Kong,
That's right, 'pgd' is a pointer to the translation table (TB) and 'address'
contains the virtual address being mapped. Shifting the virtual address
[right] by 18 (the most significant 11 bits - 31-20) yield the address of
first-level descriptor in the translation table with the first two least
significant bits (1-0) set to zero. Oddly enough, the function below is
shifts by 20. I just wondering if anyone could explain it to me...
Jason
-----Original Message-----
From: Kong Gang Feng [mailto:[EMAIL PROTECTED]]
<mailto:[mailto:[EMAIL PROTECTED]]>
Sent: Wednesday, August 18, 1999 6:51 AM
To: Chagas, Jason; '[EMAIL PROTECTED]'
Subject: Re: pgd_offset
Hi,
I think the entry of 'pgd' is in word (4bytes) instead of
byte.
So PGDIR_SHIFT=18+2=20.
Kong G.F.
At 08:52 PM 8/19/99 -0700, Chagas, Jason wrote:
>Need help understanding some code...
>
>The 'pgd_offset' function defined in
>"./include/asm-arm/proc-armv/pgtable.h" is supposed to
calculate the offset
>into the Translation Table with PGDIR_SHIFT defined as "20"
in
>./include/asm-arm/proc-armv/pgtable.h" (see attached code
snippet). It is my
>understanding the entry into the TB is calculated by
shifting the address
>right by "18" instead. It is a mystery to me how Linux
kernel still works.
>Am I missing something?
>
>Thanks,
>
>Jason
>
>--------------
>/* to find an entry in a page-table-directory */
>extern __inline__ pgd_t * pgd_offset(struct mm_struct * mm,
unsigned long
>address)
>{
> return mm->pgd + (address >> PGDIR_SHIFT);
>}
>
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