29. Segment Registers in Protected Mode

• Segment registers are called selectors when operating in protected mode.

• In protected mode, segment registers simply point to data structures called segment descriptors that contain the information needed to access a physical memory location.

  Segment selector is a segment register, containing the selector value:

  13-bit index field selects one of 8,192 segment descriptors. TI (table indicator) specifies segment descriptor, which can be in GDT - global descriptor table (one for all programs) LDT - local descriptor table (typically one for each program, more can exist.) IDT - interrupt descriptor table.

  • RPL requestor privilege level - 2-bit field, specifies if the access to the segment is allowed.

     

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