Selectors and descriptors are those items that provide the
    hardware with the software's expected image of what is located
    where.


Selectors

    A selector is a 16-bit pointer that, when loaded into a register
    or used with certain instructions, selects certain descriptors. In
    a logical address, the selector portion identifies an individual
    descriptor by first specifying the descriptor table and then
    indexing to the descriptor within that table.

    
    General selector format:
     15                                   3  2  1  0
    ┌──────────────────────────────────────┬──┬─────┐
    │                 Index                │TI│ RPL │
    └──────────────────────────────────────┴──┴─────┛

    Index       Selects one of up to 8192 descriptors in a descriptor
                table. The 80386+ multiplies this index value by eight
                (the length of a descriptor) and then adds the result
                to the base address of the descriptor table. This
                accesses the correct entry in the table.

    TI          The Table Indicator bit specifies the descriptor
                table to which the selector refers: a zero points to
                the GDT (Global Descriptor Table), and a one indicates
                the current LDT (Local Descriptor Table).

    RPL         Requested Privilege Level - used by the system
                protection mechanism.