Questions tagged [x86]
x86 is an architecture derived from the Intel 8086 CPU. The x86 family includes the 32-bit IA-32 and 64-bit x86-64 architectures, as well as legacy 16-bit architectures. Questions about the latter should be tagged [x86-16] and/or [emu8086]. Use the [x86-64] tag if your question is specific to 64-bit x86-64. For the x86 FPU, use the tag [x87]. For SSE1/2/3/4 / AVX* also use [sse], and any of [avx] / [avx2] / [avx512] that apply
17,273
questions
2430
votes
10
answers
258k
views
Why are elementwise additions much faster in separate loops than in a combined loop?
Suppose a1, b1, c1, and d1 point to heap memory, and my numerical code has the following core loop.
const int n = 100000;
for (int j = 0; j < n; j++) {
a1[j] += b1[j];
c1[j] += d1[j];
}
...
- 25.5k
1621
votes
11
answers
194k
views
Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs
I was looking for the fastest way to popcount large arrays of data. I encountered a very weird effect: Changing the loop variable from unsigned to uint64_t made the performance drop by 50% on my PC.
...
- 38.7k
935
votes
11
answers
178k
views
Why does C++ code for testing the Collatz conjecture run faster than hand-written assembly?
I wrote these two solutions for Project Euler Q14, in assembly and in C++. They implement identical brute force approach for testing the Collatz conjecture. The assembly solution was assembled with:
...
- 8,954
863
votes
17
answers
866k
views
What's the purpose of the LEA instruction?
For me, it just seems like a funky MOV. What's its purpose and when should I use it?
- 8,819
370
votes
16
answers
204k
views
How can I determine if a .NET assembly was built for x86 or x64?
I've got an arbitrary list of .NET assemblies.
I need to programmatically check if each DLL was built for x86 (as opposed to x64 or Any CPU). Is this possible?
- 58.9k
344
votes
4
answers
49k
views
Deoptimizing a program for the pipeline in Intel Sandybridge-family CPUs
I've been racking my brain for a week trying to complete this assignment and I'm hoping someone here can lead me toward the right path. Let me start with the instructor's instructions:
Your ...
- 2,507
317
votes
12
answers
288k
views
How to compile Tensorflow with SSE4.2 and AVX instructions?
This is the message received from running a script to check if Tensorflow is working:
I tensorflow/stream_executor/dso_loader.cc:125] successfully opened CUDA library libcublas.so.8.0 locally
I ...
- 6,036
309
votes
11
answers
69k
views
What does multicore assembly language look like?
Once upon a time, to write x86 assembler, for example, you would have instructions stating "load the EDX register with the value 5", "increment the EDX" register, etc.
With modern CPUs that have 4 ...
- 13.2k
308
votes
4
answers
126k
views
How to run a program without an operating system?
How do you run a program all by itself without an operating system running?
Can you create assembly programs that the computer can load and run at startup, e.g. boot the computer from a flash drive ...
- 2,079
281
votes
6
answers
253k
views
What is exactly the base pointer and stack pointer? To what do they point?
Using this example coming from wikipedia, in which DrawSquare() calls DrawLine(),
(Note that this diagram has high addresses at the bottom and low addresses at the top.)
Could anyone explain me what ...
- 102k
279
votes
3
answers
95k
views
What is a retpoline and how does it work?
In order to mitigate against kernel or cross-process memory disclosure (the Spectre attack), the Linux kernel1 will be compiled with a new option, -mindirect-branch=thunk-extern introduced to gcc to ...
- 60.7k
264
votes
5
answers
280k
views
How does the ARM architecture differ from x86? [closed]
Is the x86 Architecture specially designed to work with a keyboard while ARM expects to be mobile? What are the key differences between the two?
- 2,843
253
votes
3
answers
49k
views
How much of ‘What Every Programmer Should Know About Memory’ is still valid?
I am wondering how much of Ulrich Drepper's What Every Programmer Should Know About Memory from 2007 is still valid. Also I could not find a newer version than 1.0 or an errata.
(Also in PDF form on ...
- 33.5k
217
votes
10
answers
233k
views
What is the difference between Trap and Interrupt?
What is the difference between Trap and Interrupt?
If the terminology is different for different systems, then what do they mean on x86?
- 3,190
212
votes
5
answers
263k
views
The point of test %eax %eax [duplicate]
Possible Duplicate:
x86 Assembly - ‘testl’ eax against eax?
I'm very very new to assembly language programming, and I'm currently trying to read the assembly language generated from a binary. I'...
- 6,373
190
votes
3
answers
35k
views
Why does GCC generate such radically different assembly for nearly the same C code?
