Assembly inline

Tiago Maluta — Mon, 30 Nov 2009 02:20:11 +0000

I’ve made some code snippets about assembly inline with GCC. A quick search points to a lot of good documentation.

IBM DeveloperWorks about Inline asssembly for x86 in Linux
Linux Documentation Project HOWTO describing GCC Inline Assembly

The syntax may be confusing, if you don’t understand read the documentation available. Each example are an C function with the special construct “asm” for inline assembly code. Examples:

Increment an value:

void inc(int *value) {
 
    asm ( "incl %0"
         :"=a"(*value)
         :"0"(*value)
        );
 
}

Exchange to numbers:

void swap(int *x, int *y) {
 
    asm(""
        : "=a"(*y),"=b"(*x)
        : "a"(*x),"b"(*y)
       );
 
}

Copy an vector:

void vector_copy(int *v_src, int *v_dst, int *count) {
 
    asm("up: lodsl;"
	"    stosl;"
	"loop up;  "
	:
	: "S"(v_src), "D"(v_dst), "c"(*count)
	: "%eax"
	);
 
}

A simple copy:

void copy(int *from, int *to) {
 
     asm ("movl %1, %%eax;"
          "movl %%eax, %0;"
          :"=&r"(*to)
          :"r"(*from));
 
}

A very simple implementation for strncpy:

void _strncpy(char *src, char *dst, int *count) {
 
    asm("cld;"
        "rep movsb;"
        :
        : "S"(src), // 'S' == %esi
	  "D"(dst), // 'D' == %edi
	  "c"(*count)); // 'c' == %ecx
}

If you like to try check this example:

wget http://github.com/maluta/junk/raw/master/asm-inline.c
gcc -Wall -ggdb asm-inline.c -o asm-inline
./asm-inline

coding » asm

Assembly inline