Segment Selector
A Segment Selector is a 16-bit binary data structure that is used on x86 CPUs 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
| 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 in a segment selector is not shifted from a standard 16-bit representation.
- 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.
Uses
Segment Selectors are used in a few locations, including: