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So we have to manage a protected addressing a logical / virtual. The memory dell'x86 is of segmented type, in the sense that there is a segment register which coincides with the segment register of the real mode. The only variation is the size. As in the segmented memory is an offset. The logical address is the sum of the segment register and the offset.

The segment register is said selector, as it selects the segment, while the offset indicates the phase shift with respect to the starting address of the segment.
If the offset register is 32bit each segment is 4GB. The memory logic, namely that which is visible to the programmer, is divided into 2 ^ 16 segments each of maximum depth 4GB.

The logical memory must be brought into physical memory, and this procedure is done through the linear memory, in which the addresses are mapped into a single body of 32 bits. In fact it's in memory only the linear part of the logical memory that you are actually using. In addition, the linear memory is divided into two blocks: one for the users to the processes and one dedicated to the OS.
The transfer from linear memory to physical memory is via paging. The linear memory is broken up into pages, which are instantiated (only those employed) in physical memory. The page size can be set through hardware and varies from a few KB to a few MB.

For each resource (ie segment) is given a rank of nobility.Every time you go to manipulate the data contained within a segment must have a certification to that segment. The grades are 4, with indicators from 3 to 0, in the sense that 0 is the maximum level of nobility. Thanks to the privileges you can ensure the reliability of the code and data quality.
During decoding of each instruction, the hardware compares the privilege level of education with that of the data that needs to manage. In order to be executed, each instruction must have a level greater than the DPL DPL of the data that manipulates. At a time when a program with a certain DPL, DPL called something with a lower level, the process goes to DPL. This makes it impossible to make calls to procedures with the lowest level, as a trap handle this anomaly.

Switching from real mode to protected mode
The actual mode is active mainly in the initialization phase. All PCs have a series of instructions that allow the passage in a secure manner. In order for this to work you must initialize the kernel of the OS, initializes the GDT.Within the GDT, during ignition are inserted four descriptors (zero, data, stack, and code). The address of the GDT is placed in the GDT register.
In a secure manner, in the interrupt vector, each index is associated with the descriptor of the code segment that you have to perform.

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