Regarding enable paging code and swapper_pg_dir.

mindentropy mindentropy at
Sun Apr 3 16:54:59 EDT 2011

On Sunday 03 Apr 2011 2:22:19 am James Light wrote:
> On Sat, Apr 02, 2011 at 11:18:19PM +0530, mindentropy wrote:
> > On Saturday 02 Apr 2011 9:55:35 pm James Light wrote:
> > > To put the physical address into cr3 later.
> > > $swapper_pg_dir is not the PHYSICAL address of the PGD.
> > 
> > Correct me if I am wrong but a mov $foo,%eax would move the address of
> > foo variable to eax right? Or is there a macro somewhere where they are
> > adding PAGE_OFFSET to the address so that the $swapper_pg_dir -
> > __PAGE_OFFSET justifies?
> Right, but it uses the logical address of foo which n the case of
> swapper_pg_dir must be converted to a
> physical address. When paging is not enabled, linear addresses are
> interpreted as physical addressess.
> So it has to get this logical address
> only one step farther into a linear address.

Can you please elaborate on what one step farther is? So why not use the 
swapper_pg_dir directly i.e. mov $swapper_pg_dir,cr0? In the absence of paging 
the segment:offset  is the physical address isn't it? i.e. the  $swapper_pg_dir 
should be equal to 0x10100 as per the code fragment below?

I am having difficulty in understanding the addressing calculations here and not 
the code,

>In linux2.1.66, the physical address is used directly.
>Just for comparison:
>59 * Setup paging (the tables are already set up, just switch them on)
>60 */
>62        movl $0x101000,%eax
>63        movl %eax,%cr3          /* set the page table pointer.. */
>64        movl %cr0,%eax
>65        orl $0x80000000,%eax
>66        movl %eax,%cr0          /* ..and set paging (PG) bit */
>67        jmp 1f                  /* flush the prefetch-queue */

More information about the Kernelnewbies mailing list