Hi all!
I'm getting back into osdev and there is one thing I can't figure out about paging.
My code for getting a page table entry from a virtual address looks basically like this
Code:
uint32_t dir_index = DIRECTORY_INDEX(virt);
uint32_t table_index = TABLE_INDEX(virt);
directory_entry_t* dir = (directory_entry_t*) 0xFFFFF000;
page_t* table = ((uint32_t*) 0xFFC00000) + (0x400 * dir_index);
if (dir[dir_index] & PAGE_PRESENT) {
return &table[table_index];
}
uint32_t table_index = TABLE_INDEX(virt);
directory_entry_t* dir = (directory_entry_t*) 0xFFFFF000;
page_t* table = ((uint32_t*) 0xFFC00000) + (0x400 * dir_index);
if (dir[dir_index] & PAGE_PRESENT) {
return &table[table_index];
}
It's about the same code given in the wiki page about paging (https://wiki.osdev.org/Paging#Manipulation) and it works like a charm.
My question: why is table not calculated like in the following code? Why does my current code work?
Code:
page_t* table = ((uint32_t*) 0xFFC00000) + (dir_index << 12);
Why do we multiply the index by 0x400 (i.e left-shift by 10) instead of multiplying by 0x1000? Multiplying by 0x400 is like left-shifting by 10 bits, so the index in the page directory should be mangled by 2 bits... I don't get it. Isn't that address translated the same way every other linear address is?
That's the last thing standing in my way for understanding paging
