Question

I have the following code. And I want to complete the assembly code as indicated below:

int main(void)
{
    int x = 10;

    int i=0;
    label1:


    asm (.....) // code to add here: if i>=x then jump to label2

    printf("%d\n",i);

    i++;
    asm (.....) // code to add here: jump to label 1
    label2:

    printf("out\n");
}

My machine is x86 and the Operating system is Ubuntu 12

Answer 1

First, get yourself a list of x86 op codes, should be easy to find online.

The asm() function follows this order:

asm ( "assembly code"
           : output operands                  /* optional */
           : input operands                   /* optional */
           : list of clobbered registers      /* optional */
);

Second, one major problem you have is you can’t “jump” to a C label, you need to make your label an “assembly” label to be able to jump to it. ex:

int main()
{
  asm("jmp .end");    // make a call to jmp there
  printf("Hello ");
  asm(".end:");       //make a "jumpable" label
  printf("World\n");
  return 0;
}

Output of this program is simply “World” as we jumped over the “Hello “. Here’s the same example but with a comparative jump:

int main()
{
    int x = 5, i = 0;
    asm(".start:");
    asm("cmp %0, %1;"   // compare input 1 to 2
        "jge .end;"     // if i >= x, jump to .end
        :               // no output from this code
        : "r" (x), "r" (i));  // input's are var x and i
    printf("Hello ");
    i++;
    asm("jmp .start;");    
    asm(".end:");
    printf("World\n");
    return 0;
}