Editing High memory

{{Short description|Part of physical memory in a computer}}
{{distinguish|High Memory Area}}
{{more citations needed|date=October 2012}}

'''High memory''' is the part of [[computer memory|physical memory]] in a [[computer]] which is not directly mapped by the [[page table]]s of its [[operating system]] [[kernel (operating system)|kernel]]. The phrase is also sometimes used as shorthand for the [[High Memory Area]], which is a different concept entirely.

Some operating system kernels, such as [[Linux]], divide their [[virtual address space]] into two regions, devoting the larger to [[user space]] and the smaller to the kernel.  In current [[32-bit]] [[x86]] computers, this commonly (although does not have to, as this is a configurable option) takes the form of a 3GB/1GB split of the 4&nbsp;GB address space, so kernel virtual addresses start at 0xC0000000 and go to 0xFFFFFFFF. The lower 896&nbsp;MB, from 0xC0000000 to 0xF7FFFFFF, is directly mapped to the kernel physical address space, and the remaining 128&nbsp;MB, from 0xF8000000 to 0xFFFFFFFF, is used on demand by the kernel to be mapped to high memory. When in [[user mode]], translations are only effective for the first region, thus protecting the kernel from user [[computer program|programs]], but when in [[kernel mode]], translations are effective for both regions, thus giving the kernel an easy way to refer to the [[data buffer|buffers]] of processes—it just uses the process' own mappings.<ref>[https://lwn.net/Articles/75174/ Virtual Memory I: the problem]</ref>

However, if the kernel needs to refer to physical memory for which a userspace translation has not already been provided, it has only 1&nbsp;GB (for example) of virtual memory to use.  On computers with a lot of physical memory, this can mean that there exists memory that the kernel cannot refer to directly—this is called high memory. When the kernel wishes to address high memory, it creates a mapping on the fly and destroys the mapping when done, which incurs a performance penalty.

==See also==
* [[Physical Address Extension]] (PAE)

==References==
<references />

== External links ==
* [http://linux-mm.org/HighMemory High Memory]
* [https://lwn.net/Articles/75174/ Virtual Memory I: the problem]

{{DEFAULTSORT:High Memory}}
[[Category:X86 architecture]]
[[Category:X86 memory management]]
[[Category:Linux kernel]]