Re: [Pgubook-readers] movl _start, %eax

[Shuaib]

Thanks,I'd appreciate an explanation. I have been
trying to ask some OS folks, but I have not have a
decent explanation as to why the value at the location
pointed to by _start changes.

--- Jonathan Bartlett <address@hidden> wrote:

> Very wierd.  Looking into it...
> 
> The _start symbol is replaced correctly.  Anyway,
> I'll try to mess w/ it
> this week.
> 
> Jon
> 
> On Sun, 21 Nov 2004, Shuaib wrote:
> 
> > I dissasemble the maximum program in chapter 3.
> > (objdump maximum -D) and I get this (only a part
> of
> > it):
> >
> > 08048074 <_start>:
> >  8048074:       bf 00 00 00 00          mov
> > $0x0,%edi
> >  8048079:       8b 04 bd a8 90 04 08    mov
> > 0x80490a8(,%edi,4),%eax
> >
> > 080490a8 <data_items>:
> >  80490a8:       07                      pop    %es
> >  80490a9:       00 00                   add
> > %al,(%eax)
> >  80490ab:       00 08                   add
> > %cl,(%eax)
> >
> >
> > When I try "movl _data_items, %eax", %eax has 7 in
> it.
> > When I try "movl $_data_items, %eax", %eax has
> > 080490a8 in it. Great, everything makes sense!
> >
> > On the other hand, When I try "movl $_start,
> %eax",
> > %eax has 08048074 in it (good; makes sense since
> > $_start is the address). When I try "movl
> _data_items,
> > %eax", %eax has 0xcc in it. The address location
> > pointed to by _start contains bf000000, and I was
> > expecting bf000000 when I executed that
> instruction.
> > WHY am I getting 0xcc? I suspect other people will
> get
> > other values in the %eax.
> >
> > Here is what I'd like some elite to clarify: Does
> the
> > OS (or the loader) relocate the _start section
> > somewhere else in the memory and 0x08048074 no
> longer
> > points to the the original address that _start
> pointed
> > to? Where does this 0xcc come from? Will I be able
> to
> > see this in the dissasembly section?
> >
> > I'd very much appreciate an explanation. I do not
> > believe the reason is in the book.
> >
> >
> >
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> 



                
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