Thread: IA32 Assembly

My question is geared towards the IA32 Assembly language that a C program produces. I've been given the following and my task is to figure out what the funciton does. I've written my comments next to each line and as far as I can tell it looks to me that if local variable a >= y then 0 gets returned, if not then other actions are taken until a is > and then a value for b is returned. Any help on exactly what this function does would be greatly appreciated.

Code:

 pushl   %ebp
        movl    %esp, %ebp
        subl    $16, %esp
        movl    $0, -8(%ebp)   SET LOCAL VARIABLE B TO 0
        movl    $0, -4(%ebp)   SET LOCAL VARIABLE A TO 0
        jmp     .L2	       JUMP TO L2

			IF (A<Y)
.L3:
        movl    -4(%ebp), %eax  MOVE A TO EAX
        sall    $2, %eax	A * 2^2 IN EAX
        addl    8(%ebp), %eax   ADD X + A IN EAX
        movl    (%eax), %eax	MOVE (X + A) IN EAX
        addl    %eax, -8(%ebp)  ADD (X + A) IN B
        addl    $1, -4(%ebp)	ADD 1 TO A
.L2:
        movl    -4(%ebp), %eax  MOVE A TO EAX
        cmpl    12(%ebp), %eax	COMPARE A TO Y
        jl      .L3		IF A < Y JUMP TO L3
        movl    -8(%ebp), %eax  MOVE B TO EAX
        leave
        ret			RETURN B

That's clearly not very well optimized...

Code:

        push   %esi
        push   %edi
        mov    $12(%esp), %esi
        mov    $16(%esp), %edi
  
        movl    $0, %ecx  SET LOCAL VARIABLE B TO 0
        movl    $0, %edx   SET LOCAL VARIABLE A TO 0
        jmp     .L2	       JUMP TO L2

			IF (A<Y)
.L3:
        addl    %ecx, (%esi), %eax	
        addl     %esi, 4
        incl       %edx
.L2:
        cmpl    %edi, %edx	COMPARE A TO Y
        jl      .L3		IF A < Y JUMP TO L3
        movl    %ecx, %eax  MOVE B TO EAX
        pop     %edi
        pop     %esi
        ret			RETURN B

I actually find this easier to understand...

--
Mats

Yes.

--
Mats

It is not storing data into the array. But yes, it walks in steps of 4.

--
Mats