ARM : Kernel : Setting up of MMU in head.S

[Dave Hylands]

Hi Prakash,

On Wed, Mar 30, 2011 at 2:35 PM, Dave Hylands <dhylands at gmail.com> wrote:
> Hi Prakash,
>
> On Wed, Mar 30, 2011 at 8:19 AM, Prakash K.B. <prakashk75 at gmail.com> wrote:
>> Hello.
>>
>> Please do not hesitate to let me know if this must be posted elsewhere.
>>
>> I have been trying to understand the code that sets up the MMU. I do have a
>> fair understanding of the way MMU is meant to be setup, but something in the
>> kernel code is tripping me.

Some further explanation is due.

When the kernel starts, the MMU is off, and ther ARM is running with
an implicit identity mapping (i.e. each virtual address maps to the
same physical address).

If your physical memory starts at 0x80000000, then the PC will be 0x800xxxxx.

When the MMU table is turned on, the PC is still at 0x800xxxx, so even
though the kernel has 0xc00xxxxx mapped to 0x800xxxxx it also has to
have 0x800xxxxx mapped to 0x800xxxxx.

So this mapping of 0x800xxxxx to 0x800xxxxx is the "identity" portion
and is needed while switching the MMU on. The 0xc00xxxxx to 0x800xxxxx
mapping is what's used while the kernel is running.

-- 
Dave Hylands
Shuswap, BC, Canada
http://www.davehylands.com