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A segment selector provides the software that uses it with an index into the GDT or LDT (the offset of its associated segment descriptor), a global/local flag (determines whether the selector points to the GDT or the LDT), and access rights information."
Using these instructions, a program loads the visible part of the segment register with a 16-bit selector.
Segment descriptors are stored in descriptor tables: global (GDT) and local (LDT) and are identified with segment selectors. Segment descriptor is an 8 bytes long structure that contains one memory segment's: base address (32-bits), limit (20-bits) and access rights (see Fig. 2.4 and 2.5).
Segment Descriptor: In the Protected mode of x86, Memory Management Unit (MMU) uses the segment selector to access a descriptor, segment descriptors are a part of the segmentation unit, which provides the processor with the data it needs to translate a logical address into a linear address.
When does segment selector comes in picture. The line in Intel Guide says:
"Each segment descriptor has an associated segment selector. A segment selector provides the software that uses it with an index into the GDT or LDT (the offset of its associated segment descriptor), a global/local flag (determines whether the selector points to the GDT or the LDT), and access rights information."
I didn't quite get it.
Are there separate segment selector register(s)? How is it calculated? Why we need it.?
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