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  • masm division overflow

    - by Help I'm in college
    I'm trying divide two numbers in assembly. I'm working out of the Irvine assembly for intel computers book and I can't make division work for the life of me. Here's my code .code main PROC call division exit main ENDP division PROC mov eax, 4 mov ebx, 2 div ebx call WriteDec ret divison ENDP END main Where WriteDec should write whatever number is in the eax register (should be set to the quotient after the division call). Instead everytime I run it visual studio crashes (the program does compile however).

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  • how to know location of return address on stack c/c++

    - by Dr Deo
    i have been reading about a function that can overwrite its return address. void foo(const char* input) { char buf[10]; //What? No extra arguments supplied to printf? //It's a cheap trick to view the stack 8-) //We'll see this trick again when we look at format strings. printf("My stack looks like:\n%p\n%p\n%p\n%p\n%p\n% p\n\n"); //%p ie expect pointers //Pass the user input straight to secure code public enemy #1. strcpy(buf, input); printf("%s\n", buf); printf("Now the stack looks like:\n%p\n%p\n%p\n%p\n%p\n%p\n\n"); } It was sugggested that this is how the stack would look like Address of foo = 00401000 My stack looks like: 00000000 00000000 7FFDF000 0012FF80 0040108A <-- We want to overwrite the return address for foo. 00410EDE Question: -. Why did the author arbitrarily choose the second last value as the return address of foo()? -. Are values added to the stack from the bottom or from the top? apart from the function return address, what are the other values i apparently see on the stack? ie why isn't it filled with zeros Thanks.

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  • Multithreading and Interrupts

    - by Nicholas Flynt
    I'm doing some work on the input buffers for my kernel, and I had some questions. On Dual Core machines, I know that more than one "process" can be running simultaneously. What I don't know is how the OS and the individual programs work to protect collisions in data. There are two things I'd like to know on this topic: (1) Where do interrupts occur? Are they guaranteed to occur on one core and not the other, and could this be used to make sure that real-time operations on one core were not interrupted by, say, file IO which could be handled on the other core? (I'd logically assume that the interrupts would happen on the 1st core, but is that always true, and how would you tell? Or perhaps does each core have its own settings for interrupts? Wouldn't that lead to a scenario where each core could react simultaneously to the same interrupt, possibly in different ways?) (2) How does the dual core processor handle opcode memory collision? If one core is reading an address in memory at exactly the same time that another core is writing to that same address in memory, what happens? Is an exception thrown, or is a value read? (I'd assume the write would work either way.) If a value is read, is it guaranteed to be either the old or new value at the time of the collision? I understand that programs should ideally be written to avoid these kinds of complications, but the OS certainly can't expect that, and will need to be able to handle such events without choking on itself.

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  • How is thread synchronization implemented, at the assembly language level?

    - by Martin
    While I'm familiar with concurrent programming concepts such as mutexes and semaphores, I have never understood how they are implemented at the assembly language level. I imagine there being a set of memory "flags" saying: lock A is held by thread 1 lock B is held by thread 3 lock C is not held by any thread etc But how is access to these flags synchronized between threads? Something like this naive example would only create a race condition: mov edx, [myThreadId] wait: cmp [lock], 0 jne wait mov [lock], edx ; I wanted an exclusive lock but the above ; three instructions are not an atomic operation :(

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  • 80x86 16-bit asm: lea cx, [cx*8+cx] causes error on NASM (compiling .com file)

    - by larz
    Title says it all. The error NASM gives (dispite my working OS) is "invalid effective address". Now i've seen many examples of how to use LEA and i think i gots it right but yet my NASM dislikes it. I tried "lea cx, [cx+9]" and it worked; "lea cx, [bx+cx]" didn't. Now if i extended my registers to 32-bits (i.e. "lea ecx, [ecx*8+ecx]") everything would be well but i am restricted to use 16- and 8-bit registers only. Is here anyone so knoweledgeable who could explain me WHY my assembler doesn't let me use lea the way i supposed it should be used? Thanks.

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  • Doubts in System call mechanism in linux

    - by bala1486
    We transit from ring3 to ring0 using 'int' or the new 'syscall/sysenter' instruction. Does that mean that the page tables and other stuffs that needs to be modified for the kernel is automatically done by the 'int' instruction or the interrupt handler for the 'int 0x80' will do the required stuff and jump to the respective system call. Also when returning from a system call, we again need to go to user space. For this we need to know the instruction address in the user space to continue the user application. Where is that address stored. Does the 'ret' instruction automatically changes the ring from ring3 to ring0 or where/how this ring changing mechanism takes place? Then, i read that changing from ring3 to ring0 is not as costly as changing from ring0 to ring3. Why is this so?? Thanks, Bala

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  • Why is a 16-bit register used with BSR instruction in this code snippet?

