Question

I’m trying to simply sum up an array in assembly.

int main(){
 int a[5] = {1, 2, 3, 4, 5};
 int result;

 _asm{
   mov ecx, 5 ;set the counter for 5
   mov eax, 0 ;zero eax
 NXT: add eax, [esi*4+a] ;add array value
      inc esi ;increase esi to read next value
      LOOP NXT ;loop back to next

   mov [result], eax ;mov eax into result

  }

 printf("result: %u",result);

Regardless of what the value of the arrays are, it always sums to 2.

I’m trying to run on a Mac.

I’m compiling using:

gcc -fasm-blocks -m32 -c sum.cpp

and linking with

gcc -arch i386 -g -o sum sum.o

I’ve tried various things, but I can’t seem to get it to sum the array.

Answer 1

It looks like you aren’t initialising esi to anything, so it contains some random value at the start of your loop.

Also, since a exists at a non-fixed location (it is a local variable, after all), you should load the address of a into a register first:

lea ebx, a
...
add eax, [esi*4+ebx]

I suspect your inline assembler is doing the wrong thing with [esi*4+a].