Assembly question

Hi,

Can you please explain what the following program does?

Code:

---

Execution begins at address 0
        .pos 0
init:  irmovl Stack, %esp      # Set up Stack pointer
        irmovl Stack, %ebp      # Set up base pointer
        jmp Main                # Execute main program

# Array of 4 elements
        .align 4
array:  .long 0xd
        .long 0xc0
        .long 0xb00
        .long 0xa000

Main:  irmovl $4,%eax
        pushl %eax      # Push 4
        irmovl array,%edx
        pushl %edx      # Push array
        call rSum      # Sum(array, 4)
        halt

/* $begin rsum-ys */
        # int Sum(int *Start, int Count)
rSum:  pushl %ebp
        rrmovl %esp,%ebp
        pushl %ebx          # Save value of %ebx
        mrmovl 8(%ebp),%ebx  # Get Start
        mrmovl 12(%ebp),%eax # Get Count
        andl %eax,%eax      # Test value of Count
        jle L38              # If <= 0, goto zreturn
        irmovl $-1,%edx
        addl %edx,%eax      # Count--
        pushl %eax          # Push Count
        irmovl $4,%edx
        rrmovl %ebx,%eax
        addl %edx,%eax
        pushl %eax          # Push Start+1
        call rSum            # Sum(Start+1, Count-1)
        mrmovl (%ebx),%edx
        addl %edx,%eax      # Add *Start
        jmp L39              # goto done
L38:    xorl %eax,%eax      # zreturn:
L39:    mrmovl -4(%ebp),%ebx # done: Restore %ebx
        rrmovl %ebp,%esp    # Deallocate stack frame
popl %ebp            # Restore %ebp
        ret
/* $end rsum-ys */
        .pos 0x400
Stack:  # The stack goes here

---

Do you have a specific question?

The comments seem to make a fairly good job of explaining what is going on.

Does it add the values of the 4 element array given?

Looks like it. But it's reallty weird syntax (looks like x86, but not quite), and it's doing it in a very weird way - why would you spend that many instructions on adding 4 numbers together.

--
Mats

It's x86 assembly in AT&T syntax, as used by the GNU assembler. And it's not a very weird way. It's pretty much equivalent to this C code:

Code:

---

int rSum(int *ar, int size)
{
  return size == 0 ? 0 : rSum(ar+1, size-1) + ar[0];
}

---

i.e. a recursive array summation function. The main function then uses this function to sum up the global array.

The weird part is the recursion, not the separate function.

It may be a similar syntax to AT&T, but as far as I know, you don't differentiat different types of move instructions in standard AT&T, whilst this has irmov, rrmov to indiate "immedate to register" and "register to register" move.

And yes, I did understand that it's recursive, which is a solution, but hardly an efficient one.

--
Mats

The move differentiation is interesting indeed.

That's not standard AT&T assembly syntax. I dunno what it is.