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The Pentium is characterized by an architecture native 32bit with regard to the data registers, and addressing, even if in the last versions the part of the addressing has been extended to 64bit. To maintain compatibility with previous Intel family, you can create 16-bit architecture.
As mentioned above is able to operate in two basic ways:
Real-mode, which emulates the hardware 8086 and integrating the visibility of registers 32/64 bits. This mode starts when the reset and is then native BIOS. And 'possible to activate this mode even when operating in protected mode, activating a specific process.
Protected-mode, which makes the architecture 32/64 bit operating systems in which they operate.

80x86 addition to the normal architecture registers are present other, placed in special units. These units have been introduced to manage BitMap (MMX and SSE) and the numbers in floating point (FPU and SSE).

Applications do not have direct access to physical memory, but it, from a logical point of view, can be organized in several ways:
Flat-memory, in which the memory appears as a single block
Segmented-Memory Model, in which the memory is grouped into independent address spaces, said segments.The applications can be addressed only in a part of these segments of maximum size 2 ^ 36 bytes. This technique is an alternative to paging.
Real-address mode-Memory Model, which is the memory model used its plant 8086 compatible. This memory model is analyzed into the following parts.

Address calculation and segmentation
To calculate un'indirizzo physical, the 8086 performs a sum of the contents of a register pointer (or BX), which acts as offset, and the contents of a segment register, after the latter multiplied by 16 (shift left 4 positions).
Thanks to this method of calculation of the addresses you can think of the physical memory organized into segments, each of which begins to address a multiple of 16 and all of a size of 64Kbyte. The segments are partially overlapped. In this way, once loaded an address into a segment register, all addresses of the segment can be expressed using the offset register.
Groups of 16 byte, which begin to address a multiple of 16, are defined in paragraphs.
The segmentation allows an address space equal to 2 ^ 20, using 16 bits of address. And 'possible to divide parts of memory in the data, code, stack, completely independent.You can minimize wasted memory, overlapping segments.

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