When protection and paging is switched on the processor, it requires valid
page tables entry for every page that is accessed. Now there are only 4 GB
addresses that are available, so we have to divide it in such a way that the
kernel as well as the user can access it. So what we do is we say that first
three GB address will always be user space address. The page table entries
of these will keep on changing as the process loads, allocates and
deallocates
memory. While the last 1 GB i.e. from 3 GB to 4 GB is given to kernel and
it's page table entries are always present in the processor page tables.
The kernel
address are hence identity mapped i.e. phy_addr = (virt_addr << 4) >>
4. Since
the kernel is the program that manages all the resources including
memory, so it needs
access to all the memory that is there in the system, so for regions of
memory
above 1 GB (physical), we use special mechanism and call it as high memory.
~rpm
Dave B. Sharp wrote:
Yes, but why is only 1GB of memory "available"? The
whole address space is available to other kernels.
Dave Sharp
--- Rajendra <rpm@xxxxxxxxxxxxx> wrote:
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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