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 16-bit Protected-Mode Addressing
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     Protected mode adds a level of indirection to the addressing process.
     Instead of pointing to a base location in physical memory, the segment
     portion of a logical address becomes a selector, which is an index into
     a descriptor table.  The descriptor then refers to a physical memory
     segment.

     Each descriptor contains information about the type (code or data), the
     limit (its size), and the base address of the physical segment to which
     it refers.  If a program attempts to write beyond the segment limit, to
     write into a code segment, or to  execute data, a General Protection
     Fault (Interrupt 0x0D) occurs.  (For more information about descriptors,
     turn to the Descriptors section in this chapter.)

     Each protected-mode descriptor on an 80286 can have a limit from 0h to
     FFFFh, allowing you to access from 1 byte to 64 KB of physical memory
     through that descriptor.  On a 286, the portion of the descriptor that
     points to the base physical address contains 24 bits, so an 80286 CPU
     running in protected mode can address up to 16 MB of physical memory.

     This indirect addressing system, which protects you from inadvertently
     overwriting code or data and allows you to address more memory, may
     require a few adjustments in your real-mode programs.  The section,
     Protected-Mode Programming Restrictions, in this chapter, lists
     techniques that will not work in protected mode.