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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