Re: [i386] Questions regarding provisional page tables initialization

Ahmed S. Darwish
2007-07-02T10:23:32Z

Hi Andreas,

On Mon, Jul 02, 2007 at 11:18:08AM +0200, Andreas Schwab wrote:
> "Ahmed S. Darwish" <darwish.07@gmail.com> writes:
>
> > yes, but isn't the displacement here (0x007) a _bytes_ displacement ?. so
> > effectively, %ecx now contains physical address of pg0 + 7bytes. Is it A
> > meaningful place/address ?.
>
> It's not pg0 + 7bytes, it is pg0 plus 3 flag bits. Since a page address
> is always page aligned, the low bits are reused for flags.
>

I'm sure there's a problem in _my_ understanding, but isn't the displacement
- as specified by AT&T syntax - represented in bytes ?. I've wrote a small
assembly function to be sure:

.data
integer:
.string "%d\n"

.text
test_func:
push %ebp
mov %esp, %ebp
push 0x008(%ebp) ## 8 bytes displacement (the first arg), right ?
push $integer
call printf
mov %ebp, %esp
pop %ebp
ret

The above method works fine and prints "5" to stdout by the code:

.global main
main:
mov $5, %eax
push %eax
call test_func

movl $1, %eax
movl $0, %ebx
int $0x80

now back to head.S code:
leal 0x007(%edi),%ecx /* Create PDE entry */

Isn't the above line the same condition (bytes, not bits displacement) ?.
Thanks for your patience !.

(For other kind replies, don't understand me wrong. I did my homework and
studied the pte format before asking ;). It's just the bytes/bits issue
above that confuses me).

--
Ahmed S. Darwish
HomePage: http://darwish.07.googlepages.com
Blog: http://darwish-07.blogspot.com