void set_pd(void)
{
    //set each entry to not present
    for(size_t i=0;i<1024;i++)
    {
        // This sets the following flags to the pages:
        //   Supervisor: Only kernel-mode can access them
        //   Write Enabled: It can be both read from and written to
        //   Not Present: The page table is not present
        page_directory[i] = 0x00000002;
    }
}