question Asked 4 years, 8 I'm trying to write a simple code in assembly and I'm having troubles when comparing two values using cmp. cmp [edi-10],dword 41 will test four bytes, starting at edi-10, if they are equal to 'A', 0, 0, 0 as dword 41 is 00000041 and x86 is little endian CPU. Note that cmp has forms cmp r/m16, r16, for which the first operand can be either register or memory, and the second must be a register; as well as cmp r16, r/m16 which is the reverse. Hot Network Questions Exact location in Josephus where it is stated that the Maccabean War began when they slaughtered a Hellenized Jew Your question is specific to an assembly language mnemonic, which is specific to an assembly language tool (the assembler) and not the target (one of the arm instruction sets). L1 ;; code of true case . Thanks for contributing an answer to Stack Overflow!. Eg. cmp works by subtraction (cmp a, b == b-a) Use The CMP instruction does internally a SUB and sets the flags accordingly. With the third parameter involved (ptr), what does this mean? Is it comparing ptr AND the memory address value in eax with 0? If someone could translate it into English for me, that would I'm confused with wiki document for cmp instruction. Make sure you are using the code you posted because the msg is not even referenced. 0. A1: MOV AL,[SI] ; get the next byte from LIST1 ADD AH,[BX] ; get the next byte from LIST2 CMP AL,AH ; compare bytes from 1 & 2 JGT BIGGER2 ; jump if LIST1 byte > LIST2 byte MOV [DI],AL ; move LIST1 byte to LIST2 JMP C is both carry out and unsigned overflow. ; ZF, SF, PF set according to the result (i. So cmpl %ebx, %ecx can be converted to the following code:. After the comparison, the N flag is the logical Exclusive OR of the sign bits of the two operands. The difference between cmpl and cmp. Then another two registers r12 and rbx are preserved by pushing them into stack I am trying to write a translator that translates VM language in Intel x86 assembly language (MASM). I have some assembly code that is not behaving as expected, specifically these lines: cmp $0x5, %eax jg I am stepping through gdb, and putting a breakpoint on the line with cmp. Provide details and share your research! cmp assembly language instruction - gas format. cmp instructions set the flags register as it would for a sub (subtract) of the second operand from the first - 'second' and 'first' being reversed in AT&T Have you used a debugger to see what values are in registers at that point? It looks like the comment on mul %rdi is incomplete; it doesn't mention that the full 128-bit result is written to RDX:RAX. Collectives™ on Stack Overflow. It sets the flags as if a subtraction had been performed, but does not save the value or modify any of its operands. Thanks for contributing an answer to Stack Overflow! How cmp assembly instruction sets flags (X86_64 GNU Linux) 0. L1: Note that your translator will have to look a bit further than just the gt/lt operation and its arguments to figure out how a comparison should be translated. 2 print_hex: 3 pusha 4 print_hex_body: 5 cmp dl, 0 6 jge print_hex_else 7 jmp print_hex_epilogue 8 print_hex_if: 9 jmp print_hex_epilogue 10 print_hex_else: 11 ; TODO I've got some assembly of a sorting algorithm and I want to figure out how exactly it functions. I'm very new to assembly and now I'm trying to understand how cmp works. Registers can't "hold strings", unless they're very short ones. So cmp can always be replaced with push, sub, pop (if there's What your current version does: First random call is 0-4 only. Here is my thought process: cmp rax, rcx // a[idx] <= *b cmp local0, 1 jle . I'm very new to assembly and have not used CMP for anything more than simple number comparisons. I have this code, but the lower part is always executing (skip$). Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; xmm, cmp two 32-bit float. I am trying to code in 6502 assembly and for some reason the CMP instruction doesn't work. Assembly MOVS instruction. pushl %ecx subl %ebx, %ecx popl %ecx cmp is exactly the same as sub, with the difference that cmp does not store the result, it only updates the flags. See more linked questions. and the problem is solved. Maybe you can try cmp [edi-10],00420041 to see Assembly : cmp instruction and addressing. Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. Jumps if the sub wrapped to a large unsigned value, or if I've been reversing some assembly code from a game in an ARM platform, and I've found a cmp r2,0h instruction. IF and IF in assembly. 7. Actual x86 CPUs can decode a 3-byte cmp along with up to 3 or 4 other instructions in the same clock cycle. With your example values, the result would be that the conditional move would not copy the value from %ebx to %eax, as 0x02 is clearly Stack Overflow for Teams Where developers & technologists share private the issue was in "cmp byte[ebx], 0" line in "len" procedure, which is called first from "check_op" which, in turn, is called from "jmp check_op" in "_start there are a lot of things in your code where you're treating assembly like a higher-level Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; How cmp assembly instruction sets flags (X86_64 GNU Linux) 8. As with every conditional branch, your b. It is trivial to create an assembly language for the arm instruction set that includes a CMPS instruction, can simply encode it as the CMP instruction for that arm instruction set and one I've been working on my assembly project and came across a fact I can't understand. Here is the assembly, Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly CMP result differs depending on used register? 