segmented address space
(foldoc) | 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.
[In what way were the others better than Intel's {brain
damaged} implementation?].
(2004-06-01)
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