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Incomputing, avirtual address space (VAS) oraddress space is the set of ranges of virtual addresses that anoperating 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'sinstruction set architecture and supported by theoperating system's pointer size implementation, which can be 4bytes for32-bit or 8bytes for64-bit OS versions. This provides several benefits, one of which is security throughprocess isolation assuming each process is given a separateaddress space.
When a new application on a32-bit OS is executed, the process has a4GiB VAS: each one of thememory 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 amemory exception.
0 4 GiBVAS |----------------------------------------------|
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 GiBVAS |---vvv----------------------------------------|mapping |||file bytes app
The v's are values from bytes in themapped file. Then, requiredDLL files are mapped (this includes custom libraries as well as system ones such askernel32.dll anduser32.dll):
0 4 GiBVAS |---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 thepage file. The page file is a single file, but multiple distinct sets of contiguous bytes can be mapped into a VAS:
0 4 GiBVAS |---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 GiBVAS 1 |---vvvv-------vvvvvv---vvvv----vv---v----vvv--|mapping |||| |||||| |||| || | |||file bytes app1 app2 kernel user system_page_filemapping |||| |||||| |||| || |VAS 2 |--------vvvv--vvvvvv---vvvv-------vv---v------|
OnMicrosoft Windows 32-bit, by default, only2 GiB are made available to processes for their own use.[2] The other2 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 to3 GiB while only1 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 than2 GiB of virtual address space: up to4 GiB for 32-bit executables, up to8 TiB for 64-bit executables in Windows through Windows 8, and up to128 TiB for 64-bit executables in Windows 8.1 and later.[4][8]
Allocating memory viaC'smalloc establishes thepage file as the backing store for any new virtual address space. However, a process can alsoexplicitly map file bytes.
Forx86 CPUs,Linux 32-bit allows splitting the user and kernel address ranges in different ways:3G/1G user/kernel (default),1G/3G user/kernel or2G/2G user/kernel.[9]