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.
- Index: Bits 3-15 of 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 all zeros in the lowest 3 bits.
- TI: Specifies which descriptor table to use. If clear (0) then the GDT is used, if set (1) then the current 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: