Eax assembly example However, unless I am mistaken, %eax is a 32-bit register. ) Nov 11, 2015 · Description. What does it mean when there is only a source and no destination parameter in a sar instruction? example: sar %eax, when usually the instruction is of type sar source, destination? May 11, 2021 · So, if %rax is initially 0, and you execute the instruction movl $5, %eax, then %rax will also take on the value 5. Is this a typo? Hello, I stated learning assembly today and as usual I written my first program , hello world ( in x86 ). Including a "\0" in an immediate doesn't work; that's why you xor eax,eax and push eax where we want some zeros, or use push imm8. directivename arguments. See Intel's instruction manual entry. Jan 16, 2016 · For example, if EAX contained 0x7FFFFFFF you'd get 0 in EDX, since the most significant bit of EAX is clear. We have already used the system calls. Writing AL, AH, or AX leaves other bytes unmodified in the full AX/EAX/RAX, for historical reasons. In your case with imul edx, you get EDX:EAX = EAX * EDX. Am I correct? What you are describing is a convention used by at least the Microsoft compilers. And BTW, there are also by-one short forms of ror, rol, rcl, and rcr, as well as shl (aka sal), shr, and sar. But x86 assembly code does not have separate opcodes / instruction mnemonics for the reg,reg and reg,mem forms of these instructions. data signed_val SDWORD ? ; Data type is signed doubleword . The instruction add eax, [ebp+8] performs a similar operation but with the contents of memory at address ebp+8. Syntax idiv <reg32> idiv <mem> Examples. add eax, ebx ; adds ebx to eax Assembly Language Statements Mar 21, 2013 · rol eax,16 ; rol / ror mov ax,0b1000000000000001 rol eax,16 ; rol / ror For hardcoded values (as in the question), you can also use and with or / xor / add . i. The lower 32 bits are subtracted first, setting the Carry flag CF. So the lower 16 bits of EAX become A69B, and the upper 16 bits are zeroed: EAX = 0000A69B. You didn't say what the alternate was, it could have been the more generic 8B /r mov r32, r/m32 but that's one byte longer and shouldn't really be preferred by the assembler. section . and when we multiply it stores in eax. Explain briefly if unsure. Aug 21, 2019 · I want to know the value of eax register after this line of code in assembly . The third line copies BL into EDX. For non-hardcoded values you would need to shift the value in some other register or in memory to use this method: The high 32 bits of the answer will be written to the EDX register and the low 32 bits to the EAX register; this is represented with the EDX:EAX notation. Oct 25, 2012 · x86 Assembly - ‘testl’ eax against eax? example: TEST EAX,EAX JE some_address the CPU will jump to "some_address" if and only if ZF = 1, in other words if and We can see that the RAX register contains a 32-bit EAX sub-register, a 16-bit AX sub-register, and two 8-bit sub-registers AH and AL. For example, modifying the value in AL will also alter the lower 8 bits of AX, EAX, and Jul 8, 2015 · In 32bit or 64bit mode, it's faster to zero ax with xor eax, eax. If your assembly language function is called by C/C++ code compiled with one of the normal calling conventions, it will expect the result to be passed in eax. asm; Hello World Program - asmtutor. LEA accepts a standard memory addressing operand, but does nothing more than store the calculated memory offset in the specified register, which may be any general purpose register. Here is the classic "Hello, world" program, written in C: I/O access OUTx. In this case, EAX is used just to store something. One practical example where the push and pop instructions are useful is in function prologues and epilogues. It is also possible to address the first four registers (AX, CX, DX and BX) in their size of 16-bit as two 8-bit halves. Then mov will go to address 0x400000 and copy 4 byte of data present their to eax register. Functions are central in Assembly. e. However, we can demonstrate a similar concept using memory buffers and interrupts. I would certainly recommend to look University of Washington Example C Code Add two signed integers Assembly Add 2 4-byte integers “Long” words in GCC speak Same instruction whether signed Example Assembly Problems Problem 1: movl $1,%eax. A Slightly More Practical Example of PUSH and POP in Assembly. It may contain any printable character including blank. We’ll learn about functions with a few different examples. edx is then moved into eax. The bit shifting and rotation instructions take a 32-bit register for the value to be shifted, and the fixed 8-bit register cl for the shift count. There's nothing particular about eax in itself. An example of the bits in the FLAGS set by the program is the Direction Flag. We saw similar examples in operands subsection also. Feb 16, 2012 · movzx eax, byte ptr [eax+ecx+1] i. What is the function of the "mov eax, cr3; mov cr3, eax" in x86 assembly code? 6. Assembly Language for Intel-Based Computers, 2003. For example, on my machine, if you set all bits of %rax to 1, and then perform xor %eax, %eax, you will find that ALL of %rax is set to 0, not just the lower 32 Oct 24, 2014 · From the GAS-manual:. But I think this answers your question. So that's 8 bits into 32 bits, and zero the Variables in Assembly Language. 10. MOVSX ECX,BYTE PTR DS:[EDX] The instruction opcode is MOVSX. Nov 1, 2009 · From the "Zen of Assembly" by Abrash:. This means that eax is treated as a pointer, so the right operand is taken from the address pointed to by eax , and the result is stored to the same address. Syntax not <reg> not <mem> Example not BYTE PTR [var] — negate all bits in the byte at the memory location var. incl %ecx means ecx = ecx + 1;. (For example, mov can be rewritten as movb, movw, or movl. L4: ret Which of the assembly code fragments matches the C function shown? Page 6 of 17. For example, in all of the above A few arithmetic instructions take only one register as an argument, for example: notl %eax means eax = ~eax;. mov eax, 100 mov ebx, 200 mov ecx Assembly - System Calls - System calls are APIs for the interface between the user space and the kernel space. requires direct interfacing with hardware. The rol instruction is similar to shl except the shifted bits are rotated to the other end. These instructions perform computation on values, typically values stored in registers. remember though that eax = 2 * edx, so the result is edx * 3. Due to its low-level nature, there isn’t a direct one-to-one translation from high-level languages like Go to Assembly. Fortunately gas will optimize sar $1, %eax into the implicit by-one opcode, so you don't get inefficient code if you write it that way. Nov 10, 2015 · I'm just getting into the section about IA32 assembly, and it asks this question: For each of the following lines of assembly language, determine the appropriate instruction suffix based on the operands. An example of the bits in FLAGS set by the CPU is the Carry Flag. But wait, there’s more to the story! x86-64. In 2003 AMD effectively takes over the architectural leadership and introduces the first 64-bit processor in the x86 lineage. What EAX contains doesn't matter at all to the programmer, since it will be erased anyway. Similarly, testl %eax, %eax; jl L jumps to L if and only if %eax, considered as a signed 32-bit number, is less than zero. The a modifier enforces val to be placed in the eax register before the asm command is issued and Nd allows for one-byte constant values to be assembled as constants, freeing the edx register for other cases. Jun 12, 2019 · As an example, assume EDX has the value 0x80000000. 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. LEA, the only instruction that performs memory addressing calculations but doesn't actually address memory. text global _start _start: mov edx, 13 ; number of bytes to write - one for each Ask Computer Science Questions And Get Answers! This subreddit is intended for questions about topics that might be taught by a computer science department at a university. Assembly language is a low-level programming language that is specific to a particular computer architecture. All use radically different assembly languages. However, we can simulate a basic timer using a loop and a system call to sleep. . l1: . For example, a programmer may consider using an assembly language to write a device driver or optimize part of a game program. For example: shll %cl, %ebx means ebx = ebx << cl;. cdq sign-extends eax into edx:eax, i. It can add registers, multiply registers with the constants 2, 4 and 8 for address calculations of words, integers and doubles. For other registers like eax, ecx, edx, there is no guarantee of them not being used by the C library functions. or. I am learning x86 assembly, and have some troubles with the lea instruction. Jul 21, 2014 · My nasm (version 2. Data-movement and control-flow instructions do not modify flags. Labels look like labelname: or labelnumber:; directives look like . 