While writing an optimized ftol function I found some very odd behaviour in GCC 4.6.1. Let me show you the code first (for clarity I marked the differences):
fast_trunc_one, C:
int fast_trunc_one(...
- 113k
190
votes
4
answers
28k
views
What happens when a computer program runs?
I know the general theory but I can't fit in the details.
I know that a program resides in the secondary memory of a computer. Once the program begins execution it is entirely copied to the RAM. Then ...
- 2,146
184
votes
1
answer
82k
views
What is the best way to set a register to zero in x86 assembly: xor, mov or and?
All the following instructions do the same thing: set %eax to zero. Which way is optimal (requiring fewest machine cycles)?
xorl %eax, %eax
mov $0, %eax
andl $0, %eax
- 2,202
182
votes
12
answers
240k
views
What is the difference between MOV and LEA?
I would like to know what the difference between these instructions is:
MOV AX, [TABLE-ADDR]
and
LEA AX, [TABLE-ADDR]
- 53.5k
182
votes
3
answers
116k
views
How do you use gcc to generate assembly code in Intel syntax?
The gcc -S option will generate assembly code in AT&T syntax, is there a way to generate files in Intel syntax? Or is there a way to convert between the two?
- 7,555
173
votes
4
answers
46k
views
Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?
In the x86-64 Tour of Intel Manuals, I read
Perhaps the most surprising fact is that an instruction such as MOV EAX, EBX automatically zeroes upper 32 bits of RAX register.
The Intel documentation (...
- 3,512
172
votes
5
answers
94k
views
Header files for x86 SIMD intrinsics
Which header files provide the intrinsics for the different x86 SIMD instruction set extensions (MMX, SSE, AVX, ...)? It seems impossible to find such a list online. Correct me if I'm wrong.
- 257k
172
votes
4
answers
10k
views
An expensive jump with GCC 5.4.0
I had a function which looked like this (showing only the important part):
double CompareShifted(const std::vector<uint16_t>& l, const std::vector<uint16_t> &curr, int shift, int ...
- 1,113
170
votes
9
answers
170k
views
"No such file or directory" error when executing a binary
I was installing a binary Linux application on Ubuntu 9.10 x86_64. The app shipped with an old version of gzip (1.2.4), that was compiled for a much older kernel:
$ file gzip
gzip: ELF 32-bit LSB ...
- 57.4k
160
votes
3
answers
192k
views
What does `dword ptr` mean?
Could someone explain what this means? (Intel Syntax, x86, Windows)
and dword ptr [ebp-4], 0
- 5,520
158
votes
3
answers
51k
views
What is the meaning of "non temporal" memory accesses in x86
This is a somewhat low-level question. In x86 assembly there are two SSE instructions:
MOVDQA xmmi, m128
and
MOVNTDQA xmmi, m128
The IA-32 Software Developer's Manual says that the NT in ...
- 123k
153
votes
6
answers
105k
views
What is the purpose of XORing a register with itself? [duplicate]
xor eax, eax will always set eax to zero, right? So, why does MSVC++ sometimes put it in my executable's code? Is it more efficient that mov eax, 0?
012B1002 in al,dx
012B1003 push ...
- 102k
152
votes
5
answers
262k
views
Purpose of ESI & EDI registers?
What is the actual purpose and use of the EDI & ESI registers in assembler?
I know they are used for string operations for one thing.
Can someone also give an example?
- 61.8k
147
votes
7
answers
43k
views
How does this milw0rm heap spraying exploit work?
I usually do not have difficulty to read JavaScript code but for this one I can’t figure out the logic. The code is from an exploit that has been published 4 days ago. You can find it at milw0rm.
...
- 137k
144
votes
5
answers
436k
views
What is the function of the push / pop instructions used on registers in x86 assembly?
When reading about assembler I often come across people writing that they push a certain register of the processor and pop it again later to restore it's previous state.
How can you push a register? ...
- 1,459
137
votes
6
answers
17k
views
Why does integer overflow on x86 with GCC cause an infinite loop?
The following code goes into an infinite loop on GCC:
#include <iostream>
using namespace std;
int main(){
int i = 0x10000000;
int c = 0;
do{
c++;
i += i;
...
- 465k
135
votes
6
answers
119k
views
What is the "FS"/"GS" register intended for?
So I know what the following registers and their uses are supposed to be:
CS = Code Segment (used for IP)
DS = Data Segment (used for MOV)
ES = Destination Segment (used for MOVS, etc.)
SS = Stack ...
- 205k
131
votes
9
answers
178k
views
What does "int 0x80" mean in assembly code?
Can someone explain what the following assembly code does?
int 0x80
- 10.2k
131
votes
3
answers
44k
views
CPU Privilege Rings: Why rings 1 and 2 aren't used?
