In the given example 5000 is the offset within the file and not the address. Given, the offset 5000, kernel anyway could allocate a physical page and place the contents from the starting of the page and map it to the virtual address.
For E.g. it could allocate a physical page of base address 0x2abd1000 and copy the 1000 bytes of contents from the offset 5000 and map the physical address to some virtual address say 0x08054000.
Q. How does the offset not being page aligned affect the execution ?
The 2nd point wrt fragmentation is convincing.
Regards,
Prabhu
On Mon, Sep 20, 2010 at 10:27 PM, Mulyadi Santosa <mulyadi.santosa@xxxxxxxxx> wrote:
hi..
I think it is due to two reasons:
On Mon, Sep 20, 2010 at 19:30, mohit verma <mohit89mlnc@xxxxxxxxx> wrote:
> thanks guys,
> but the problem is :
> what is the need of page aligned address? as we have given starting
> address 5000 and the length 1000 ( in the example by prabhu) so it should
> start from that location.
1. on some architecture, non page aligned access could mean
trap/exception. Well, x86 is immune in this case...but if you wanna
make your code safely portable, I think it's a must.
2. By making it page aligned, at least you reduce fragmentation.
How come? Let's assume you just have 8K address space. You allocate a
memory starting at 3K as large as 4K. That means you occupy 3K up to
7K address. What's left? 0-1 K and 7-8 K, right?
And what if you wanna allocate another 4K? Yes, a page can fit
that...but not mapping. We can't split below page granularity, so at
least we need two page. one is mapped to 0-1 K, the other is for 7-8K.
It's 2 page vs 3 page case.
Yes, there's slab...but if we can allocate straight in page size
granularity, things would be simpler, right?
--
regards,
Mulyadi Santosa
Freelance Linux trainer and consultant
blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com
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