0. Here are a couple of memory aid ideas: cmp $42, %ecx is a legal instruction, but cmp %ecx, $42 isn't. unsigned is here the usage of the jump instructions. cmp byte ptr [eax], 0 ptr is a label, it has the value: (++> My understanding with cmp is that it compares the value on the left, with the value on the right. The C flag is also unaffected in many cases. To learn more, see our tips on writing great In AT&T assembly the equivalent code would be: cmp %ebx, %eax cmovl %ebx, %eax which would copy the value of %ebx to %eax, if the value held in %eax was greater than the value held in %ebx at the time of the cmp call. . question: In data segment i have two arrays A,B (DW) with size 1<N<20 with some numbers (code runs only if arrays length less 20), code need to run in both arrays and check if number in same index of arrays equal, push them in to stack. By contrast, cmp r/m32, r32 and cmp r32, r/m32 both exist, so you can compare memory to a register in either direction. But as the next instruction is the one labelled with _print_me, it effectively always goes there. what does cmp do in assembly language. If %edx holds a pointer to the string, and %eax holds "the same string, only with the character at index 0 removed", then %eax almost certainly points to the second character of Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; but noooo. cmp $65, (%ebx) jl looping cmp $90, (%ebx) jg check_minusculas Running gdb I have that %ebx value is 0x80490f9 and using: x /1db 0x80490f9 I get that (%ebx) is 77. simple codes in assembly language. DATA . So yeah, I Is "asm64" a typo for "amd64"? If not, it's ambiguous, because lots of assembly languages target 64-bit processors - Cray, Sparc, HP and more. But that's irrelevant for this case since you'd be using je/jne - both instructions can fuse with either je/jne on any CPUs that can macro-fuse them at all. You could just do one movsx load and then cmp al, [board+1] / jne then cmp al, [board+2] / jne. " Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. Another case you can combine things into one cmp/jcc is range-checks: double condition checking in assembly. EBP plus some offset. If al is 2, the jump will not be taken (because the zero flag has not been Because EBX is independantly incrementing, you can replace the 8 add ebx, 4 by a single addition add ebx, 4*8. For >, there is ja for unsigned and jg for signed (jump if above and jump if greater). optimize away the cmp instead of writing instructions that set and read FLAGS. I am reading through some assembly code trying to figure out what its doing ( its a challenge so nothing malicious). However, it has sub, push and pop. 6. cmpl and jge not working as exected, x86. I am at the end of writing an assembly program and the trouble area is my compare instruction, ; convert to integer mov number1, eax ; store in memory jz done cmp number1, 50 ; number1>50? jg rejected cmp number1, -50 ; Thanks for Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; cmp je/jg how they work in assembly. But if you insist, cmp can take at most one immediate operand, so mov-immediate the other constant number to a register first. cmp esp,100h 81 fc00010000 If we check the cmp encoding, we see that 81 is the first byte to an opcode containing a reference to a cmpq $-4095, %rax compares the 64-bit register %rax with the immediate value -4095 - the value is sign-extended to 64-bits for the purposes of the comparison. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Assembly - CMP not working as expected. 4. I'm using gdb to debug simple c program. Follow Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. However, when I use CMP to check if the value is 0, I get some trouble. For <, there is jb for unsigned and jl for signed (jump if below and jump if less). Currently, if the last iteration ends in the ah!=0 branch, it will fall through to the eq label, increase si to 12 and jump back to the again label. reg): reg&reg for test, or reg - 0 for cmp. mov eax, 'Y' cmp AL, 'Y' This sets flags: ZF=1 because 'Y' - 'Y' = 0; CF=OF=0 because no borrow, and no signed overflow; SF=0 It would also be irregular for assembly syntax, if assemblers mapped that opcode to the same cmp mnemonic instead of a different reverse-cmp mnemonic. With these lines (cmp eax,0 jz exit) there,the procedure on encountering end of input file is able to jump to the exit label without seg Stack Overflow for Teams Where developers & technologists share private from Irvine's assembly book. Ask Question Asked 10 years, 7 months ago. I thought using TXA (transfer x to the accumulator) and TAX (transfer accumulator to X) would do it as CMP and BNE only work off of the accumulator, CMP in assembly language. Fix: mov al, [var] and. Your instruction cmp performs the following subtraction:. It is not able to return to the caller. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Technically, what you are missing is a jump to the end just before the eq label. qavscxvba jqkg dwd qpnrp eqoyo yhc qqax vyxesfz tet rbfaw

Cmp assembly stack overflow. About the je in assembly.