Basic HighMeM Question

[Prabhu nath]

Please see inline.

On Mon, Jun 27, 2011 at 2:42 PM, piyush moghe <pmkernel at gmail.com> wrote:

> I have very basic some question's related to HighMem Memory Mapping:
>
> 1) Why can't we directly map memory in highmemory?
>
        This question is incorrect ? Typically on a intel architecture, the
physical address space is divided into LOWMEM and HIGHMEM region. Lower 896
MB is marked as LOWMEM region and >896MB as HIGHMEM. More likely in the
intel architecture the memory is always decoded from 0x00000000. For Eg. If
you have memory of 1GB, then 896MB is decoded to LOWMEM and the rest is
decoded in HIGHMEM.

>
> 2) As documents at many places why is the limit of 896MB for ZONE_NORMAL?
>
        Since this 896 MB of physical address space is directly mapped to
the Kernel linear virtual address space i.e from (0xC0000000 to 0xF8000000).
Also, there is a Fixed constant offset relation between VA to PA i.e. VA =
PA + 0xC000000. This has been done to avoid any page table walk for
translating kernel virtual address to physical address.
Thus when kernel virtual address is generated for execution, PA is
calculated by MMU directly, thus making kernel code execution faster.

All the above explaination strictly holds good for Intel architecture on
Desktop machines.

Regards,
Prabhunath

>
>
> Regards,
> Piyush
>
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