0x080486f7 <+21>: lea eax,[esp+0x18] Can anybody explain what happens in this line? In my understanding, it takes the value at [esp+0x18] and interprets the value as an address, and puts the value of what is int the address into eax. In 16b mode on a modern CPU, probably xor eax,eax is still better, because of the many special benefits of using a recognized zeroing idiom – The assembly as follows, I am interested in the testl line: 000319df 8b4508 movl 0x08(%ebp), %eax 000319e2 8b4004 movl 0x04(%eax), %eax 000319e5 85c0 testl %eax, %eax 000319e7 7407 je 0x000319f0 I am trying to understand that point of testl between %eax and %eax? I think the specifics of what this code isn't important, I am just trying to May 10, 2015 · For example, if you have this code: cmp eax,ebx jne . so if I call the function multiply( 3 , 8 ), the value of eax register after that line should be 120. com ; Compile with: nasm -f elf helloworld. quotient in eax, remainder in edx. In this example for matching constraints, the register %eax is used as both the input and the output variable. When you manipulate the contents of one of these sub-registers, it affects the corresponding portion of the larger register. This data can be stored in memory and accessed from thereon. 5 Specifying an Offset The offset part of a memory address can be specified directly as a static value (called a displacement) or through an address computation made up of one or more of the following components: Nov 28, 2015 · The idiv instruction divides the contents of the 64-bit integer EDX:EAX by the specified operand value. And AX now refers to the lower half of EAX, while AH and AL continue to refer to the two AX bytes. However, reading data from and storing data into memory slows down the processor, as it involves complicated processes of sending the data request across the control bus and into the mem %eax %ax %al %ah. In this case, your assembly code is. For example, the instruction addq %rax, %rbx performs the computation %rbx := %rbx + %rax. data msg db 'Hello, world!', 0xa len equ $ - msg In this Assembly language version: We define a vals routine that simulates returning two values by putting them in the eax and ebx registers. [3] We will now use the GNU C compiler to generate assembly code, for the purposes of examining the GAS assembly language syntax. Bit 31 is 1. In x86-64 assembly, when a function is called, it needs to set up a stack frame to store its local variables and other information. 01) assembles your example to A1. Assembly Language doesn’t have direct equivalents for concepts like channels or buffering as they exist in higher-level languages. text global _start ; Here's a function that takes two integers and returns their sum plus: ; Function parameters are passed on the stack push ebp mov ebp, esp mov eax, [ebp+8] ; First parameter add eax, [ebp+12] ; Second parameter pop ebp ret ; A function that takes three parameters and Jul 6, 2014 · int main(int argc, char* argv[]) { return 0; } what's the shortest assembly example to do the same that can be compiled into an executable by gcc? I came across this example but there're too many Jan 29, 2019 · Based on these specifications, I can manually create this procedure to fit C calling convention:;-----_ProcC_CallWithStackFrame PROC near ; For __cdecl, manually making C calling convention with Stack Frame; Receives: x and y on the Stack Frame ; Returns: EAX, the result x-y;-----push ebp mov ebp, esp mov eax,[ebp+8] ; first argument x sub eax,[ebp+12] ; second argument y pop ebp ret _ProcC Assembly Language doesn’t have built-in timer functionality or high-level concepts like channels. For example, if you add 1 to the AL register that contains 255, you will get AL=0, Carry = 1. All binary numbers which not have set the 13th bit on memory address [eax], or all numbers present like b'xxxxxxxx xxxxxxxx xx0xxxxx xxxxxxxx', where x is 0 or 1, there is exactly 2^31 numbers. Not to be confused with cdqe, the 64-bit instruction that is a more compact form of movsxd rax, eax. Ah, ok, I was wondering how that div eax, ebx got in there in an example based on working code! (and yes, I could tell it must not have been copy/pasted verbatim). Example: movl %edi, %eax. The point of CDQ is to set up EDX prior to a division by a 32-bit operand, since the