问题
I was looking through some code and found 2 lines that perplexed me:
add -0x4(%esi,%ebx,4),%eax
cmp %eax,(%esi,%ebx,4)
I am accustomed to the standard add src,dst and cmp x1,x2 and I'm not really sure what these lines are actually doing.
I believe that it is compiled with GCC
回答1:
That's using the Base + (Index * Scale) + Displacement addressing mode. At least, I think so. I'm not real familiar with the AT&T syntax. I think the Intel syntax would be:
add eax,[esi + ebx*4 - 4]
cmp [esi + ebx*4],eax
This looks like it's indexing into an array of integers (4-byte values). Imagine in C that you want to add the value from some array element to a total, like this:
int a[100];
int i = 10;
int total = 0;
total += a[i-1];
Now, make esi hold the address of the array, ebx hold the value of i, and eax hold the value 33. You'd get:
add eax,[esi + ebx*4 - 4]
The comparison instruction is testing to see if the result (in eax) is equal to the next value in the array. In the C example, that would be equivalent to comparing total to a[i].
来源：https://stackoverflow.com/questions/10159264/confusing-add-command-in-x86-assembly