Cmp assembly x86. even though I have NONE.

Cmp assembly x86 failcheck1: mov eax, val1 add ebx, 4 cmp eax, 40 jl failinc1 mov eax, passes inc eax mov passes, eax jmp failcheck2 failinc1: mov eax, fails inc eax mov fails, eax failcheck2 : mov eax, val2 add ebx, 4 cmp eax, 40 jl failinc2 mov eax, passes inc eax mov passes, eax jmp failcheck3 failinc2: mov eax, fails How to work with the cmp parameter in assembly x86. Basically, it's: Set the flags as if I had subtracted zero from something. The CMP Instruction: The CMP instruction operates by performing an implied subtraction of the two operands. The [E]FLAGS register (and what the arithmetic flags denote) are described in section 3. Your code has %st(0) == 9 and %st(1) == 10. data array db 0Fh, 45h, 0A1h, 78h, 0CFh, 0AAh, 8Fh, 19h ; Array p db 0 ; Number of even numbers in the array . Oct 5, 2017 · cmp b,a calculates b-a and sets the flags according to the result; given that b-0 == b, cmp b,0 just sets the flags according to b, exactly as test b,b. The value pointed to by eax , if you think it is in fact a pointer, isn't compared to the constant at all. May 9, 2021 · Note that you didn't fix the OP's bug of not checking the first byte (perhaps [rdi+rax-1], or init RAX to -1 before the loop). Mar 25, 2020 · CMP instruction. The test and cmp instructions are frequently seen before a conditional branch. A "arquitetura" ao qual me refiro aqui é a ISA (Instruction Set Architecture) onde ela cria um modelo abstrato de um computador e tem diversas instruções que são computadas pelo processador, essas instruções são o que é conhecido como código de máquina. So cmp low_, high_ would be attempting to assemble an instruction which does not exist, and your assembler will reject it. Just as you can perform 8- or 16-bit multiplications in 32-bit mode. So. CMN makes the same comparison but with the second operand negated, i. 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. The comparison is performed by subtracting the second operand from the first operand and then setting the status flags in the same manner as the SUB instruction. 31 Purpose of cmove instruction in x86 assembly? 0 Understanding cmp in assembly The condition codes used by the Jcc, CMOVcc, and SETcc instructions are based on the results of a CMP instruction. . Apr 13, 2019 · forEachVector: cmp bl, vector jge endforEachVector mov rax, QWORD[array1+rbx] ; move array[i] to rax mov rdx, QWORD[array2+rbx] ; move array[i] to rdx if1Equals2: cmp dl, al jne fi1Equals2 inc cl fi1Equals2: inc bl jmp forEachVector endforEachVector: Jun 30, 2021 · I'm trying to compare a value in the data segment (an item in an array) to the ASCII character value for a space (20h, stored in a register) using a cmp instruction in MASM. May 31, 2015 · How to work with the cmp parameter in assembly x86. This principle reaches its bounds in some of the newest x86_64 SIMD instructions. If an operand greater than one byte is compared to an immediate byte, the immediate byte value is first sign-extended. mov ax, 5 mov bx, 6 cmp ax, bx There you go. You have to put one or both of them in a register. I know that it's because 2's complement format makes the -1 bigger than 1 in underlying binary. Sep 19, 2019 · cmp just sets FLAGS according to dst - src, exactly like sub. e. The dst operand is the first operand in the Intel notation, thus, the flags will be updated according to the result of [ebp+var_8]-eax. intel_syntax, dword is a constant with value 4, so mov dword [rdi], eax is actually mov [rdi + 4], eax. These operations perform arithmetic but throw away the result, except for condition codes. test rax, rax jz is_zero You can get the assembly output from a compiler and check or view it in an online tool like gcc godbolt Here is a simple example of an if/else that stores to one of two different locations, depending on ebx <= 10. mov eax, [var_1] cmp eax, dword[var_2] is valid, because each instruction only has one memory operand. Am I missing something, or does cmp a, b - jg execute if b > a and not a>b? Nov 11, 2015 · It performs a signed comparison jump after a cmp if the destination operand is greater than or equal to the source operand; Syntax jg destination, source Examples cmp bl, 78h jg short loc_402B1D ; if bl ≥ 78h, jump to loc_402B1D Comments We will uses the standard AT&T syntax for writing x86 assembly code. What am I comparing to? No CMP instruction in assembly code before JNE. If memory at -0x10(%ebp) equals immediate 0x7 then the flag ZF is set. However they gave us this sample