OSDev.org

The Place to Start for Operating System Developers
It is currently Thu Mar 28, 2024 4:28 am

All times are UTC - 6 hours




Post new topic Reply to topic  [ 4 posts ] 
Author Message
 Post subject: Question about 32-bits paging
PostPosted: Fri Sep 20, 2019 10:54 am 
Offline

Joined: Fri Sep 20, 2019 10:37 am
Posts: 2
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];
}

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 :(


Top
 Profile  
 
 Post subject: Re: Question about 32-bits paging
PostPosted: Fri Sep 20, 2019 11:12 am 
Offline
Member
Member

Joined: Tue Mar 04, 2014 5:27 am
Posts: 1108
NotSchur wrote:
Code:
page_t* table = ((uint32_t*) 0xFFC00000) + (0x400 * dir_index);

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.


It's C. You're adding to a pointer to uint32_t. Adding 1 advances the pointer to the next uint32_t (4 bytes away).


Top
 Profile  
 
 Post subject: Re: Question about 32-bits paging
PostPosted: Fri Sep 20, 2019 3:32 pm 
Offline
Member
Member

Joined: Wed Mar 09, 2011 3:55 am
Posts: 509
NotSchur wrote:
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);


To expand on alexfru's response, if this code snippet were instead:

Code:
page_t* table = (uint32_t*)((0xFFC00000) + (dir_index << 12));


So that you cast to a pointer *after* adding dir_index, it should work identically to your code.


Top
 Profile  
 
 Post subject: Re: Question about 32-bits paging
PostPosted: Fri Sep 20, 2019 4:33 pm 
Offline

Joined: Fri Sep 20, 2019 10:37 am
Posts: 2
That makes perfect sense now! Kicking myself for not having spotted this #-o
Thanks a lot to both of you!


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 6 hours


Who is online

Users browsing this forum: DotBot [Bot] and 57 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group