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Incomputing, avirtual address space (VAS) is an area of contiguousvirtual memory locations, calledvirtual addresses, which anoperating system makes available to a process for executing instructions and storing data, and which it maps to theaddress space ofphysical addresses in a computer's hardware memory.[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 including security throughprocess isolation, assuming each process is given a separate address 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 GiB VAS |----------------------------------------------|
Then the application's executable file is mapped into the VAS. Addresses in the process VAS are mapped to bytes in theEXE file. The OS manages the mapping:
0 4 GiB VAS |---vvv----------------------------------------| mapping ||| file bytes app
The symbolv represents values from bytes in themapped file. RequiredDLL files are then mapped (this includes custom libraries in addition to system libraries such askernel32.dll anduser32.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 thepage 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 |---vvvv-------vvvvvv---vvvv---vv-----v----vvv-| mapping |||| |||||| |||| || | ||| file bytes app1 app2 kernel user system_page_file mapping |||| |||||| |||| || | 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 asIMAGE_FILE_LARGE_ADDRESS_AWARE and enabling the/3GB switch in theboot.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,PowerPC, andARM 32-bit CPUs,Linux 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]