Virtual address space

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Virtual address space and physical address space relationship.svg

In computing, a virtual address space (VAS) or address space is the set of ranges of virtual addresses that an operating system makes available to a process.[1] The range of virtual addresses usually starts at a low address and can extend to the highest address allowed by the computer's instruction set architecture. This provides several benefits, one of which is, if each process is given a separate address space, security through process isolation.

Example[edit]

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

When you run a new application on a 32-bit OS, the process has a 4 GiB VAS: each one of the memory addresses (from 0 to 232−1) in that space can have a single byte as value. Initially, none of them have values ('-' represents no value). Using or setting values in such a VAS would cause a memory exception.

           0                                            4GB
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                                            4GB
VAS        |---vvvvvvv------------------------------------|
mapping        |-----|
file bytes     app.exe

The v's are values from bytes in the mapped file. Then, required DLL files are mapped (this includes custom libraries as well as system ones such as kernel32.dll and user32.dll):

           0                                            4GB
VAS        |---vvvvvvv----vvvvvv---vvvv-------------------|
mapping        |||||||    ||||||   ||||
file bytes     app.exe    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                                            4GB
VAS        |---vvvvvvv----vvvvvv---vvvv----vv---v----vvv--|
mapping        |||||||    ||||||   ||||    ||   |    |||
file bytes     app.exe    kernel   user   system_page_file

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

           0                                            4GB
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 a 32-bit Microsoft Windows installation, by default, only 2 GiB are made available to processes for their own use.[2] The other 2GB 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 64-bit Microsoft Windows, processes running 32-bit executables that were linked with the /LARGEADDRESSAWARE:YES option have access to 4 GiB of virtual address space;[5][4] without that option they are limited to 2GB. By default, 64-bit processes have 8TB of user-mode virtual address space; Linking with /LARGEADDRESSAWARE:NO artificially limits the user-mode virtual address space to 2 GB.[6][7]

Allocating memory via system calls such as C's malloc implicitly maps bytes of the page file into the VAS. However, a process can also explicitly map file bytes.

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