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The Intel 8086 family of CPUs provided four segments: the code segment, the data segment, the stack segment and the extra segment. Each segment was placed at a specific location in memory by the software being executed and all instructions that operated on the data within those segments were performed relative to the start of that segment.
Four registers are used to refer to four segments on the 16-bit x86 segmented memory architecture. DS ( data segment), CS ( code segment), SS ( stack segment), and ES (extra segment). Another 16-bit register can act as an offset into a given segment, and so a logical address on this platform is written segment : offset , typically in ...
Segments can be defined to be either code, data, or system segments. Additional permission bits are present to make segments read only, read/write, execute, etc. In protected mode, code may always modify all segment registers except CS (the code segment selector). This is because the current privilege level (CPL) of the processor is stored in ...
The term "segment" comes from the memory segment, which is a historical approach to memory management that has been succeeded by paging.When a program is stored in an object file, the code segment is a part of this file; when the loader places a program into memory so that it may be executed, various memory regions are allocated (in particular, as pages), corresponding to both the segments in ...
In a system using segmentation, computer memory addresses consist of a segment id and an offset within the segment. [3] A hardware memory management unit (MMU) is responsible for translating the segment and offset into a physical address, and for performing checks to make sure the translation can be done and that the reference to that segment and offset is permitted.
Some segments are implicitly treated as code segments, dedicated for instructions, stack segments, or normal data segments. Although the usages are different, the segments do not have different memory protections reflecting this. In the flat memory model all segments (segment registers) are generally set to zero, and only offsets are variable.
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. [8]
The LDT also cannot store certain privileged types of memory segments (e.g. TSSes). Finally, the LDT is actually defined by a descriptor inside the GDT, while the GDT is directly defined by a linear address. Creating shared memory through the GDT has some drawbacks. Notably such memory is visible to every process and with equal rights.