Re: few doubts in mm

Ravi K
2001-12-26T21:15:54Z

Hi,

There is some good documentation on the Linux MM site
(http://linux-mm.org) which might help you. Here are
some details that I know.

The virtual-to-physical conversion on x386 happens
like this:

- 10 (most significant) bits are used to index into
the page global directory (PGD).
- The PGD entry points to a page containing 1024 page
table entries (PTE).
- The next 10 bits of the address are used to get the
correct PTE.
- PTE points to a page of size 4096 bytes.
- The last 12 bits are used as offset within the page.

That is:
10 bits for PGD + 10 bits for PTE + 12 bits offset

Note:
If the PSE (Page Size Extension) flag is set in the
PGD entry, the second level indexing (PGD->PTE) does
not happen. Instead, the PGD entry points to a 4MB
page and the last 22 bits are directly used as offset
within the page.

Intel provides a 'Physical Address Extension'
(PAE)mode on some processors (Pentium Pro and above).
In this mode, the address size is 36-bits allowing for
up to 64GB of memory can be addressed. Three-level
page table is used in case of PAE mode.

If PAE is enabled when building the Linux kernel,
virtual-to-physical conversion happens like this:

- 2 bits are used to index into the page global
directory.
- The PGD entry points a page containing 512 page
middle directory (PMD) entries.
- The next 9 bits of the address are used to get the
correct PMD.
- PMD entry points to a page containing 512 PTEs.
- The next 9 bits of the address are used to get the
correct PTE.
- The last 12 bits are used as offset within the page.

So: 2 bits for PGD + 9 bits for PMD + 9 bits for PTE +
12 bits offset

Note:
Even though the address size is 36-bits, the virtual
address space is still limited to 4GB (32-bits).
PGD, PMD and PTEs are 64-bit wide in PAE mode.
In PAE mode, the PSE flag will be set in the PMD.

Hope this helps
Ravi.

PS: I feel your question would be more appropriate on
the kernel newbies list (kernelnewbies@nl.linux.org)

--- Vishwanath <vishwanath@inablers.net> wrote:
> Hi all
> I am new to this mailing list and also Memory
> management in linux.
> I have a doubt that how exactly the convertion of
> virtual address to
> physical addr
> happens. As for i have read in Linux Kernel
> Internals, it says there is
> some thing
> called page directory, page middle dir and page
> table, The virtual addr
> is divided into
> 4 parts(len not mentioned), The first part is index
> to page dir.
>
> What exactly the page dir entry contains an address
> or a number index to
> page middle dir, if index,
> then where the base addr of page dir is stored, and
> base addr of page
> middle dir, and page tabe are
> stored.
>
> How exactly this happens, The book also says that
> x86 supports only 2
> level convertion.
> How do i find out what my machine supports, i have
> linux kernel code of
> v2.4.x
>
> Please do help me out.
>
> Thanx in advance
> Vishy

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