Re: [Pgubook-readers] Exercise Chapter 4 - Maximum modified

[Justis Durkee]

Horacio wrote:

> Hi!, I'm going to introduce myself before starting with the questions. My name 
> is Horacio and I live in Argentina. I've been reading the book and making all 
> the exercises, but there's one that keep me asleep at night.
> Chapter 4. modify the maximun program from chapter 3, replace the loop with a 
> function that recives a pointer to the data_items. (Actualy, you'll have to 
> pass the function 3 lists and it' will return the maximum number from the 
> last one, but I'm putting it shorter in order to get the point)
>
> Here is my program, but I'll mark the point were i cant understand what to do
>
> #Purpouse: Finds the maximum number on a list
> #
> #Function maximo recives the pointer to the data_items
> #
> #               %eax = holds number being examined
> #               %ebx = holds maximum number
> #		%ecx = pointer to data 
>
> .section .data
>
> data_items:
> .long 3,67,34,222,45,54,34,44,33,22,11,66,0
>
> .section .text
>
> .globl _start
> _start:
>
> movl $data_items, %ecx          #makes %ecx pointer
>
> cmpl $0, %ecx                           #checks if we have reached the end of 
> the list
>  
You have set %ecx to a pointer, yet here you are comparing a constant(0) 
to %ecx.  This loop will most likely end in a segmentation fault, as it 
will increment %ecx into unknown memory locations.  You probably want to 
indirectly address %ecx as (%ecx).

> je salir_maximo                         #if it's true we exit de function
>
> addl $4, %ecx                           #moves the pointer to the next value
>
> movl (%ecx), %eax                       #moves the value to %eax, this could be 
> ommited
>                                                 #compared directly to ebx
> cmpl %eax, %ebx
> jle ############                #This is the point were I don't know how to do 
> it
>                                                 #I'll need to jump to the 
> beggining of the function and
>                                                 #make all the loop again. I'd 
> tried with je call maximo but
>                                                 #it failed, also je maximo and 
> also.
>
>  
Here you want to jump to the beginning of the loop instead of the 
function as you wouldn't want to adjust the base pointer again.  This 
would require a label just before the beginning of the loop, before the 
cmpl instruction.  The jump should occur if %eax is smaller the %ebx, so 
you would need a jge.  Check out page 39 for why jge.


> movl %eax, %ebx                 #makes %eax the maximum value
> #############                   #Here I will also need to make all the loop 
> again
>
>  
Since this is the end of the loop instructions, control should go to the 
beginning of the loop unconditionally.

> salir_maximo:
> movl %ebp, %esp
> popl %ebp
> ret
>
> I've tried a lot of thing before calling for help, but I feel that the answer 
> is easy and I've been missing something.
>
> Thank You!
>
>  
I too am working through the book.  Perhaps we can help each other out 
now and again.

Hope that helps,
Justis

> _______________________________________________
> Pgubook-readers mailing list
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/pgubook-readers
>
>