assembly code: 080485fa <check_pin>: 80485fa: 55 push ebp 80485fb: 89 e5 mov ebp,esp 80485fd: 81 7d 08 bf 07 00 00 cmp DWORD PTR [ebp+0x8],0x7bf 8048604: 0f 94 c0 sete al 8048607: 0f b6 c0 movzx eax,al 804860a: 5d pop ebp 804860b: c3 ret Jul 31, 2021 · These both work, but test edx, edx does the same and is more idiomatic than either one of them. Nov 24, 2018 · The conditional jumps check the flags (eflag register), they are not aware of which instruction did produce the flag. If a cmp was performed and the first operand was greater than or equal to the other, the carry flag will not be set (will be 0). I know that test sets the ZF flag and jne checks that. b instruction do? 1. To my understanding, my cmp command will set the carry flag to 1 or zero, then adc will Jul 20, 2014 · When encoding an assembly instruction, immediate data is typically encoded in the instruction itself. Apr 13, 2020 · mov ecx, 0 ; index and loop counter . I'm very new to assembly and now I'm trying to understand how cmp works. Actually, assembly doesn't work as simply as return true. May 31, 2014 · How to work with the cmp parameter in assembly x86. So it compares its input operands, of course. Just for starting I made the program ch Aug 11, 2017 · The flag CF is set when we compare two digits and one number is less than another: mov eax,1 cmp eax,2 jb truWay In binary form: 1=00000001 2=00000010=11111101+1(add. This SO answer describes some (possible) exceptions to the May 6, 2018 · You can use the conditional jump any time, it will check the value in FLAGS register, so one old optimization from Pentium-like era was to do CMP few instructions ahead to give CPU time to write the change to FLAGS and execute between some instructions not affecting flags (like MOV and similar), but on modern x86 this is counter-performance Apr 22, 2021 · For example, if you have a loop that runs 10 times, instead of setting a register to 1 and incrementing it to 10, doing cmp ecx, 10 in each loop, compilers will set the register to 10, then at the bottom of the loop they do dec ecx ; jnz top (at least if the loop doesn't actually use the value of ecx). Aug 12, 2009 · 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. 04 Dec 13, 2022 · Let's look at a while loop in C. stc jb somewhere will take the branch to somewhere always, because stc is "set carry flag" - yet it doesn't have anything to do with human concept of "below" term (and reasonable programmer would rather write the jc alias there to not confuse reader by "below", but make it Jan 3, 2014 · cmp eax, edx ja somewhere ; will go "somewhere" if eax >u edx ; where >u is "unsigned greater than" cmp eax, edx jg somewhere ; will go "somewhere" if eax >s edx ; where >s is "signed greater than" >u and >s agree for values with the top bit zero, but values with the top bit set are treated as negative by >s and as big by >u (of course if both Oct 21, 2018 · I'm doing x86 asembly code and I keep getting this Error: operand type mismatch for `cmp's The line of code it appears at is: cmpb %rdi, $0 Oct 30, 2013 · When tackling to translate the following segment of code, can someone tell me the purpose of ptr being there? cmp byte ptr [eax], 0 ptr is a label, it has the value: (++&gt; My understanding wi cmp is typically executed in conjunction with conditional jumps and the setcc instruction. The jnz (or jne) instruction is a conditional jump that follows a test. Here are two comparisons I have done to see how it works: # a &gt; b -- 0b01 mov $4, %rax mov $3, %rbx cmp %rax, %rbx Feb 19, 2014 · JS will jump if the sign flag is set (by an earlier instruction). 40. Therefore, I would conclude that there is no situation where test eax, eax will give you something that cmp eax, 0 will not, nor vice versa. The conditional move instructions are shown on page 172 of the combined volume. The difference in signed vs. or. The second instruction throws away the result of the first instruction by overwriting all the flags without depending on them. Should I use the jl instruction, or the cmp instruction, or some other instruction? Nov 15, 2015 · AMD CPUs can only merge test and cmp with a JCC. Nov 9, 2015 · How to work with the cmp parameter in assembly x86. The logic would be pretty much identical if this was 64-bit x86 code, or even ARM or MIPS assembly. Description. incorrect ; if not, the password is incorrect inc ecx ; advance index cmp ecx, 6 ; reached the end of the string? Jan 1, 2019 · I wrote my own implementation of strlen and strcmp from C in x86 FASM and I would like