Virtual address space

Source: Wikipedia, the free encyclopedia.
"Virtual address" and "virtual addressing" redirect here. For other uses, see Virtual address (disambiguation).

In

64-bit OS versions. This provides several benefits, one of which is security through process isolation assuming each process is given a separate address space
.

Example

In the following description, the terminology used will be particular to the Windows NT operating system, but the concepts are applicable to other virtual memory operating systems.

When a new application on a

GiB VAS: each one of the memory addresses (from 0 to 232 − 1) in that space can have a single byte as a value. Initially, none of them have values ('-' represents no value). Using or setting values in such a VAS would cause a memory exception
. 

           0                                           4 GiB
VAS        |----------------------------------------------|

Then the application's executable file is mapped into the VAS. Addresses in the process VAS are mapped to bytes in the exe file. The OS manages the mapping: 

           0                                           4 GiB
VAS        |---vvv----------------------------------------|
mapping        |||
file bytes     app

The v's are values from bytes in the

DLL
files are mapped (this includes custom libraries as well as system ones such as kernel32.dll and user32.dll): 

           0                                           4 GiB
VAS        |---vvv--------vvvvvv---vvvv-------------------|
mapping        |||        ||||||   ||||
file bytes     app        kernel   user

The process then starts executing bytes in the EXE file. However, the only way the process can use or set '-' values in its VAS is to ask the OS to map them to bytes from a file. A common way to use VAS memory in this way is to map it to the

page file
. The page file is a single file, but multiple distinct sets of contiguous bytes can be mapped into a VAS: 

           0                                           4 GiB
VAS        |---vvv--------vvvvvv---vvvv----vv---v----vvv--|
mapping        |||        ||||||   ||||    ||   |    |||
file bytes     app        kernel   user   system_page_file

And different parts of the page file can map into the VAS of different processes: 

           0                                           4 GiB
VAS 1      |---vvvv-------vvvvvv---vvvv----vv---v----vvv--|
mapping        ||||       ||||||   ||||    ||   |    |||
file bytes     app1 app2  kernel   user   system_page_file
mapping             ||||  ||||||   ||||       ||   |
VAS 2      |--------vvvv--vvvvvv---vvvv-------vv---v------|

On Microsoft Windows 32-bit, by default, only 2 GiB are made available to processes for their own use.[2] The other 2 GiB are used by the operating system. On later 32-bit editions of Microsoft Windows, it is possible to extend the user-mode virtual address space to 3 GiB while only 1 GiB is left for kernel-mode virtual address space by marking the programs as IMAGE_FILE_LARGE_ADDRESS_AWARE and enabling the /3GB switch in the boot.ini file.[3][4]

On Microsoft Windows 64-bit, in a process running an executable that was linked with /LARGEADDRESSAWARE:NO, the operating system artificially limits the user mode portion of the process's virtual address space to 2 GiB. This applies to both 32- and 64-bit executables.[5][6] Processes running executables that were linked with the /LARGEADDRESSAWARE:YES option, which is the default for 64-bit Visual Studio 2010 and later,[7] have access to more than 2 GiB of virtual address space: up to 4 GiB for 32-bit executables, up to 8 TiB for 64-bit executables in Windows through Windows 8, and up to 128 TiB for 64-bit executables in Windows 8.1 and later.[4][8]

Allocating memory via

malloc
establishes the page file as the backing store for any new virtual address space. However, a process can also explicitly map file bytes.

Linux

For x86 CPUs, Linux 32-bit allows splitting the user and kernel address ranges in different ways: 3G/1G user/kernel (default), 1G/3G user/kernel or 2G/2G user/kernel.[9]

See also

Notes

  1. ^ IBM Corporation. "What is an address space?". Retrieved August 24, 2013.
  2. MSDN
    . Microsoft.
  3. MSDN
    . Microsoft.
  4. ^
    MSDN
    . Microsoft.
  5. MSDN
    . Microsoft.
  6. MSDN
    . Microsoft.
  7. MSDN
    . Microsoft.
  8. MSDN
    . Microsoft.
  9. ^ "Linux kernel - x86: Memory split".

References

  • "Advanced Windows" by Jeffrey Richter, Microsoft Press
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