Memory segmentation

Memory segmentation is the division of a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifies a segment and an offset(memory location) within that segment. Segments or sections are also used in object files of compiled programs when they are linked together into a program image and when the image is loaded into memory.

Segments usually correspond to natural divisions of a program such as individual routines or data tables so segmentation is generally more visible to the programmer than paging alone. 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 be shared between programs.

In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment. A hardware memory management unit (MMU) is responsible for translating the segment and offset into a physical 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.

Each segment has a length and set of permissions (for example, read, write, execute) 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 if 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.

Segments may also be used to implement virtual memory. In this case each segment has an associated flag indicating whether it 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.

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