Re: Why "high memory" in x86?

[Christophe Lucas <clucas@xxxxxxxxxxxxx>]

Rajat Jain (rajat.noida.india@xxxxxxxxx) 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?)

Perhaps I will say some mistakes, please correct me if it is.
Kernel map hardware memory addresses as identical in 3GB->4GB in address
space by means of paging. So 4GB - 3GB = 1G, if you have more than 1G, kernel
must use high memory = mapping and unmapping pages for memory above 1GB.

Sorry if it is ugly mistake ;-)

> 2) Has the "high Memory" concept got anything to do with PAE (Page
> Address Extention) feature of x86?

I don't know.

> 3) Do any other architectures than x86 have the concept of high memory?

Ditto.

Have a nice day,
-- 
			 - Christophe -


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