inline-assembly

I'm writing a performance-critical, number-crunching C++ project where 70% of the time is used by the 200 line core module. I'd like to optimize the core using inline assembly, but I'm completely new to this. I do, however, know some x86 assembly languages including the one used by GCC and NASM. All I know: I have to put the assembler instructions in _asm{} where I want them to be. Problem: I have no clue where to start. What is in which register at the moment my inline assembly comes into play? You can access variables by their name and copy them to registers. Here's an example from MSDN: int

I want to cause an ARM Cortex-M3 Undefined Instruction exception for the test of my test fixture. The IAR compiler supports this with inline assembly like this: asm("udf.w #0"); Unfortunately the GNU CC inline assembler does not know this opcode for the NXP LPC177x8x. It writes the diagnostic: ccw3kZ46.s:404: Error: bad instruction `udf.w #0' How can I create a function that causes a Undefined Instruction exception? Building on Masta79 's answer: There is a "permanently undefined" encoding listed in the ARMv7-M architecture reference manual - ARM DDI 0403D ( documentation placeholder,

While going through some C code having inline assembly I came across the .byte (with a Dot at the beginning) directive. On checking the assembly reference on web I found that it is used to reserve a byte in memory. But in the code there was no label before the statement. So I was wondering what is use of an unlabeled .byte directive or any other data storage directive for that matter. For e.g. if i code .byte 0x0a , how can i use it ? There are a few possibilities... here are a couple I can think of off the top of my head: You could access it relative to a label that comes after the .byte

Why does this print garbage instead of exiting my program gracefully? I use system calls this way on BSD, and I wonder what would I need to make it work in Linux. int main(int argc, char **argv) { __asm ("movq $1,%rax; movq $0,%rdi; syscall"); /* exit(0) ? */ return 0; } Thanks. Why does this print garbage instead of exiting my program gracefully? Per CESA-2009-001 , "Syscall 1 is exit on i386 but write on x86_64". what would I need to make it work in Linux Use the syscall ordinals from the current unistd_64.h Hope this helps! Syscall 1 is exit on i386 but write on x86-64 I believe. EDIT: this

In x86 assembly, the overflow flag is set when an add or sub operation on a signed integer overflows, and the carry flag is set when an operation on an unsigned integer overflows. However, when it comes to the inc and dec instructions, the situation seems to be somewhat different. According to this website , the inc instruction does not affect the carry flag at all. But I can't find any information about how inc and dec affect the overflow flag, if at all. Do inc or dec set the overflow flag when an integer overflow occurs? And is this behavior the same for both signed and unsigned integers? =

I am trying to write a simple compare and swap inline assembly code. Here is my code #include <stdio.h> #include <stdlib.h> #include <stdint.h> static inline unsigned long cas(volatile unsigned long* ptr, unsigned long old, unsigned long _new) { unsigned long prev=0; asm volatile("lock cmpxchg8b %0;" : "=m"(prev) : "m"(*ptr),"a"(old),"c"(_new) ); return prev; } int main() { unsigned long *a; unsigned long b=5,c; a=&b; c=cas(a,b,6); printf("%lu\n",c); return 0; } This code should ideally print 5 but it is printing 0. What is wrong in my code ?Please help. Let me start by saying "Using inline

what does this line in assembly arm does? mrc p15, 0, %0, c9, c13, 0" : : "r" (counter) who is p15 isn't it should be r15 what are all the others? what is :: who are c9, c1 what is the role of each argument? Whilst MRC is a generic co-processor inter-op instruction, cp15 is the control processor - which all modern ARM CPUs have and this has been used by ARM was a means of extending the instruction set for on-chip units such as the cache, MMU, performance monitoring and lots else besides. Taking your instruction a bit at a time: mrc p15, 0, %0, c9, c13, 0" : : "r" (counter) According to the ARM