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In Protected mode, when CPU translates addresses, it checks every memory reference to insure it behaves properly.
Effective address is the physical address of the data in memory.
If the effective address (segment base plus offset) is greater than limit field in segment descriptor, the 386 aborts the instruction and generates a protection violation exception.
Programmer can also specify that a segment is read-only, then any write attempt will create the same exception.
If the memory access isn't valid, the CPU generates either a General Protection Fault (GPF).
A GPF indicates that program is trying to access memory that it doesn't have access to.
A page fault indicates that program is trying to access a memory location that doesn't exist.
More: by The Ganssle Group.
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