OSDev.org

Posted: **Wed Feb 13, 2008 12:18 pm**

Hi everyone!

I'm making a minimal OS in C, based in James M. and Brandon F. tutorials.

I want to add paging to it, so I got some questions.

As far as I know, paging is another way of accessing memory which provides memory protection and virtual address, Am I wrong?

If I want to enable it I'll do the following:

Load my kernel in a direction which can be divided by 4096
Add padding to all kernel with Align(4096) in the linker file in each section
Make a symbol at the end so I know where can I start allocating memory
Alloc page_directory and page_table aligned at 4K
Set all kernel pages to be read and writeable by the kernel but not user
Make kernel virtual address the same linear address
Set video framebuffer page/s read and write only by kernel
Add a Page fault handler at interrupt 14
Send the linear address of page_directory to CR3
Set on the paging bit in CR0

Am I missing something? Is something innecesary?

Thanks,

Gonzalo

Posted: **Wed Feb 13, 2008 1:00 pm**

Load my kernel in a direction which can be divided by 4096

Not strictly necessary. Only saves you a few computations and bytes.

Add padding to all kernel with Align(4096) in the linker file in each section

not necessary either. The only real reason to do this is that pages start on a 4096-byte boundary and you might waste a page if the kernel doesn't start on a boundary, or you want different permissions for code or data parts of the kernel.

Make a symbol at the end so I know where can I start allocating memory

that's for memory management. It's not necessarily part of initializing paging itself, but something you can use in the process. It's handy to have nevertheless as it allows you to dynamically determine how much space to reserve for the kernel

Alloc page_directory and page_table aligned at 4K

if you plan on using 4M pages, you don't necessarily need page tables. Conversely, you might want more than one page table to start with.

Set all kernel pages to be read and writeable by the kernel but not user

See the note above, you may want to map parts (code, readonly data) as not-writable.

Make kernel virtual address the same linear address

The main reason for this is that when paging is enabled the code that is currently being executed doesn't get "changed". For higher-half kernels the virtual address does not match the physical address.

Set video framebuffer page/s read and write only by kernel

Handy to have, but not strictly necessary either. My kernel for one has a different way of doing things.

Add a Page fault handler at interrupt 14

You haven't got a default handler yet?

Send the linear address of page_directory to CR3

it seems to me that you're confusing virtual, linear and physical addresses:

Code: Select all

virtual address -----------> linear address ------------> physical address
                segmentation                   paging

Set on the paging bit in CR0

Obviously

Posted: **Wed Feb 13, 2008 1:48 pm**

Lots of thanks Combuster!

You answered all my questions.

Combuster wrote:
Send the linear address of page_directory to CR3
it seems to me that you're confusing virtual, linear and physical addresses

I meant to send the physical address of it

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Questions about paging

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