Segmentation: Difference between revisions

DS, ES, FS, GS, SS are used to form addresses when you want to read/write to memory. They don't always have to be explicitly encoded, because some processor operations assume that certain segment registers will be used.

:''Segmentation is considered obsolete memory protection technique in protected mode by both CPU manufacturers and most of programmers. It is no longer supported in long mode. The information here is required to get protected mode working; also 64 bit GDT is needed to enter long mode and segments are still used to jump from long mode to compatibility mode and the other way around. If you want to be serious about OS development, we strongly recommend using flat memory model and [[Paging|paging]] as memory management technique. For more information, consult [[x86-64]].''

* The segment type (Code/Data/Read/Write/Accessed/Conforming/Non-Conforming/Expand-Up/[[Expand_Down|Expand-Down]])

*In this model all the segments cover the full address space (Usuallyusually 0->4Gb on x86). In essence this means that we completely ignore the A part of our A:B logical address. The reason for this is that most processors don't actually have segmentation (Plusand it's a hell of a lotmuch easier for the compiler to optimize).

*This leaves you with 2 descriptors per privilege level (usually Ring 0 and Ring 3 normally), one for code and one for data, which both describe precisely the same segment. The only difference being that the code descriptor is loaded into CS, and the data descriptor is used by all the other segment registers. The reason you need both a code and data descriptor is that the processor will not allow you to load CS with a data descriptor (This is to help with security when using a segmented memory model, and although useless in the flat-memory model it is still required because you can't turn off segmentation).