OSDev.org

Posted: **Wed Jan 25, 2012 3:40 pm**

I write my os mostly in c, but sometimes I NEED to use assembly. if i define a variable in c:

main()
{
  long var = 0;
}

and then i want to do something with it in assembly:

asm("movl $1, var\n");

the gcc compiler gives me this error:

/tmp/ccpKcN36.o: In function `main':
test.c:(.text+0xf): undefined reference to `var'
collect2: ld returned 1 exit status

why?

Posted: **Wed Jan 25, 2012 4:51 pm**

The wiki has a section about inline assembly, and examples of correct syntax:
http://wiki.osdev.org/Inline_Assembly

Posted: **Fri Jan 27, 2012 9:51 am**

This is GCC specific. In MSVC it is perfectly valid to use local variable names in inline assembly directly. This is mainly a matter of syntax.

c and assembly problem