The processor privilege level when accessing data segments see Intel documentation for details

Figure 2-3. Segment Selector and Segment Descriptor

Figure 2-3. Segment Selector and Segment Descriptor

Since a Segment Descriptor is 8 bytes long, its relative address inside the GDT or the LDT is obtained by multiplying the 13-bit index field of the Segment Selector by 8. For instance, if the GDT is at 0x00020000 (the value stored in the gdtr register) and the index specified by the Segment Selector is 2, the address of the corresponding Segment Descriptor is

The first entry of the GDT is always set to 0. This ensures that logical addresses with a null Segment Selector will be considered invalid, thus causing a processor exception. The maximum number of Segment Descriptors that can be stored in the GDT is 8,191 (i.e., , 2131).

Continue reading here: Segmentation Unit

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