Memory segmentation is the division of computer memory into segments or sections. Segments or sections are also used in object files 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 segments. Certain segments may even be shared between programs.[1]
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Memory segmentation is one of the most common ways to achieve memory protection; 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.[1]
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.
The memory segmentation used by early x86 processors, beginning with the Intel 8086, 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.
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 DOS there are different memory models that specify the names of special segments and whether or not they may be combined.[2]
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