Finding the PDE offset and the PTE offset from a virtual add

[0xBADC0DE]

How do you find the PDE offset and PTE offset from a virtual address?

[paulbarker]

PDE_OFFSET = (addr / PAGE_SIZE / PAGES_PER_PDE) * sizeof(PDE_ENTRY)
PTE_OFFSET = (addr / PAGE_SIZE % PAGES_PER_PDE) * sizeof(PTE_ENTRY)

PAGE_SIZE = 4096
PAGES_PER_PDE = 1024
I think the size of each entry is 4 bytes but I'm not sure of that.

Disclaimer: This is off the top of my head and not checked against documentation.

[Candamir]

Wouldn't it be like this?

Code: Select all

/** Gets the page directory entry for a given virtual address.
 */

unsigned long get_pde(unsigned long *va)
{
   unsigned long first_ten       = ((unsigned long)va) >> 22; // Top ten bits tell index of pde
   return page_directory[first_ten]; // Use global pointer to page directory
}

/** Gets the page table entry for a given virtual address.
 */

unsigned long get_pte(unsigned long *va)
{
   unsigned long pde             = get_pde(va);
   unsigned long *pde_address    = (unsigned long *)(pde & 0xFFFFF000); // Clear all flags in bits 0-11 to obtain addr
   
   unsigned long next_ten        = (((unsigned long)va) >> 12) & 0x3FF; // Next ten bits tell index of pte in table
   return pde_address[next_ten];
}

The code is heavily based on the fact that all PDEs start addresses are 4MB-aligned and thus only rely on the last ten bits of the address, and the PTEs are always 4KB-aligned and so we use the next/previous (depends on how you look at it) 10 bits.

[paulbarker]

I'm pretty sure thats equivalent to my code. The point of mine was to explain how its done without doing it for him .

[Candamir]

Oops ;D Forgot about the MT-philosophy for a moment