to know is there anything that should be changed or improved. It does both, in a way. Sep 1, 2014 · Gladir. mov $0x11 %rcx test %cl %cl jne c Suppose that %rcx holds a long. code)=11111110и 00000001+ Oct 24, 2013 · You need multiple x86 instructions for one "LT" instruction. So if you think of the second operand as "the variable I'm testing" and the Dec 18, 2011 · cmp BYTE PTR [rax+rcx*1], 0 So it computes the address using rax + rcx, What does this Assembly x86 line do? 1. test rdx,rdx cmovg rax,rcx is equivalent to. This is all coming from Volume 2 of Intel 64 and IA-32 Architectures Software Developer's Manuals. Jul 4, 2022 · I am having trouble finding a way to compare a positive number and a negative number in x86 assembly code. unsigned is here the usage of the jump instructions. Hot Network Questions Unable to install libncurses5 in Ubuntu Server 24. with js. The only guidebook to x86 Assembly programming you will ever need. I. (A good C compiler might turn your switch into a table lookup for the data instead of a jump table). The cmpl instruction compares the contents of general-purpose register (GPR) RA with the contents of GPR RB as unsigned integers and sets one of the bits in Condition Register Field BF. If it jumps to where then it will skip the jne somewhere and the lea NEQUAL. Trouble with cmpsb in x86 Assembly. TEST sets the zero flag, ZF, when the result of the AND operation is zero. com - Manuel de langage de programmation Assembleur 80x86. Using the 16-bit programming model can be quite complex. Generally speaking, conditional execution is usually based on a status register. If you want it to be simpler to understand for people with limited knowledge of the ISA, use cmp al, 0 / jne. cmp, in this case, is comparing the value in eax to a constant 5. Feb 16, 2013 · cmp word [eax],ecx is wrong because operand sizes don't match (ecx is a dword, not word). Sep 1, 2023 · If we take the sub (which is basically what a cmp does) instruction as an example, we have: minuend - subtrahend. What is going on in this x86 instruction? 0. Nov 11, 2015 · It performs a signed comparison jump after a cmp if the destination operand is less than or equal to the source operand. In more pseudo-code fashion, your two instructions amount to: Feb 17, 2021 · Build your x86 assembly skills with six courses covering the basics of computer architecture, how to build and debug x86, x86 assembly instructions and more. Which condition flags CMP instruction sets and how, if the result is equal and if Nov 15, 2013 · So this means that there can't be a general cmp r/m32, r/m32 instruction, and we need two different opcodes: cmp r/m32, r32 and cmp r32, r/m32. Here is the portion of code I wrote, but obviously does not work: CMP DS:[BX], FFFF0H JGE LeaveLoop I'm very new to Jun 19, 2017 · How cmp assembly instruction sets flags (X86_64 GNU Linux) 0. I saw few examples on google but I am still little bit confused. FCOMI sets only some of the flags that CMP does. After subtracting them, it does a few quick tests, updating the Z,O,C,S, and P flags. It may also be more efficient because I would guess that either or or and would appear to the CPU as though they possibly modify the value of the register, since or edx, XXX would do so in general, and the CPU probably doesn't check for this special case. For example, cmpb $0, (%esi) to check for a terminating zero byte at the end of an implicit-length C-style string. But if you insist, cmp can take at most one immediate operand, so mov-immediate the other constant number to a register first. In other words, if you wanted to check if bit 6 (01000000 b = 2 6 = 64) is set in register ch, then you'd use test ch, 64. will carry flag be set after CMP? 1. The mov store instructions are the if and else blocks. Mar 4, 2011 · Related: Reverse-engineering asm using sub / cmp / setbe back to C? My attempt is compiling to branches example of GCC compiling range checks to sub/cmp/unsigned condition. 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 Mar 16, 2020 · i. So cmpl %ebx, %ecx can be converted to the following code: 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. 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. Here are a couple of memory aid ideas: cmp $42, %ecx is a legal instruction, but cmp %ecx, $42 isn't. With respect to sub and cmp there are several cases that fulfill this criterion: minuend < subtrahend and the operation does not have overflow; minuend > subtrahend and the operation has an overflow The CMP instruction is typically used in conjunction with a conditional jump (Jcc), condition move (CMOVcc), or SETcc instruction. Dec 20, 2019 · No. 1. Not knowing the specific syntax, and agreeing that CMP instructions don't have 3 operands, I'm guessing the "d" means "dword [32 bits]". (This is AT&T syntax, so cmp %al, %cl is the same as Intel syntax cmp cl, al) 這樣做可確保重複不超過較短字串的字元數，否則會導致錯誤的結果。請記住，Assembly 不執行自動邊界檢查。 還有更多的字串指令可以繼續看： x86 Assembly 的字串指令、方向指令與重覆指令（續） Nov 27, 2014 · You have to specify the assembler syntax you are using; there's quite a number of varieties for the x86 (which this likely is) and even for others. CMP Instruction. Mar 30, 2015 · 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. For >, there is ja for unsigned and jg for signed (jump if above and jump if greater). Apr 11, 2015 · cmp %eax, %ebx jg < something > would jump to < something > if eax was greater than ebx. Almost all simple ALU ops (bitwise boolean, add/sub, etc. The condition codes used by the Jcc, CMOVcc, and SETcc instructions are based on the results of a CMP instruction. CMPXCHG8B example using Nasm Assembler. May 19, 2016 · In x86 assembly how can you set the zero flag (ZF) without doing a compare operation? 17 Why is the Carry Flag set during a subtraction when zero is the minuend? Mar 23, 2014 · You didn't specify the assembly notation used, so I can just guess it's esp + 4*ebx + 0x18. model small . Syntax jl destination, source Examples cmp bl, 78h jl short loc_402B1D ; if bl < 78h, jump to loc_402B1D Comments Jan 11, 2016 · mov esi,[eax+12] //third argument mov eax,esi cmp eax,'+' What you're doing here is comparing a character (which typically is a single byte) with the 32-bit address of the string that is the third argument. 0. Most x86 instructions with two operands can only work with operands of the same size. Cmp instruction mismatch. Jul 26, 2012 · xor edx, edx ; make sure edx is 0 to start with LOOP: mov al, [esi + edx] mov bl, [edi + edx] inc edx ; prepare for next char cmp al, bl ; compare two current characters jne DIFFERENT ; not equal, get out, you are DONE! Since there are plenty of good responses below I'll just add this here. $',10,13 data ends code segment start: mov ax,data mov ds Jan 20, 2017 · I thought CMP was agnostic, i. g. Apr 2, 2014 · 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 在 assembly 中，若要進行資料的比較，必須使用 CMP 指令，以下是 CMP 指令的使用方式： cmp vleft, vright CMP 指令作的，其實就是 vleft - vright，並將比較後的結果存入 EFLAGS register 中(使用到 zero、carry、sign 三個 flag)，以下進行詳細的說明： Sep 11, 2020 · cmpl subtracts -0x10(%ebp) from $0x7 and modifies flags: AF CF OF PF SF ZF. Note that other architectures are different, but the basic principle stays as far as I know. Apr 21, 2015 · cmp rax, 0 je equal_zero However since cmp is longer if you look at the output binary, test or sometimes and, or is preferred. ; It jumps to the specified location if the Zero Flag (ZF) is cleared (0). 44ac: Jump to location The goal is to have the Z flag high. Mar 3, 2018 · I have the following code to compare numbers and increment a variable if true or false. Below I have shown a part of assembly code that I am trying to convert to C language and the corresponding C code. Here is what's written in wiki: Performs a comparison operation between arg1 and arg2. Oct 25, 2018 · @E. I'm using gdb to debug simple c program. strlen needs string in eax and is returning length of that string into ebx. Or even C. Start Learning Using comparison instructions to control applications at the x86 level Dec 9, 2015 · この記事について. Feb 18, 2012 · Assembly x86 call after 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 Feb 16, 2016 · cmp byte ptr [edi],00 add [eax],al The first one goes to memory address pointed by edi register & fetches the first byte starting from that address , then comparing it to 00=0 . cmp subtracts the second operand from the first without actually modifying the first operand. . Dec 24, 2016 · How to work with the cmp parameter in assembly x86. test can macro-fuse in some cases where cmp can't, e. I simply do not understand how it accesses the command line parameters. g tasm, masm or nasm) you cannot compare immediates or variables with each other. If you look closely at your disassembly you'll realize that the JE could very well be considered to be executed right after the CMP (not logically), the instruction compares EAX with contents located at [EBP+0xC], the instructions that follow never modify [EBP+0xC] and no other instruction will modify the EFLAG registers. This means that the result is not stored in memory. Jun 8, 2019 · Assembly language has no concept of "null" and a cmp is generally the same as a sub (subtract) but without actually changing the value. But yes, code-size is the disadvantage of cmp reg,imm8 vs. Purpose of cmove instruction in x86 assembly? 5. Jan 3, 2015 · In x86 assembly according to your assembler(e. The instruction i'm attempting to use is: cmp [ds:bp + si + 1],cx Feb 3, 2015 · Each instruction set or processor design arm vs x86 vs powerpc, etc has different designers that do different things. 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. the cmovg is an instance of the cmovCC instruction, with condition code g, i. Nov 11, 2015 · It performs a signed comparison jump after a cmp if the destination operand is less than the source operand. 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 making that a 1 means +1 May 7, 2024 · I am very new to assembly and trying to create a very simple script to practice using stdout, stdin and cmp, I ran into the problem of my script not working when using cmp. l1 mov eax,edx . like this: mov ax, 5 cmp ax, 6 or. In Intel syntax, this would be cmp al, bl. The two MASM uses the standard Intel syntax for writing x86 assembly code. Jun 7, 2010 · @CiroSantilliOurBigBook. Interesting to note is that CMP alone cannot compare two memory operands. com: In MASM and GAS . The cmp instruction updates the flags according to the result of the subtraction of src operand from the dst operand. 31. De forma resumida Assembly é uma notação em formato de texto das instruções do código de máquina de uma determinada arquitetura. code start : mov ebx , 20 ( we insert 20 in ebx ) CMP ebx , 10 ( we compare 20 with 10 ) JBE there ( Jump if below or equal to there ) mov [1020h] , ebx ( else we move ebx to [1020h] ) JMP exit ; don't fall into the See Extended Mnemonics of Fixed-Point Compare Instructions for more information. cmp [ebp+argc], 3 mov eax, [ebp+argv] I understand what this code does: mov ecx, [esp+4] ; argc mov edx, [esp+8] ; argv Nov 11, 2015 · Description. when al = 0), just as cmp al, 0 does. – Adriano Repetti Mar 22, 2018 · I have been stuck on trying to understand the following parts of assembly. $',10,13 gata db 10,13,'The directory was created. data ;roll number 2435 data1 db 66h, 2, 045h, 4, 040h, 3, -025h, 5, -010h, 011h swap db 0 . C code to assembler. Oct 27, 2012 · On x86, test does a binary AND between the operands, just does not save the result anywhere. For example: when I compare -1 and 1 I always get -1 as greater. Aug 5, 2015 · mov al, byte ptr[esi + ecx]; move the first character to al cmp al, 0 ; compare al with null which is the end of string je done ; if yes, jump to done cmp al, 0x41 ; compare al with "A" (upper bounder) jl next_char ; jump to next character if less cmp al, 0x7A ; compare al with "z" (lower bounder) jg next_char ; jump to next character if May 10, 2015 · The purpose of cmov is to allow software (in some cases) to avoid a branch. ) Jan 15, 2017 · assume cs:code,ds:data data segment mesaj1 db 10,13,'First dir $',10,13 mesaj2 db 10,13,'Second dir $',10,13 dir1 db 30 dup(?) dir2 db 30 dup(?) pnf db 10,13,'Path not found. Sep 18, 2016 · But in the case of cmp eax,0 the AF will always be cleared regardless of the value of eax, so there is nothing that you can learn from a cmp eax, 0 that you would not learn from a test eax, eax. The P, or parity, flag is rarely used, so we'll ignore it in this article for the purpose of brevity. Regarding cmp / jg, jle, etc in AT&T syntax assembly. cmp cl, dl JE where JNE somewhere somewhere: lea dx, NEQUAL where: lea dx, EQUAL follow your code. JG vs JA). This sucks a lot (silent wrong-code), but so do other MASM syntax design decisions: Confusing brackets in MASM32 - part of the need for ptr is to specify that it's a memory operand at all when there's no register involved: brackets don't do that. Oct 25, 2012 · CMP subtracts the operands and sets the flags. Understanding cmp instruction. CMP :Cette instruction offre la possibilité essentielle de comparer 2 registres ou emplacements de mémoire. Try Teams for free Explore Teams It's a peephole optimization for cmp reg,0 that you can use regardless of the semantic meaning. The instructions, including cmp and jae (or j<cond>) are described in: Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 2. Feb 26, 2013 · It seems that Y86 does not have cmp instruction. This is the case for most x86 arithmetic instructions: one of the operands may be a memory reference, but not both. Syntax jle destination, source Examples cmp bl, 78h jle short loc_402B1D ; if bl ≤ 78h, jump to loc_402B1D Comments Jan 10, 2013 · JE and JNE are more appropriate after a CMP instruction: cmp edx, 42 je the_answer_is_42 (A CMP instruction performs a subtraction, and throws the value of the result away, while keeping the flags; which is why you get ZF=1 when the operands are equal and ZF=0 when they're not. The only common reason for using CMP with an immediate 0 is when you want to compare against a memory operand. The cmp instruction compares the contents of general-purpose register (GPR) RA with the contents of GPR RB as signed integers and sets one of the bits in Condition Register Field BF. Some instructions assume registers implicitly. code xor si, si xor cx, cx mov cl, 2 again: xor ax, ax mov al, array[si] div cl cmp ah, 0 je eq inc si cmp si, 11 jne again eq: inc p inc si cmp si, 11 jne again Do you have any ideas how to fix this code? Feb 24, 2013 · jnb instruction jumps when the carry flag is zero. Aug 4, 2021 · But there is nothing like cmp r/m16, r/m16 or cmp m16, m16. code mov ax, @data mov ds, ax start: mov swap, 0 mov bx, 0 loop1: mov al, [bx+data1] mov cl, [bx+data1+1] cmp al, [bx+data1+1] ;here is the problem when compare 66h with -025h jbe noswap mov dl, [bx+data1+1] mov [bx Sep 16, 2022 · So I've just started writing x86-64 assembly (AT&T syntax) and this is the code for one of my first assignments. This instruction basically subtracts one operand from the other for comparing whether the operands are equal or not. stack 100h . Nov 23, 2015 · I encountered the following assembly version of a C code (Practical Malware Analysis by Michael Sikorski, Chapter 5). (Or lea/cmp if you just need the condition without modifying the original value, and the start of the range is a constant. or reg,reg. Covers fundamental concepts, instruction sets, & mem management. cmp assembly language instruction - gas format x86 Assembly x86-assembly-cheat-sheet: A must-have for quick reference in x86 Assembly. CMP will always modify the flags by performing a subtraction, in this case %cl - %al. The comparison is performed by a (signed) subtraction of arg2 from arg1, the results of which can be called Temp. Would this code set ZF/SF/PF flags using the AND result? Or would this code just AND two operands and do nothing about flags? I know there is TEST and CMP instructions that set flags, but the code would be shorter if I can just use AND to set flags. l1: . 2. Feb 15, 2012 · Ask questions, find answers and collaborate at work with Stack Overflow for Teams. Let us discuss the CMP instruction before discussing the conditional instructions. Unless you're good at reverse engineering, you would most likely have to find the original prototype of the function which states where/how/which Oct 22, 2015 · 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. Jul 17, 2015 · I want to leave a loop if the current address I'm looking at is at least 0xFFFF0. loop: mov al, [passwd+rcx] ; load a character from passwd cmp al, [input+rcx] ; is it equal to the same character in the input? jne . cmp is typically executed in conjunction with conditional jumps and the setcc instruction. 5. It's just one of the many tricks for x86 asm. However, CMPS can. Therefore, the value returned / compared depends on the function signature/language, calling convention, etc. Apr 1, 2013 · Is it possible to mimic an if-statment in x86 assembly language (using masm syntax)? I want to do something like this in x86 assembly language, but I'm not sure which operator I should use to mimic an if-else statement. Assembly-Guidebook 5 CMP AL, BL ; Al = BL = 5 ==> ZF is set to 1 RET I'm new to x86 and I'm trying to put the larger integer ( at location 4(%esp) and 8(%esp) ) into register %eax. For example, if you have this code: cmp eax,ebx jne . BF can be Condition Register Field 0-7; programmers can specify which Condition Register Field will indicate the result of the operation. A CMP instruction performs a subtract operation on both operands, and the status flags ZF and CF will be set according to the result. "move if greater". Compares the equality of two operands CMP AX,BXSYNTAXCMP operand1, operand2NOTE – result is not stored anywhere, flags are set (OF, SF, ZF, AF, PF, CF) according to result. 