Segment Selector

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A Segment Selector is a 16-bit binary data structure specific to the IA-32 and x86-64 architectures. It is used in Protected Mode and Long Mode. Its value identifies a segment in either the Global Descriptor Table or a Local Descriptor Table. It contains three fields and is used in a variety of situations to interact with Segmentation.

For more information, see Section 3.4.2: Segment Selectors and Figure 3-6: Segment Selector of the Intel Software Developer Manual, Volume 3-A.

Segment Selector

Segment Selector
15    3 2 1   0
Index TI RPL
  • Index: The Index of the GDT or LDT entry referenced by the selector. Since Segment Descriptors are 8 bytes in length, the value of Index is never unaligned and contains no data in the lowest 3 bits. As such Index is not shifted from a standard 16-bit representation (as if the other fields were not present).
  • TI: Specifies which descriptor table to use. If clear (0) then the GDT is used, if set (1) then the LDT is used.
  • RPL: The requested Privilege Level of the selector, determines if the selector is valid during permission checks and may set execution or memory access privilege.


Segment Selectors are used in a few locations, including:

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