Virtual address space
Encyclopedia
Virtual address space is a memory mapping mechanism available in modern operating systems such as OpenVMS
, UNIX
, Linux
, and Windows NT
. This is beneficial for different purposes, one is security through process isolation
.
is easiest to comprehend if one thinks in terms of the VAS, and not the physical memory of the machine nor the size of its page file. Byte values in the VAS come only from byte values in a file
. The OS manages the mapping between the VAS and the files that hold its values.
Each time an application is run on an operating system
(OS), the OS creates a new process and a new VAS for this process.
Physical memory comes in various flavors: on-chip cache, off-chip cache
, and system memory. As far as the process is concerned, system memory is just another level of cache used by the OS. System memory has a lot to do with performance, but nothing to do with the architecture of a process. The process architecture is based on the VAS. Physical memory is used by the OS to map values from file bytes to VAS addresses: process memory is VAS memory, not physical memory.
When you run a new application on a 32-bit
OS, the process has a 4 GB
VAS: each one of the memory address
es (from
.
0 4GB
VAS |----------------------------------------------|
Then the application's EXE
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 2GB are made available to processes for their own use. 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 3GB while only 1GB 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.
On 64-bit Microsoft Windows, processes running 32-bit executables that were linked with the /LARGEADDRESSAWARE:YES option have access to 4GB of virtual address space; 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.
Allocating memory via system call
s such as C
's malloc
implicitly maps bytes of the
page file into the VAS. However, a process can also explicitly map
file bytes.
OpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...
, UNIX
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
, Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, and Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
. This is beneficial for different purposes, one is security through process isolation
Process isolation
Process isolation is a set of different hardware and software technologies designed to protect each operating system process from other processes...
.
Overview
Virtual memoryVirtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
is easiest to comprehend if one thinks in terms of the VAS, and not the physical memory of the machine nor the size of its page file. Byte values in the VAS come only from byte values in a file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
. The OS manages the mapping between the VAS and the files that hold its values.
Each time an application is run on an operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
(OS), the OS creates a new process and a new VAS for this process.
Physical memory comes in various flavors: on-chip cache, off-chip cache
CPU cache
A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations...
, and system memory. As far as the process is concerned, system memory is just another level of cache used by the OS. System memory has a lot to do with performance, but nothing to do with the architecture of a process. The process architecture is based on the VAS. Physical memory is used by the OS to map values from file bytes to VAS addresses: process memory is VAS memory, not physical memory.
Example
- 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
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
OS, the process has a 4 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
VAS: each one of the memory address
Memory address
A digital computer's memory, more specifically main memory, consists of many memory locations, each having a memory address, a number, analogous to a street address, at which computer programs store and retrieve, machine code or data. Most application programs do not directly read and write to...
es (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 exceptionPage fault
A page fault is a trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual address space, but not loaded in physical memory. In the typical case the operating system tries to handle the page fault by making the required page accessible at a location...
.
0 4GB
VAS |----------------------------------------------|
Then the application's EXE
EXE
EXE is the common filename extension denoting an executable file in the DOS, OpenVMS, Microsoft Windows, Symbian, and OS/2 operating systems....
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
Memory-mapped file
A memory-mapped file is a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on-disk, but can also be a device, shared memory object, or other resource...
. Then, required DLL
Dynamic-link library
Dynamic-link library , or DLL, is Microsoft's implementation of the shared library concept in the Microsoft Windows and OS/2 operating systems...
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 2GB are made available to processes for their own use. 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 3GB while only 1GB 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.
On 64-bit Microsoft Windows, processes running 32-bit executables that were linked with the /LARGEADDRESSAWARE:YES option have access to 4GB of virtual address space; 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.
Allocating memory via system call
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
s such as C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
's malloc
Malloc
C dynamic memory allocation refers to performing dynamic memory allocation in the C via a group of functions in the C standard library, namely malloc, realloc, calloc and free....
implicitly maps bytes of the
page file into the VAS. However, a process can also explicitly map
Memory-mapped file
A memory-mapped file is a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on-disk, but can also be a device, shared memory object, or other resource...
file bytes.