7. CMP EAX, XYZ ; compare register EAX with contents of memory location named XYZ. Apr 10, 2014 · Addendum to previous comment: CMP peforms comparison via subtraction, 66 ('B') - 65 ('A') will give 1 (above) so JAE will always jump (0 if equals, 1 or more for any character > 65). continue) the loop. Nov 11, 2015 · It performs a signed comparison jump after a cmp if the destination operand is greater than the source operand; Syntax jg destination, source Examples cmp bl, 78h jg short loc_402B1D ; if bl > 78h, jump to loc_402B1D Comments Mar 31, 2016 · I'm learning assembly language and encountered a problem that I don't know even how to ask Google. optimize away the cmp instead of writing instructions that set and read FLAGS. interpreting the result of the compare as signed/unsigned was something the following branch instruction would do (e. To do this with a cmp, both src and dst must be equal. What does the MSP430 cmp. After this point, the following jumps would be taken: JB, because 0 < 255; JNA, because !(0 > 255) JNL, because !(0 < -1) JG, because 0 > -1 Oct 4, 2015 · According to my understanding cmpl compares unsigned. ; jnz is commonly used to explicitly test for something not being equal to zero whereas jne is commonly found after a cmp instruction. Oct 5, 2016 · Using tables instead of a chain of conditional jumps is extremely widely applicable. Then that's reasonable, but normally you want to use movzx ecx, byte [rdi + rax] for byte loads, not create a false dependency and take an ALU uop by merging into the low byte of RBX. In this case they're both qword registers holding pointers. The CMP instruction compares two operands. Appendix B, “EFLAGS Condition Codes,” in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, shows the relationship of the status flags and the condition codes. Mar 8, 2012 · */ cmp %bl, %al Note that AT&T syntax is "backwards": mov src, dst. The x86 architecture in general only allows one memory operand per instruction. I will use Intel x86 architecture in this explanation. It should be noted that the CMP instruction also does not affect the operands. - 7etsuo/x86 Oct 17, 2019 · I am making A Tic-Tac-Toe game and when making the function to check if a player won. Nov 8, 2018 · You can use for example the Intel documentation as a reference for the x86-64/AMD64 instruction set. The string instructions fall into that category. Linuxでx86アセンブラ、第8回目．今回はアセンブラでの条件分岐について書いていきます．条件分岐とは条件にしたがって行う処理を切り替えることです．Cなどの高級言語では、if、switchなどで行っています．アセンブラでの条件分岐はCなどの条件分岐と比べて少し複雑です Jan 15, 2013 · cmp $0, 8(%rsp) If the instruction had been one of the following, the assembler could have figured out the size of the memory operand to fetch and compare based upon the register: cmp %rax, 8(%rsp) Nov 28, 2015 · Logic: CMP (DS:SI), (ES:DI) ; Sets flags only if DF = 0 SI SI + 1 DI DI + 1 else SI SI - 1 DI DI - 1 Example The following example compares BUFFER1 against BUFFER2 for the first mismatch. 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. This is wrong. cmp word [eax],word ecx is wrong because ecx is a dword, not word. The CMP instruction is another example of conditionals. So you have to mentally reverse the operands for the condition codes to make sense with cmp. ,e. test performs binary-and, cmp performs subtraction. It impacts the Zero Flag (ZF) as well as the Carry Flag (CF) as follows: Compares the first source operand with the second source operand and sets the status flags in the EFLAGS register according to the results. It is generally used in conditional execution. Apr 27, 2017 · The reason the CMP instruction is not working for single word comparison is because a word must contain 2 bytes. Often one can write complex "expressions" in the operand field that enable the instruction, if it has the capability, to address memory in variety of ways. 4. Example. while ( condition ) { <loop-body> } The C semantics of a while loop say when to stay in (i. The useful difference between the two operations is that or al, al has a smaller encoding on x86 architectures. cmn a, b means a - (-b) - which under two's complement arithmetic is exactly equivalent to a + b. cmp is hard to grasp: remember that subq %rax, %rbx performs %rbx := %rbx - %rax—the Jan 17, 2011 · . ) run in a Mar 21, 2017 · Actual x86 CPUs can decode a 3-byte cmp along with up to 3 or 4 other instructions in the same clock cycle. It's identical to the sub instruction except it does not affect operands. – davmac Aug 28, 2020 · I think I'm not quite understanding what the PF is in the eflags register. The comparison is done by subtraction - in your case, that means 5 is subtracted from the value in eax , and several flags (CF, OF, SF, ZF, AF, and PF, according to the Oct 26, 2018 · My uni course requires some basic assembly knowledge even though I have NONE. vanPutten: I have no experience of writing 64-bit x86 assembly code. Jun 6, 2016 · Sounds like your lecture slides are slightly mixed up. For testing purposes, I made the program simply exit when A player won. Feb 23, 2020 · or al, al sets the zero flag if the result is 0 (i. 83F800 cmp eax, 0 09C0 or eax, eax 85C0 test eax, eax The resulting code will be. See x86_64 - Assembly - loop conditions and out of order, or just refer directly to Agner Fog's microarch docs for the details of which CPU can macro-fuse what. 