Segment Selector: Difference between revisions

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

• 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 address calculated from the Index is always 8-byte aligned, and the lowest 3 bits of the Index are ignored by the hardware.
• 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.

Uses

Segment Selectors are used in a few locations, including:

• Segmentation Registers
• The Interrupt Descriptor Table
• The Task State Segment

• X86
• Memory Segmentation