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.
So, this "swapper_pg_dir" is a symbol in a section of code. That section of code has an associated segment and this symbol lives at a particular location in that section and thus in that segment, and that particular location, relative to the beginning of the section is it's offset. Combine those two and you have the logical address of swapper_pg_dir.
Ok I think I got where the addresses are assigned. Its in the linker script vmlinux.lds. I got the precise answer here http://tldp.org/HOWTO/Linux-i386- Boot-Code-HOWTO/kernel_head.html I had completely forgotten about linker scripts. I was getting confused as the ENTRY(swapper_pg_dir) has ORG 0x10000 as the offset. Was struggling to find where the .text address was.