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 32bit register, and a 4 byte immediate. For example. Jan 3, 2013 · You can put a label (or comment) on the fall-through path and still write a simple loop with a standard test/jnz at the bottom. What does it mean "Temp is then discarded"? Nov 11, 2015 · The cmp instruction is used to perform comparison. Apr 16, 2022 · Here is a code to sort the given array in assembly language. The CPU is fine, check in debugger that it does evaluate everything correctly, exactly as described in Intel instruction manual, it's not fault of CPU that you provided different values than you intended. I have following code: cmp eax,DWORD PTR [rbp-0xc] j Dec 6, 2013 · I would like to understand how cmp and je/jg work in assembly. However, it has sub, push and pop. Is it implemented in the right way or do I have a wrong understanding of how cmp works? Jan 28, 2015 · 'call' transfers the control to the address of the library routine, strcmp, which is not an intrinsic assembly instruction (like cmp). then when a modern CPU sees the jne branch it will take a guess about whether the branch will be taken or not taken, and then start speculatively executing instructions based on the guess. Assembly x86 - jnz instruction. $',10,13 acd db 10,13,'Acces denied, or pathname already exists. It is a very simple program: it asks for a base and an exponents and it makes the calculation. b instruction do? 0. There are plenty of cases where a 3-byte cmp would be better than a 2-byte or, though, on out-of-order exec Oct 14, 2022 · x86_64 cmp instruction invalid combination of opcode and operands (1 answer) Closed 2 years ago . Mar 10, 2019 · I am trying to figure out the behavior of conditional jumps (JE/JNE, JZ/JNZ) in the x86 instruction set familly. Temp is then discarded. Feb 24, 2016 · Long story short: CMPS performs a CMP with DS:ESI and ES:EDI as operands. Namely, it sets the zero flag if the difference is zero (operands are equal). Compare the 8-bit constant, 0xff, with the content of the AL register: cmpb $0xff, %al Jul 14, 2009 · For the Microsoft X86 assembler, you can write: CMP EAX, 23 ; compare register EAX with the constant 23. But I assume that the addresses are 64 bits, and the operands (in this instruction) are 32 bits. So, your. ) – May 14, 2018 · I'm pretty sure the cmp does work as it should, so your code just isn't written to fulfil your expectations, it's not "cmp doesn't work properly". It is this condition which jae branches on. May 19, 2016 · In x86 assembly how can you set the zero flag (ZF) without doing a compare operation? 17 Why is the Carry Flag set during a subtraction when zero is the minuend? Sep 7, 2015 · is carried out, with the above assembly code in GAS syntax. I am trying to compare the values of 2 variables to each other in assembly. cmp rdx,0 cmovg rax,rcx Jul 19, 2022 · @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. For example, there is a 16-bit subset of the x86 instruction set. 3 of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1 . The compare is followed by a JNE which is taken because ZF=0 and is causing incorrect results. Example: pop ax ; this is X pop cx ; this is Y xor dx,dx ; set edx to 0 cmp cx,ax jle some_label mov dx,1 some_label: push dx Using 32-bit code you may use the "setgt" instruction: Let's say I have the MSP430 assembly segment below: r15: 439c Memory map: 4390: 6045 0200 9c43 6400 8844 5044 363a 0000 Code: 448a: cmp #0x363a, 0x0(r15) 4490: jnz $+0x1c 4492: Code continues . In order to translate this into assembly language's if-goto-label style, we would tell the processor when to exit the loop rather than when to stay in the loop (there are other options, but this is the most direct translation). As a side effect, this creates some redundancy when comparing two registers. This is below EBP so it's probably a local variable, if this is an un-optimized build using EBP as a frame pointer. fxnqpj pnftn xwhj kfxdbd exso drjxa wwtue zsvy zjp qmkfpg