segmented address space

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segmented address space

An addressing scheme where all memory references are formed by adding an offset to a base address held in a segment register.

The effect is to segment memory into blocks, which may overlap either partially or completely, depending on the contents of the segment registers but normally they would be distinct to give access to the maximum total range of addresses. In this case the scheme does provide some degree of memory protection within a single process since, for example, a data reference cannot affect an area of memory containing code. However, compilers must either generate slower code or code with artificial limits on the size of data structures.

The best known implementation is that used on the Intel 8086 and later Intel microprocessors, where a 16-bit offset is added to a 16-bit base address held in one of four segment base registers. Each instruction has a default segment (code (CS), data (DS), stack (SS), ? (ES)) which determines which segment register is used. Special prefix instructions allow this default to be overridden.

Other computers, such as GE-645/Honeywell Multics, Burroughs large systems (B-5500, B-6600), and others, have used segmentation to good effect.

Opposite: flat address space. See also addressing mode.

segmented address space

Memory addressing in which each byte is referenced by a base number (the segment) plus an offset. An x86-based PC running in 16-bit mode (Real Mode and Virtual 8086 Mode) uses 64KB segments, and a segment register always points to the base of the segment that is currently being addressed. Although the PC 32-bit mode is considered a flat address space, it too uses segments. However, the segments are 4GB, huge by comparison to 64KB segments, and more than enough to handle all the memory most users require. See Real Mode and Virtual 8086 Mode.