    - by sharptooth
    In this hardcore article there's a function find_maskwidth() that basically detects the number of bits required to represent itemCount dictinct values: unsigned int find_maskwidth( unsigned int itemCount ) { unsigned int maskWidth, count = itemCount; __asm { mov eax, count mov ecx, 0 mov maskWidth, ecx dec eax bsr cx, ax jz next inc cx mov maskWidth, ecx next: } return maskWidth; } the question is why do they use ax and cx registers instead of eax and ecx?

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  • Segmentation Fault when using "mov" in Assembly

    - by quithakay207
    I am working on a simple assembly program for a class, and am encountering an odd segmentation fault. It's a pretty simple program to convert bytes into kilobytes. However, within the function that does the conversion, I get a segmentation fault when I try to move the value 1024 into the ebx register. I've never had this kind of problem before when working with registers. Does someone know what could be causing this? I imagine it is something simple that I'm overlooking. Thank you! asm_main: enter 0,0 pusha mov eax, 0 mov ebx, 0 call read_int push eax call functionA popa mov leave ret functionA: mov eax, [esp + 4] call print_int call print_nl mov ebx, 1024 ;segmentation fault occurs here div ebx call print_int ret UPDATE: One interesting discovery is that if I delete the lines interacting with the stack, push eax and mov eax, [esp + 4], there is no longer a segmentation fault. However, I get a crazy result in eax after performing div ebx.

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  • Problem with asm program (nasm)

    - by GLeBaTi
    org 0x100 SEGMENT .CODE mov ah,0x9 mov dx, Msg1 int 0x21 ;string input mov ah,0xA mov dx,buff int 0x21 mov ax,0 mov al,[buff+1]; length ;string UPPERCASE mov cl, al mov si, buff cld loop1: lodsb; cmp al, 'a' jnb upper loop loop1 ;output mov ah,0x9 mov dx, buff int 0x21 exit: mov ah, 0x8 int 0x21 int 0x20 upper: sub al,32 jmp loop1 SEGMENT .DATA Msg1 db 'Press string: $' buff db 254,0 this code perform poorly. I think that problem in "jnb upper". This program make small symbols into big symbols.

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  • help understanding differences between #define, const and enum in C and C++ on assembly level.

    - by martin
    recently, i am looking into assembly codes for #define, const and enum: C codes(#define): 3 #define pi 3 4 int main(void) 5 { 6 int a,r=1; 7 a=2*pi*r; 8 return 0; 9 } assembly codes(for line 6 and 7 in c codes) generated by GCC: 6 mov $0x1, -0x4(%ebp) 7 mov -0x4(%ebp), %edx 7 mov %edx, %eax 7 add %eax, %eax 7 add %edx, %eax 7 add %eax, %eax 7 mov %eax, -0x8(%ebp) C codes(enum): 2 int main(void) 3 { 4 int a,r=1; 5 enum{pi=3}; 6 a=2*pi*r; 7 return 0; 8 } assembly codes(for line 4 and 6 in c codes) generated by GCC: 6 mov $0x1, -0x4(%ebp) 7 mov -0x4(%ebp), %edx 7 mov %edx, %eax 7 add %eax, %eax 7 add %edx, %eax 7 add %eax, %eax 7 mov %eax, -0x8(%ebp) C codes(const): 4 int main(void) 5 { 6 int a,r=1; 7 const int pi=3; 8 a=2*pi*r; 9 return 0; 10 } assembly codes(for line 7 and 8 in c codes) generated by GCC: 6 movl $0x3, -0x8(%ebp) 7 movl $0x3, -0x4(%ebp) 8 mov -0x4(%ebp), %eax 8 add %eax, %eax 8 imul -0x8(%ebp), %eax 8 mov %eax, 0xc(%ebp) i found that use #define and enum, the assembly codes are the same. The compiler use 3 add instructions to perform multiplication. However, when use const, imul instruction is used. Anyone knows the reason behind that?

