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Access to memory is qualified by the descriptor selected by the current segment register.
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Selector is a 12-bit number indicating which entry to use in the descriptor table:
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If the selector is 0, the first descriptor is taken
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Selector of 1 takes the second, etc.
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The 80386 multiplies the selector by 8 (8 bytes per entry), and adds this to the base address of the table of descriptors (contained in an internal 386 register loaded by the programmer before switching to protected mode.)
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Segment descriptor format:
