SEGMENTATION OF 80386 PROCESSOR

The memory management unit (MMU) consists of a segmentation unit and a paging unit. The
segmentation unit allows the use of two address components, such as segment and offset for relocabilityand sharing of code and data. The segmentation unit allows segments of size 4Gbytes at maximum. Thesegmentation unit provides a four level protection mechanism for protecting and isolating the system's code and data from those of the application program.
Descriptors
Every segment has a descriptor. When the programmer creates a logical segment, the system software
creates the corresponding descriptor with the help of compilers and loaders, linkers. It is the job of
operating system. Descriptors contain the information about segment. The 80386 descriptors have a 20- bit segment limit and 32-bit segment address. The descriptors of 80386 are 8-byte quantities containing access right or attribute bits along with the base and limit of the segments.
Descriptor Tables
In any system, there will be number of segments of various types created for various applications. Thus,there should be as many descriptors too. All these descriptors are stored in tables called descriptor tables. These tables are created by system software and stored in memory. The segmentation scheme provides a way of offering protection to different types of data and code.
Descriptor Attribute Bits

•  The A (accessed) attribute bit indicates whether the segment has been accessed by the CPU or not.
•  The TYPE field decides the descriptor type and hence the segment type.
•  The S bit decides whether it is a system descriptor (S=0) or code/data segment descriptor (S=1).
•  The DPL field specifies the descriptor privilege level.
•  The D bit specifies code segment operation size. If D=l, the segment is a 32-bitoperand segment,      else, it is a 16-bit operand segment.
•  The P-bit (present) signifies whether the segment is present in the physical memory or not. If P=l,    the segment is present in the physical memory.
•  The G (granularity) bit indicates whether the segment is page addressable. The zero-bit must remain  zero for compatibility with future processors.
•  The AVL (available) field specifies whether the descriptor is available for user or for the operating  system.

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