dividend is EDX:EAX. broadcasts the sign bit of eax into every bit of edx. Ex: pushl %eax and popl %eax – Examples and $0x0f, %eax — clear all but the last 4 bits of EAX. From what I understand "%eax" returns the address, for example movl $0x4050, %eax, moves the value 0x4050 at address 0x100. Size Directives In general, the intended size of the of the data item at a given memory address can be inferred from the assembly code instruction in which it is referenced. Irvine, Kip R. There's no form of div / idiv that ignores edx in the input. While you're at it, I'd suggest a comment to point out that it's intentional that you don't zero EDX inside the loop; that the remainder from the previous division is the upper half of the input to the next limb. #this is in a file first. Ordering of operands: Unlike Intel convention (first operand is destination), the order of operands is source(s) first, and destination last. xor %edx, %edx — set the contents of EDX to zero. MOVSBL and MOVZBL * MOVSBL sign-extends a single byte, and copies it into a double-word destination * MOVZBL expands a single byte to 32 bits with 24 leading zeros, and copies it into a double-word destination Example: %eax = 0x12345678 %edx = 0xAAAABBBB MOVB %dh, %al %eax = 0x123456BB MOVSBL %dh, %eax %eax = 0xFFFFFFBB May 4, 2017 · If eax was a pointer, then a is now a pointer to that same memory location. 32-bit signed multiplication example: . • mov [esi+eax], cl ; Move the contents of CL into the byte at address ESI+EAX • mov edx, [esi+4*ebx] ; Move the 4 bytes of data at address ESI+4*EBX into EDX Some examples of incorrect address calculations include: • mov eax, [ebx-ecx] ; Can only add register values • mov [eax+esi+edi], ebx ; At most 2 registers in address computation 3. var input is read to %eax and updated %eax is stored in var again after increment. The assembly output makes sense to me (mostly) except for the line xorl %eax, %eax. With assembly language, what you see is what you get. Similarly, in the 64-bit version, the 'E' is replaced with an 'R' ( register ), so the 64-bit version of 'EAX' is called 'RAX'. Apr 7, 2016 · div / idiv: divides edx:eax by the src. It's equivalent to a single sub instruction, except a sub requires the destination to be the same as one of the sources. It is set when you have a carry after an arithmetic operation. s . For example, in the adder2 function shown in the previous example, the compiler references component register %eax instead of %rax since int types typically take up 32 bits (four bytes) of space on 64-bit systems. mov [eax], 10 means: move the value 10 into the address stored in EAX register. The code uses esi and edi to store counters as they will be unchanged by the C library function printf. Assembly language comment begins with a semicolon (;). So the answer is yes. It's fine for the explicit source operand to be one of the implicit operands, even EAX to square into EDX:EAX. mov eax, [ebx] and. In optimized code typically values can live in registers if not optimized away so it's hardly "normal" to see fastcall args getting spilled and reloaded, that would defeat the purpose of fastcall. section:[base + index*scale + disp] is translated into the AT&T syntax. If the guess is wrong there's a performance penalty, because the CPU has to Channel Buffering in Assembly Language. globl main main: movl $20, %eax movl $10, %ebx ret Mar 29, 2023 · Not only does this give you an assembly file that compiles correctly, but it also ensures that the assembly routine does exactly what you intended it to. If the guess is wrong there's a performance penalty, because the CPU has to May 10, 2015 · For example, if you have this code: cmp eax,ebx jne . An Intel syntax indirect memory reference of the form. It can also add an offset. Return value (accumulator) For example, the instruction addq %rax, Assembly generated by a compiler contains instructions as well as labels Jan 3, 2011 · Warning: all the current answers are given in Intel's assembly syntax; push-pop in AT&T syntax for example uses a post-fix like b, w, l, or q to denote the size of the memory being manipulated. It can appear on a line by itself, like −; This program displays a message on screen or, on the same line along with an instruction, like −. The first argument to the function has type int or unsigned int (but not long, unsigned long, or a pointer type). not — Bitwise Logical Not Logically negates the operand contents (that is, flips all bit values in the operand). Traditional names are outb, outw and outl respectively. QUESTION ASM-23A. (Such as objdump -Mintel or gcc -S -masm=intel, although the latter affects the syntax used when compiling inline-asm). We usually see them used in control flow instructions, as in jmp L3 (“jump to L3”). /helloworld SECTION . eax is then added to itselt (eax * 2), and then is added back to edx (edx + eax). This is different from add eax, ebp, which adds the contents of ebp to eax. The example logic: Sets EDX: EAX to 00000001h:00000000h. Whereas lea will copy the address 0x400000 into eax. o -o helloworld ; Run with: . For example, the bitwise-and of %rax and %rax is zero if and only if %rax is zero, so testq %rax, %rax; je L jumps to L if and only if %rax is zero. 7. Labels are markers in the generated assembly, used to compute addresses. lea eax, [ebx] Suppose value in ebx is 0x400000. sys_write and sys_exit, for writing into the screen and exiting from the program, respectively. lea can do a lot more. Branches •x86 uses condition codes for branches • Arithmetic ops set the flags register • carry, parity, zero, sign, overflow 8 Instruction Meaning cmpl %eax %ebx Compute %eax - %ebx, set flags register work backwards from input constraints on inline assembly ò I didn’t do this in the previous slide for the purposes of illustration ò Not always possible to avoid moving registers around or saving values before inline assembly Example (More Sophisticated) %eax= 0; // “a”(x_0), %ebx = %eax + 1; // “b”(y_0) “imul %ebx, %eax” int bitnot(int a) { return ~a; } mov eax, edi not eax ret int logical_not(int a) { return !a; } xor eax, eax test edi, edi sete al ret int booleanize(int a) { return !!a; } xor eax, eax test edi, edi setne al ret Jul 14, 2009 · CMP EAX, 23 ; compare register EAX with the constant 23. The topic of x86 assembly language programming is messy because: There are many different assemblers out there: MASM, NASM, gas, as86, TASM, a86, Terse, etc. However, in your example, (%eax) is a memory operand; that's what parentheses mean in AT&T syntax (like square-brackets in NASM syntax). Problem 5: mov edx, 1 ; upper half mov eax, 0 ; lower half sub eax, 1 ; subtract 1 from the lower half, set CF. This then is clearly the y parameter, which is furthest on the stack, and was therefore the first pushed. hmm eax == 32bits ax == 16bits ah|al == 8bits, it's always like this, x64 adds new registers, back in the 16 bit days we only had ax & al & ah then when 32 bit addressing came round it was added in a way that didn't really effect how you address the 16bit or 8bit registers, the new registers in x64 (64 bit registers that overlap, eax that overlaps ax, etc) start with an r so rax rbx and so on. Feb 15, 2014 · When making a system call sys_read (int80h with the value 3 in eax) in the assembler, what will be stored in the register eax after the call is made? Is it the number of characters in the string th Oct 31, 2015 · To quote the intel basic architecture manual: 3. In the main routine, we call vals and then use the values in eax and ebx as if they were returned values. However, they are not entirely independent, so one should be careful. BTW, most GNU tools now have a switch or a config option to prefer Intel syntax. For example. Subtracts 1 from the value in EDX: EAX. But if you had EAX = 0x80000000 you'd get EDX = 0xFFFFFFFF since the most significant bit of EAX is set. Nov 4, 2012 · SAR command in X86 assembly with one parameter. sbb edx, 0 ; subtract carry CF from the upper half. Since edx is 0, edx:eax is a positive number. it has to merge a new AL into the full RAX, for example. Sends a 8/16/32-bit value on a I/O location. In your example the address calculation is very simple, because it just adds a offset to ebx and stores the result in eax: eax = ebx + 0x10. Mar 30, 2010 · As Mihai says, it is just a naming convention. As a beginner I don't know edx,ecx,ebx or eax mean. Here is the code. Consider the following example: top:cmp eax,ebx ; check loop condition jae next ; false? exit loop Assembly example 0000000000401210 < add > : 401210: 8 d 04 3 e leal ( %rsi , %rdi ), %eax 401213: c3 retq 401214: 66 2 e 0 f 1 f 84 00 00 00 00 00 nopw %cs :( %rax , %rax ) 40121 e: 66 90 nop Left: Address or offset at which code appears The basic kinds of assembly instructions are: Computation. globl main main: movl $20, %eax movl $10, %ebx ret mov examples mov eax, ebx ; copy the value in ebx into eax mov byte ptr [var], 5 ; store 5 into the byte at location var mov eax, [ebx] ; Move the 4 bytes in memory at the address Branches •x86 uses condition codes for branches • Arithmetic ops set the flags register • carry, parity, zero, sign, overflow 8 Instruction Meaning cmpl %eax %ebx Compute %eax - %ebx, set flags register Assembly - Registers - Processor operations mostly involve processing data. CMP EAX, XYZ ; compare register EAX with contents of memory location named XYZ. xor edx, edx — set the contents of EDX to zero. Assembly generated by a compiler contains instructions as well as labels and directives. Shorter encoding, and no false dependency on the previous value of the upper 16b. Mar 12, 2024 · The examples store the result in either %eax for data or %rax for pointers The notation M[addr] refers to the data stored at addr in memory As a side note, this memory access syntax is so convenient that compilers often use lea to perform arithmetic unrelated to memory access. This document contains very brief examples of assembly language programs for the x86. The quotient result of the division is stored into EAX; The remainder is placed in EDX. This example will use a simplified x86 assembly language syntax. Syntax not <reg> not <mem> Example not %eax — flip all the bits of EAX neg — Negate In this case, it's doing a simple numeric subtraction: leal -4(%ebp), %eax just assigns to the %eax register the value of %ebp - 4. That should hopefully auto-vectorize nicely, although I wouldn't be surprised if compilers will be stupid and unpack bytes to dwords before adding even though the loop bound means there's no chance for ove Dec 6, 2009 · Jeff Duntemann in his assembly language book has an example assembly code for printing the command line arguments. For example add eax, [ebp] reads the contents of memory at the address pointed to by ebp and adds the value to register eax. May 10, 2012 · Which of the following lines of pseudo C code performs the same operation as the assembly statement lea 0xffffffff(%esi), %eax ? Options: a) *(esi-1) = eax b) esi = eax + 0xffffffff c) eax = esi - 1 d) eax = *(esi -1) Because it's a "lea" operation, I feel like the answer should be (c), but apparently the answer key says (a). . text global _start _start: mov edx,len mov ecx,msg mov ebx,1 mov eax,4 int 0x80 mov eax,1 int 0x80 section . For example, Intel syntax "mov eax, edx" will look like "mov %edx, %eax" in AT&T assembly. l1 mov eax,edx . For each question, write one to three assembly instructions that might imply that the containing function has the given property. Therefore, modifications to a 32-bit register are reflected in the corresponding 64-bit register, and vice versa. not — Bitwise logical not Logically negates the operand contents (that is, flips all bit values in the operand). 3. Feb 11, 2019 · If you actually wanted to help compilers optimize this, you'd do unconditional notcounted += visited[i]; inside the loop, and counter = 100-notcounted; outside the loop. If the adder2 function had a long parameter instead of a int, the compiler would store a in register %rax instead of register %eax. Jul 7, 2015 · The second line copies BX into EAX; but BX is 16 bits and EAX is 32 bits - what about the extra 16 bits in EAX? MOVZX produces zero-extension, which fills the rest with zeroes. Dec 16, 2010 · Instruction test works like and instruction, the only difference is that result is not stored back in to the destination operand. helloworld. load a byte from memory at eax+ecx+1 and zero-extend to full register. push eax and eax, 1 xchg ebx, eax ; swap contents, could also use `mov` here pop eax Note that both solutions do not change the value in eax, so you can still use the value in eax freely. section:disp(base, index, scale) Nov 27, 2014 · Let's say that register %eax contains address 0x100 and at address 0x100 there is a value 0x3999. lea eax, DWORD PTR [eax+eax*4] I know when add integers in assembly, it stores result in the destination. data msg db 'Hello World!', 0Ah ; assign msg variable with your message string SECTION . Sep 5, 2018 · a normal requirement for shellcode is that the machine code not contain any 00 bytes, so strcpy doesn't stop when overflowing a buffer. "0" here specifies the same constraint as the 0th output variable. However, we can demonstrate some basic concepts using x86 assembly syntax. Mar 20, 2020 · EAX stood for extended AX. Whether