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3. 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.
Every protected mode program must include a table of descriptors, which are 4-word data structures that defines the start and end of a segment.

Depending on the type of a segment, the descriptor stores other information such as access rights and the like.
In a multitasking operating system, where numerous programs can run simultaneously, programs cannot access the code and data of another process without permission.

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