Re: Why "high memory" in x86?

[Rajendra <rpm@solidcore.com>]

The reason for the high memory is this.

   o Linux divides the address space into two parts, user and kernel.
   o Kernel gets 1 GB of address space while user gets 3GB virtual 
address space.
   o Kernel needs to access all of the  memory so ideally it needs 4 GB 
of virtual addresses.
   o But since only 1 GB (i.e. beyond 0xc000 0000) is available, so we 
call the rest as
      high memory (approx 3 GB)
   o The high memory is accessed using temp. page table entries that map 
the high memory
      areas in kernel address space.
   o The high memory region is mostly allocated to the user space programs.
  
hope it answers the question !

regd,
~rpm
Rajat Jain wrote:

> Hi list,
>
> I recently read that the concept of "High Memory" was introduced
> because certain architectures are capable of physically addressing
> larger amounts of memory than they can virtually address (physical
> address space > virtual address space). I also read that nowadays
> "high Memory" exists only in x86.
>
> 1) Why is virtual memory > 896 MB on x86 designated as high memory?
> AFAIK x86 has 4 GB of virtual address space (=physical address space?)
>
> 2) Has the "high Memory" concept got anything to do with PAE (Page
> Address Extention) feature of x86?
>
> 3) Do any other architectures than x86 have the concept of high memory?
>
> TIA,
>
> Rajat
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