an operand is a register or a memory location is indicated, in the assembler, by assembly syntax. However, given that 'X' is often used for "fill in your value" and is commonly used by mathematicians as the first variable name of choice in equations, and that those particular registers are general purpose (as opposed to say ESP which is the extended (32-bit) stack pointer or EIP the extended instruction pointer) that is perhaps why X is chosen Nov 17, 2015 · Then add eax, 200 adds 200 to eax, making eax contain 300. If you add 1 to AL=254, the Carry will be 0. To understand the assembly code, Let us consider the simple code below. Jul 7, 2017 · Note that the 32-bit and the 64-bit registers are not separate registers since they overlap: the 64-bit rax, for example, has eax as its bottom 32-bits, and so on for rbx and ebx, r8 and r8d and so on. Apr 24, 2015 · Top web hit for movsbl is this page, and if you search for movsbl it says. Syntax rol destination, count Example mov eax, 0xA ; set EAX to 0xA (00001010 in binary) rol eax, 2 ; rotate 2 bits left in EAX, now equal to 0x28 (00101000 in binary) Jun 11, 2021 · One example is movzx rax,eax, where it's obvious the programmer wanted something smaller to be extended with zeros to become something larger; but where it's better for the assembler to generate a mov eax,eax given that the CPU will zero extend by default. 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. We use the printf function to print these values, which simulates the behavior of the original Mar 2, 2017 · A little example using registers and pointers: mov eax, 10 means: move into the register EAX the value 10. From googling, I gather that the purpose of this is simply to set %eax to zero, which in this case corresponds to my iterator long i;. You are dividing 4294901760 by -61184, giving -70196 with a remainder of 29696. But "(%eax)" returns the value at the address, meaning 0x3999. Jun 3, 2014 · No reason to assume these are register args spilled to the stack (because of debug mode); just as likely to be un-optimized code with int x, y; initialized somehow. (In 32 or 64-bit code, prefer a movzx eax, byte [mem] or movzx eax, word [mem] load if you don't specifically want this merging: Why doesn't GCC use partial registers? Examples and eax, 0fH — clear all but the last 4 bits of EAX. So what does movl $0x4050, (%eax) do? mov 16(%r12), %rdi # argv[2] call atoi # y in eax cmp $0, %eax # disallow negative exponents jl error2 mov %eax, %r13d # y in r13d mov 8(%r12), %rdi # argv call atoi # x in eax mov %eax, %r14d # x in r14d mov $1, %eax # start with answer = 1 check: test %r13d, %r13d # we're counting y downto 0 jz gotit # done imul %r14d, %eax # multiply in Apr 16, 2020 · the value at offset +12 from ebp is moved into edx. Also note that the commented values are for eax having the value of 3 as mov eax, 3 was used in the question. 17 WHILE Loops while( eax < ebx) eax = eax + 1; A WHILE loop is really an IF statement followed by the body of the loop, followed by an unconditional jump to the top of the loop. Jan 1, 2024 · For example, 'EAX' is the accumulator register as a 32-bit value. If you want to set a to the value that eax is pointing to, then you will need to do: mov eax, DWORD PTR [eax] ; eax = *eax mov DWORD PTR [a], eax ; a = eax Of course, this clobbers the pointer and replaces it with the dereferenced value. Intel x86 Assembly Language Since Intel x86 processors are ubiquitous, it is helpful to know how to read assembly code for these • EAX, EBX, ECX, EDX Lets understand this with a example. Oct 14, 2006 · Register Naming: Register names are prefixed with %, so that registers are %eax, %cl etc, instead of just eax, cl. Most have zero or one source operands and one source/destination operand, with the source operand coming first. Here’s a simplified version of the timer concept in x86 Assembly for Linux: Jan 2, 2019 · idiv divides edx:eax by the explicit source operand. examples. asm ; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 helloworld. • mov eax, [ebx-ecx] ; Can only add register values • mov [eax+esi+edi], ebx ; At most 2 registers in address computation 3. code mov eax, +4823424 mov ebx, -423 imul ebx ; EDX: EAX = FFFFFFFFh:86635D80h, OF = 0 move signed_val, eax ; store the result EDX is a sign extension of EAX, so the Overflow flag is cleared. And that’s how EAX got its name. iyj uhfaoso tfrj vqmck gyuxrja wqudcv wbli qyiqkfj kirs iwn