Array access in assembly language

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.

Greg Hewgill · Accepted Answer · 2012-02-23 02:26:48Z

3

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].

edited Feb 23, 2012 at 2:26

answered Feb 23, 2012 at 1:38

Greg Hewgill

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6 Comments

lcs Over a year ago

It is initialized to zero. I've tested this by just performing mov [result], esi in the asm statement.

paxdiablo Over a year ago

@millsj, you shouldn't rely on esi being set to 0, you should explicitly enforce it.

lcs Over a year ago

@paxdiablo understandable, I added a line at the start to set it to 0, there is no change on the output however, it still reads 2.

lcs Over a year ago

Adding the mov ebx, a line is now causing a bus error. gdb disassemble shows it occurring on the add line: 0x00001ecd <main+109>: add (%ebx,%esi,4),%eax. Is there any way to stop the value of a from shifting around?

Greg Hewgill Over a year ago

Actually, you might need lea, I'll update my answer (it all depends on how your assembler interprets memory references). Or, you could make a a global variable.

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