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  • Is it safe to change the 'Security.salt' line to a more lengthy string {64 hex key}

    - by Gaurav Sharma
    Hi everyone, I have changed the Configure::write('Security.salt', '############'); value in the file config/core.php file to a '256-bit hex key'. Is it safe or a good practice to change these lines for every different installation of cakephp application or shall I revert back to the original ? I also changed the Configure::write('Security.cipherSeed','7927237598237592759727'); to a different one of more length. Please throw some light on this. Thanks

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  • Setting processor to 32-bit mode

    - by dboarman-FissureStudios
    It seems that the following is a common method given in many tutorials on switching a processor from 16-bit to 32-bit: mov eax, cr0 ; set bit 0 in CR0-go to pmode or eax, 1 mov cr0, eax Why wouldn't I simply do the following: or cr0, 1 Is there something I'm missing? Possibly the only thing I can think of is that I cannot perform an operation like this on the cr0 register.

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  • How to detect Windows 64 bit platform with .net?

    - by Marc
    In a .net 2.0 C# application I use the following code to detect the operating system platform: string os_platform = System.Environment.OSVersion.Platform.ToString();<br/> This returns "Win32NT". The problem is that it returns "Win32NT" even when running on Windows Vista 64bit. Is there any other method to know the correct platform (32 or 64bit)? Note that it should also detect 64bit when run as 32bit app on Windows 64bit.

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  • use callback function to report stack backtrace

    - by user353394
    Assume I have the following: typedef struct { char *name; char binding; int address; } Fn_Symbol //definition of function symbol static Fn_Symbol *fnSymbols; //array of function symbols in a file statc int total; //number of symbol functions in the array and file static void PrintBacktrace(int sigum, siginfo_t * siginfo, void *context) { printf("\nSignal received %d (%s)\n", signum, strsignal(signum)); const int eip_index = 14; void *eip = (void *)((struct ucontext *)context)->uc_mcontext.gregs[eip_index]; printf("Error at [%p] %s (+0x%x), eip, fnName, offset from start); //????? exit(0); } I have this so far, but what is the best way using the fnSymbols static global pointer to identify the function where the error occured and then back trace through the stack to identify each calling function by address, name, and offset?

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  • How do you use printf from Assembly?

    - by bobobobo
    I have an MSVC++ project set up to compile and run assembly code. In main.c: #include <stdio.h> void go() ; int main() { go() ; // call the asm routine } In go.asm: .586 .model flat, c .code go PROC invoke puts,"hi" RET go ENDP end But when I compile and run, I get an error in go.asm: error A2006: undefined symbol : puts How do I define the symbols in <stdio.h> for the .asm files in the project?

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  • What am I doing wrong? (Simple Assembly Loop)

    - by sunnyohno
    It won't let me post the picture. Btw, Someone from Reddit.programming sent me over here. So thanks! TITLE MASM Template ; Description ; ; Revision date: INCLUDE Irvine32.inc .data myArray BYTE 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 .code main PROC call Clrscr mov esi, OFFSET myArray mov ecx, LENGTHOF myArray mov eax, 0 L1: add eax, [esi] inc esi loop L1 call WriteInt exit main ENDP END main Results in: -334881242

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  • Where is the bottleneck in this code?

    - by Mikhail
    I have the following tight loop that makes up the serial bottle neck of my code. Ideally I would parallelize the function that calls this but that is not possible. //n is about 60 for (int k = 0;k < n;k++) { double fone = z[k*n+i+1]; double fzer = z[k*n+i]; z[k*n+i+1]= s*fzer+c*fone; z[k*n+i] = c*fzer-s*fone; } Are there any optimizations that can be made such as vectorization or some evil inline that can help this code? I am looking into finding eigen solutions of tridiagonal matrices. http://www.cimat.mx/~posada/OptDoglegGraph/DocLogisticDogleg/projects/adjustedrecipes/tqli.cpp.html

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  • Calling assembly in GCC????

    - by rbr200
    include static inline uint xchg(volatile unsigned int *addr, unsigned int newval) { uint result; asm volatile("lock; xchgl %0, %1" : "+m" (*addr), "=a" (result) : "1" (newval) : "cc"); return result; } Can some one tell me what this code does exactly. I mean I have an idea or the parts of this command. "1" newval is the input, "=a" is to flush out its previous value and update it. "m" is for the memory operation but I am confused about the functionality of this function. What does the "+m" sign do? Does this function do sumthing like m=a; m = newval; return a

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  • NASM shift operators

    - by Hudson Worden
    How would you go about doing a bit shift in NASM on a register? I read the manual and it only seems to mention these operators , <<. When I try to use them NASM complains about the shift operator working on scalar values. Can you explain what a scalar value is and give an example of how to use and <<. Also, I thought there were a shr or shl operators. If they do exist can you give an example of how to use them? Thank you for your time.

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