Segmentation (memory)
Encyclopedia
Memory segmentation is the division of computer memory
into segments or sections. Segments or sections are also used in object file
s of compiled programs when they are linked together into a program image, or when the image is loaded
into memory. In a computer system using segmentation, a reference to a memory location includes a value that identifies a segment and an offset within that segment.
Different segments may be created for different program modules, or for different classes of memory usage such as code
and data segment
s. Certain segments may even be shared between programs.
; another common one is paging
, and they can be combined. The size of memory segments is generally not fixed and is usually less than any limitation imposed by the computer, otherwise segmentation could be treated the same as paging. Also, segmentation is generally more visible than paging because the programmer or compiler has to define the segments.
A segment has a length and set of permissions associated with it. A process
is only allowed to make a reference into a segment if the type of reference is allowed by the permissions, and the offset within the segment is within the range specified by the length of the segment. Otherwise, a hardware exception such as a segmentation fault
is raised.
Information is also associated with a segment that indicates where the segment is located in memory. This might be the address of the first location in the segment, or the address of a page table
for the segment if the segmentation is implemented with paging
. In the first case, if a reference to a location within a segment is made, the offset within the segment will be added to address of the first location in the segment to give the address in memory of the referred-to item; in the second case, the offset of the segment is translated to a memory address using the page table.
Segments may also have a flag indicating whether the segment is present in main memory or not; if a segment is accessed that is not present in main memory, an exception is raised, and the operating system
will read the segment into memory from secondary storage.
When a segment does not have a page table associated with it, the address of the first location in the segment is usually an address in main memory; in those situations, no paging is done. In the Intel 80386
and later, that address can either be an address in main memory, if paging is not enabled, or an address in a paged address space, if paging is enabled.
A memory management unit
(MMU) is responsible for translating a segment and offset within that segment into a memory address, and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.
, does not provide any protection. Any program running on these processors can access any segment with no restrictions. A segment is only identified by its starting location; there is no length checking.
Segmentation in the Intel 80286
and later provides protection: with the introduction of the 80286, Intel retroactively named the sole operating mode of the previous x86 CPU models "real mode" and introduced a new "protected mode" with protection features. For backward compatibility, all x86 CPUs start in "real mode" with no memory protection, fixed 64 KiB segments, and only 20-bit (1024 KiB) addressing. An 80286 or later processor must be switched into another mode by software in order to use its full address space and advanced MMU features.
there are different memory model
s that specify the names of special segments and whether or not they may be combined.
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
into segments or sections. Segments or sections are also used in object file
Object file
An object file is a file containing relocatable format machine code that is usually not directly executable. Object files are produced by an assembler, compiler, or other language translator, and used as input to the linker....
s of compiled programs when they are linked together into a program image, or when the image is loaded
Loader (computing)
In computing, a loader is the part of an operating system that is responsible for loading programs. It is one of the essential stages in the process of starting a program, as it places programs into memory and prepares them for execution...
into memory. In a computer system using segmentation, a reference to a memory location includes a value that identifies a segment and an offset within that segment.
Different segments may be created for different program modules, or for different classes of memory usage such as code
Code segment
In computing, a code segment, also known as a text segment or simply as text, is one of the sections of a program in an object file or in memory, which contains executable instructions....
and data segment
Data segment
A data segment is a portion of virtual address space of a program, which contains the global variables and static variables that are initialized by the programmer...
s. Certain segments may even be shared between programs.
Hardware implementation
Memory segmentation is one of the most common ways to achieve memory protectionMemory protection
Memory protection is a way to control memory access rights on a computer, and is a part of most modern operating systems. The main purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it. This prevents a bug within a process from affecting...
; another common one is paging
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
, and they can be combined. The size of memory segments is generally not fixed and is usually less than any limitation imposed by the computer, otherwise segmentation could be treated the same as paging. Also, segmentation is generally more visible than paging because the programmer or compiler has to define the segments.
A segment has a length and set of permissions associated with it. A process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
is only allowed to make a reference into a segment if the type of reference is allowed by the permissions, and the offset within the segment is within the range specified by the length of the segment. Otherwise, a hardware exception such as a segmentation fault
Segmentation fault
A segmentation fault , bus error or access violation is generally an attempt to access memory that the CPU cannot physically address. It occurs when the hardware notifies an operating system about a memory access violation. The OS kernel then sends a signal to the process which caused the exception...
is raised.
Information is also associated with a segment that indicates where the segment is located in memory. This might be the address of the first location in the segment, or the address of a page table
Page table
A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are those unique to the accessing process...
for the segment if the segmentation is implemented with paging
Paging
In computer operating systems, paging is one of the memory-management schemes by which a computer can store and retrieve data from secondary storage for use in main memory. In the paging memory-management scheme, the operating system retrieves data from secondary storage in same-size blocks called...
. In the first case, if a reference to a location within a segment is made, the offset within the segment will be added to address of the first location in the segment to give the address in memory of the referred-to item; in the second case, the offset of the segment is translated to a memory address using the page table.
Segments may also have a flag indicating whether the segment is present in main memory or not; if a segment is accessed that is not present in main memory, an exception is raised, and the 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...
will read the segment into memory from secondary storage.
When a segment does not have a page table associated with it, the address of the first location in the segment is usually an address in main memory; in those situations, no paging is done. In the Intel 80386
Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
and later, that address can either be an address in main memory, if paging is not enabled, or an address in a paged address space, if paging is enabled.
A memory management unit
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
(MMU) is responsible for translating a segment and offset within that segment into a memory address, and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.
x86 architecture
The memory segmentation used by early x86 processors, beginning with the Intel 8086Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...
, does not provide any protection. Any program running on these processors can access any segment with no restrictions. A segment is only identified by its starting location; there is no length checking.
Segmentation in the Intel 80286
Intel 80286
The Intel 80286 , introduced on 1 February 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and 8088...
and later provides protection: with the introduction of the 80286, Intel retroactively named the sole operating mode of the previous x86 CPU models "real mode" and introduced a new "protected mode" with protection features. For backward compatibility, all x86 CPUs start in "real mode" with no memory protection, fixed 64 KiB segments, and only 20-bit (1024 KiB) addressing. An 80286 or later processor must be switched into another mode by software in order to use its full address space and advanced MMU features.
Object files
Segments or sections are also defined in object files. Segments in different object files may be combined by the linker according to rules specified when the segments are defined. Conventions exist for segments shared between object files; for instance, in DOSDOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
there are different memory model
Memory model (computing)
In computing, a memory model describes the interactions of threads through memory and specifies the assumptions the compiler is allowed to make when generating code for segmented memory or paged memory platforms.-History and significance:...
s that specify the names of special segments and whether or not they may be combined.
See also
- Data segmentData segmentA data segment is a portion of virtual address space of a program, which contains the global variables and static variables that are initialized by the programmer...
- Text segment
- BSS Segment
- Virtual address spaceVirtual address spaceVirtual address space is a memory mapping mechanism available in modern operating systems such as OpenVMS, UNIX, Linux, and Windows NT...
- Virtual memoryVirtual memoryIn 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...
- x86 memory segmentation
- Segmentation faultSegmentation faultA segmentation fault , bus error or access violation is generally an attempt to access memory that the CPU cannot physically address. It occurs when the hardware notifies an operating system about a memory access violation. The OS kernel then sends a signal to the process which caused the exception...