A couple of questions regarding the x86 CPU privilege rings:
Why aren't rings 1 and 2 used by most operating systems? Is it just to maintain code compatibility with other architectures, or is there a ...
- 205k
129
votes
8
answers
93k
views
`testl` eax against eax?
I am trying to understand some assembly.
The assembly as follows, I am interested in the testl line:
000319df 8b4508 movl 0x08(%ebp), %eax
000319e2 8b4004 movl 0x04(%eax), %eax ...
- 5,039
126
votes
11
answers
133k
views
Floating point vs integer calculations on modern hardware
I am doing some performance critical work in C++, and we are currently using integer calculations for problems that are inherently floating point because "its faster". This causes a whole ...
- 5,039
123
votes
3
answers
197k
views
Difference between JE/JNE and JZ/JNZ
In x86 assembly code, are JE and JNE exactly the same as JZ and JNZ?
- 4,831
122
votes
9
answers
249k
views
How to write hello world in assembly under Windows?
I wanted to write something basic in assembly under Windows. I'm using NASM, but I can't get anything working.
How do I write and compile a hello world program without the help of C functions on ...
- 3,079
122
votes
19
answers
159k
views
System.BadImageFormatException: Could not load file or assembly (from installutil.exe)
I am trying to install a Windows service using InstallUtil.exe and am getting the error message
System.BadImageFormatException: Could not load file or assembly '{xxx.exe}' or one of its ...
- 38.3k
116
votes
11
answers
428k
views
How to install ia32-libs in Ubuntu 14.04 LTS (Trusty Tahr)
I installed Ubuntu 14.04 (Trusty Tahr) yesterday. Everything seems OK. But when I tried to compile some C code, I encounter the following error. The error seems to be due to the OS lacking the 32-bit ...
- 1,593
113
votes
2
answers
42k
views
How does x86 paging work?
This question is meant to fill the vacuum of good free information on the subject.
I believe that a good answer will fit into one big SO answer or at least in a few answers.
The main goal is to give ...
112
votes
6
answers
51k
views
Why is SSE scalar sqrt(x) slower than rsqrt(x) * x?
I've been profiling some of our core math on an Intel Core Duo, and while looking at various approaches to square root I've noticed something odd: using the SSE scalar operations, it is faster to take ...
- 40.6k
112
votes
10
answers
40k
views
Why is x86 ugly? Why is it considered inferior when compared to others? [closed]
I've been reading some SO archives and encountered statements against the x86 architecture.
Why do we need different CPU architecture for server & mini/mainframe & mixed-core? says
"PC ...
- 52.3k
105
votes
3
answers
130k
views
Using gdb to single-step assembly code outside specified executable causes error "cannot find bounds of current function"
I'm outside gdb's target executable and I don't even have a stack that corresponds to that target. I want to single-step anyway, so that I can verify what's going on in my assembly code, because I'm ...
- 1,798
103
votes
8
answers
104k
views
What are IN & OUT instructions in x86 used for?
I've encoutered these to instructions IN & OUT while reading "Understanding Linux Kernel" book. I've looked up reference manual.
5.1.9 I/O Instructions
These instructions move data ...
- 52.3k
103
votes
5
answers
59k
views
What is the purpose of the EBP frame pointer register?
I'm a beginner in assembly language and have noticed that the x86 code emitted by compilers usually keeps the frame pointer around even in release/optimized mode when it could use the EBP register for ...
- 67.6k
101
votes
7
answers
83k
views
How do I disassemble raw 16-bit x86 machine code?
I'd like to disassemble the MBR (first 512 bytes) of a bootable x86 disk that I have. I have copied the MBR to a file using
dd if=/dev/my-device of=mbr bs=512 count=1
Any suggestions for a Linux ...
- 28.7k
101
votes
7
answers
53k
views
Why does Intel hide internal RISC core in their processors?
Starting with Pentium Pro (P6 microarchitecture), Intel redesigned it's microprocessors and used internal RISC core under the old CISC instructions. Since Pentium Pro all CISC instructions are divided ...
- 5,197
100
votes
6
answers
30k
views
Enhanced REP MOVSB for memcpy
I would like to use enhanced REP MOVSB (ERMSB) to get a high bandwidth for a custom memcpy.
ERMSB was introduced with the Ivy Bridge microarchitecture. See the section "Enhanced REP MOVSB and ...
- 32.7k
97
votes
7
answers
44k
views
Limitations of Intel Assembly Syntax Compared to AT&T [closed]
To me, Intel syntax is much easier to read. If I go traipsing through assembly forest concentrating only on Intel syntax, will I miss anything? Is there any reason I would want to switch to AT&T (...
- 1,246